Free Synastry Calculator: Relationship Chart Compatibility

What is synastry?

Synastry is the astrological practice of overlaying two natal charts and reading the contacts between them. Each partner's planets land somewhere in the other partner's chart, in a sign, in a house, and in aspect to the other partner's planets. The synastry reading is the description of those contacts, what they suggest about how each person experiences the other, and what tasks the relationship is asking of both people. In traditional Western astrology, synastry is the most-used relationship technique in modern practice.

The technique is ancient. Hellenistic astrologers practiced chart-overlay synastry as early as the first century BCE, and Vettius Valens, the second-century author of the Anthology, includes synastry contacts in his case studies. Chris Brennan, in Hellenistic Astrology (2017), documents the classical roots of the method. Medieval Arabic astrologers refined the technique. The modern Western revival was led by Stephen Arroyo, Relationships and Life Cycles (1979); Liz Greene, Relating (1976); Howard Sasportas; and Robert Hand, Planets in Composite (1975), where Hand frames synastry as the necessary first step before composite interpretation.

According to Stephen Arroyo, synastry describes the symbolic landscape of a connection, not its outcome. The contacts between two charts show what energies meet, what attracts, what soothes, what challenges, and what feels fated, but the chart suggests pattern, not verdict. Liz Greene's framing in Relating goes further: synastry names the inner relationship the two people are already having with each other at the unconscious level, before words and choices ever enter the picture. The work of the relationship is to bring that unconscious landscape into conscious awareness.

To run a synastry reading, you need both partners' full birth data: date, time, and location. Each partner's free birth chart feeds the calculation. Synastry pairs naturally with the composite chart, the midpoint chart of the relationship itself, and many practitioners read both for a single couple.

The 5 most important synastry contacts

Every synastry chart produces dozens of inter-aspects, but five contact types carry most of the interpretive weight in modern Western astrology. Stephen Arroyo treats these as the synastry's structural backbone in Relationships and Life Cycles. Liz Greene confirms the priority in Relating. Begin every reading with these five before turning to secondary contacts.

The five priority contacts at a glance

Sun-Moon contacts

The conscious-unconscious meeting. Widely considered the strongest indicator of relational fit. Read first.

Venus-Mars contacts

Magnetic attraction. What each partner finds beautiful meeting what each partner pursues. The classical chemistry signature.

Moon-Moon contacts

Emotional rapport. How easy or strained the daily-living emotional layer feels. Often more predictive of long-term ease than romantic chemistry.

Saturn contacts

The bonding force. Saturn-Sun, Saturn-Moon, Saturn-Venus from one chart to the other. Heavy at first, often the longest-lasting.

North Node contacts

Fated growth. When one partner's planets touch the other's North Node, the relationship often feels purposeful and developmental.

Arroyo's sequence is deliberate. The Sun-Moon contacts describe the most fundamental layer of relational fit. The Venus-Mars contacts describe the chemistry, which is often what brings two people together but is the weakest predictor of long-term ease. The Moon-Moon contacts are arguably more important than Venus-Mars for sustainability, because daily emotional rapport is what couples actually live inside. Saturn and the North Node describe the longer arc, what bonds the connection and where it is asking each partner to grow.

Sun-Moon contacts: the “soulmate” aspect

A close Sun-Moon contact between two charts, especially a conjunction or a trine, is widely considered the single strongest indicator of a “right partner” sensation in traditional Western astrology. Liz Greene describes Sun-Moon contacts as the meeting of conscious will and unconscious need, the day-self of one person meeting the night-self of the other. Stephen Arroyo, in Relationships and Life Cycles, treats Sun-Moon aspects as the synastry's foundation layer.

The Sun person's conscious identity and the Moon person's emotional life become deeply legible to each other in this contact. The Sun person feels recognized in who they are becoming. The Moon person feels their emotional nature met by a counterpart who already understands it. In Greene's framing, this is the aspect that creates the feeling of “coming home” that couples report when they describe their relationship as fated.

The aspect type matters. A Sun-Moon conjunction is the classic soulmate aspect, the two luminaries meeting at the same degree across two charts. A trine flows easily. A sextile carries opportunity. A square or opposition is more complicated: the same recognition is there, but with productive friction built in. Greene notes that Sun-Moon squares can produce the most intense relationships in practice precisely because the recognition coexists with real difference, requiring conscious work to integrate.

In my practice with couples carrying tight Sun-Moon contacts, the recognition arrives in the first conversation. There is a sense of having met before, of being immediately legible to each other. The chart suggests this pattern, not a verdict on duration. To read each partner's Sun and Moon separately, use the Big 3 calculator (Sun, Moon, Rising) and the dedicated Moon sign calculator, then bring both readings into the synastry above.

Venus-Mars contacts: attraction

Venus-Mars contacts are the classical signature of romantic and sexual chemistry in synastry. Venus signifies what you find beautiful, what you value, and how you express affection. Mars signifies what you pursue, how you act on desire, and your assertive style. When Person A's Venus sits on Person B's Mars, or vice versa, the magnetic aspect of attraction often becomes the loudest single signal in the chart. Stephen Arroyo treats this as the synastry's chemistry layer, distinct from the Sun-Moon emotional layer.

The aspect type modulates how the chemistry feels. A conjunction or trine creates instant, effortless attraction. A square produces friction-charged attraction, where the chemistry is present but accompanied by tension or conflict. An opposition mirrors the chemistry across the relationship axis, often producing the “opposites attract” dynamic. According to Liz Greene, even challenging Venus-Mars aspects often persist as attraction long after the relationship's other layers shift, which is why couples sometimes stay magnetically drawn together even when day-to-day life is hard.

Venus-Mars contacts are particularly important in romantic synastry, but less central in friendship or family readings, where Mercury and Moon contacts often outweigh the Venus-Mars chemistry layer. Robert Hand notes that long-running marriages frequently show Venus-Mars contacts, but so do shorter, more turbulent connections. The chemistry layer is real but is not, on its own, a predictor of duration.

Moon-Moon contacts: emotional rapport

Moon-Moon contacts between two charts describe the emotional rapport layer of the relationship: how easy or strained the daily emotional life feels, what soothes both partners, and whether the inner emotional rhythms align. Liz Greene and Howard Sasportas, in The Luminaries (1992), treat the Moon-Moon contact as one of the most predictive indicators of long-term ease in a relationship, often more so than the louder Venus-Mars chemistry signature.

A Moon-Moon conjunction means both partners have their Moons in the same sign, with the same emotional language. A trine means their Moons sit in compatible elements (fire-fire, earth-earth, air-air, water-water) and share an emotional rhythm even when the signs differ. A sextile means their Moons sit in complementary elements (fire-air or earth-water), where each partner's emotional style naturally supports the other. These three aspects often correlate with effortless daily-life compatibility.

A Moon-Moon square or opposition is more demanding but is not, in Greene's framing, an incompatibility signal. The two emotional languages are different (often water vs fire or earth vs air), and the partners must consciously learn to translate for each other rather than coasting on instinct. According to Howard Sasportas, Moon-Moon squares can produce the most vivid emotional life of any synastry pattern because difference is always loud, and conscious translation is always richer than easy resonance.

Moon-Moon contacts depend on both partners' exact birth times, since the Moon moves about 13 degrees per day and changes sign every two and a half days. Without precise times for both, the Moon-Moon contact is approximate. For a precise reading, confirm both birth certificates and run the calculator with full data. In my practice with couples, the Moon-Moon contact is often the most useful single piece of synastry data for predicting day-to-day ease.

Saturn contacts: commitment and lessons

Saturn contacts in synastry, especially Saturn-Sun, Saturn-Moon, and Saturn-Venus from one partner's chart to the other's, are traditionally described as the bonding force of a long-lasting relationship. Liz Greene, in Saturn: A New Look at an Old Devil, calls Saturn the architect of commitment. According to Greene, Saturn contacts produce the sensation of weight and importance in a connection, the felt sense that this person matters and that the relationship is serious. The Saturn person often becomes a teacher figure for the other, offering structure, boundary, and reality.

Saturn-Sun contacts often describe a relationship where one partner's identity comes into clearer focus because of the other. The Saturn person, sometimes older or in a structural position, helps the Sun person see what they are actually becoming. Saturn-Venus contacts often describe a relationship where the Venus person's romantic life becomes more grounded and accountable. Saturn-Moon contacts describe a relationship where the Moon person's emotional life finds anchor and steadiness in the Saturn person.

These contacts can feel heavy at first. Stephen Arroyo notes that early in a Saturn-touched relationship, both partners may feel inhibited or restricted by each other. Over time, the same contact often resolves into one of the longest-lasting bonds in a person's life. According to Greene, divorce statistics from astrological case studies show Saturn contacts disproportionately represented in long-running marriages, even when other synastry layers suggest difficulty.

Saturn squares and oppositions are not danger signs, despite the modern tendency to read them as conflict. In traditional framing, a Saturn-Sun square between two people often describes a relationship where the Saturn person's presence consistently surfaces the Sun person's growth edges, sometimes uncomfortably, sometimes productively. The chart suggests pattern. The work is consciousness, not avoidance. Saturn contacts traditionally pair with natal Saturn placement awareness for both partners.

North Node contacts: purpose and fate

The North Node, also called the True Node or the Dragon's Head, represents the soul's growth direction in modern Western astrology. The South Node represents what the soul is already familiar with, the comfortable past. The North Node represents what the soul is being asked to develop, the growth edge in this lifetime. When one partner's planets contact the other partner's North Node, the relationship often feels fated or purposeful in a way that goes beyond chemistry or emotional rapport.

Stephen Arroyo, in Astrology, Karma and Transformation (1978), treats North Node contacts as the karmic backbone of a connection. According to Arroyo, the contact creates the sense that “this person is here to help me grow into what I am becoming.” A Venus on the partner's North Node often describes a relationship that develops the Venus person's capacity for love. A Saturn on the North Node often describes a relationship that develops the Saturn person's discipline. A Sun on the North Node often describes a relationship that develops the Sun person's sense of identity and direction.

Liz Greene's framing in The Astrology of Fate (1984) is more nuanced. Greene notes that North Node contacts produce the feeling of fate, but the feeling is not the same as inevitable destiny. The relationship offers a growth path. Whether either partner takes that path is their choice. The chart suggests opportunity, not determination. In my practice with couples carrying tight North Node contacts, both partners typically describe the relationship as “meant to be” even when the relationship is difficult, which is a useful diagnostic on its own.

The asteroid Chiron, sometimes called the wounded healer, carries a related but distinct fate-flavored signal in synastry. Chiron contacts between two charts often describe a relationship that engages both partners' deepest wounds and offers a path of healing. For the Chiron layer of a synastry reading, see the dedicated asteroid synastry tool.

House overlays: where each partner lands

After planet-to-planet aspects, the second major layer of the synastry reading is the house overlay: where each partner's planets fall in the other partner's natal houses. The houses are the twelve life-areas of the chart: self, resources, communication, home, creativity, work, partnership, depth, meaning, vocation, community, hidden life. When one partner's planets activate specific houses in the other's chart, those life areas light up for the second partner whenever the first is present.

Robert Hand frames house overlays as the spatial complement to the aspectual reading. Aspects describe the energetic quality of the contact. House overlays describe where in life the contact plays out. A Venus aspect between two charts says “there is affection here.” The overlay says where that affection lives, in the kitchen (4th), in conversations (3rd), in creative projects (5th), or in marriage itself (7th). Both layers belong in the reading.

Twelve house overlays at a glance

Planets in partner's 1st

Strong physical presence to the partner; immediate sense of impact and visibility in the partnership.

Planets in partner's 2nd

Activates the partner's resources and self-worth; bonding around money, comfort, and material life.

Planets in partner's 3rd

Activates daily conversation and exchange; the partner experiences this person through words and ideas.

Planets in partner's 4th

Activates home and family roots; brings up the partner's lineage and emotional foundations.

Planets in partner's 5th

Activates creativity, romance, and play; the partner experiences this person as a source of joy and self-expression.

Planets in partner's 6th

Activates daily routine and service; the partner integrates this person into the structure of their daily life.

Planets in partner's 7th

Partnership material; this person represents marriage and committed bond to the partner. Sun in 7th is especially classical.

Planets in partner's 8th

Activates depth, intimacy, shared resources, and transformation; brings the partner into intense bonding.

Planets in partner's 9th

Activates meaning, travel, philosophy, and worldview; the partner experiences this person as a horizon-expander.

Planets in partner's 10th

Activates vocation and public life; the partner experiences this person as relevant to career and reputation.

Planets in partner's 11th

Activates community, friendship, and shared causes; bonding through chosen family and group life.

Planets in partner's 12th

Activates the hidden life, the unconscious, and depth dreaming; the partner experiences this person as both close and elusive.

Stephen Arroyo notes that the partner's 7th house is classically the “marriage house” in synastry: a Sun, Moon, Venus, or Mars in the partner's 7th house is a strong indicator that the second partner experiences the first as partnership material. The partner's 5th house is the romance house: planets here activate the dating and creative-expression layer. The partner's 8th house is the depth house: planets here activate deep intimacy and transformation. Combine the aspectual layer (what energies meet) with the overlay layer (where in life they meet) for the full reading.

Synastry vs composite vs Davison

Three relationship-astrology techniques are widely practiced in modern Western astrology, and the three are complementary, not interchangeable. Robert Hand, in Planets in Composite (1975), recommends reading all three together for any serious relationship reading.

Synastry is the chart-overlay technique covered in this guide. Two natal charts, two sets of planets, two sets of aspects. The synastry reading describes how each person experiences the other and what tasks the relationship is asking of both people. Synastry is the foundation; every modern reading starts here.

Composite is the midpoint chart, developed by Robert Hand and John Townley in the 1970s. The composite is a third chart, made by averaging the midpoints of the two natal charts planet by planet. The composite Sun is the midpoint of the two natal Suns, the composite Moon is the midpoint of the two natal Moons, and so on. The composite chart represents the relationship itself, the “us,” rather than either partner individually. For the full composite reading, use the dedicated composite chart calculator, read the full composite chart guide, and bring its findings back to this synastry above.

Davison is the relationship chart developed by Ronald Davison in the 1970s. The Davison chart uses the midpoint in time AND space, the actual halfway date and geographic midpoint between the two birth events. Where the composite is a symbolic construction, the Davison is an actual moment in time at an actual location on Earth. Davison charts can be cast with the same tools as natal charts. Many practitioners treat the Davison as more astronomically real than the composite, though both are widely used.

Read all three together for the fullest picture. The synastry shows how each partner experiences the other. The composite chart shows what the relationship is in itself. The Davison shows the relationship as a third event in time and space. Robert Hand, John Townley, and Liz Greene all recommend reading the three together, never one alone.

The Vertex and other “fated” points

Beyond the planets, several specialty points carry strong fated-feeling signals in synastry. The most widely used is the Vertex, the western intersection of the ecliptic and the prime vertical, sometimes called the third angle of the chart (after the Ascendant and Midheaven). Contacts to the Vertex, especially conjunctions from another person's Sun, Moon, Venus, or Mars, are widely associated with fated-feeling encounters in modern Western astrology.

Beyond the Vertex, several asteroids carry relationship- relevant signals. Juno is associated with committed partnership and is traditionally read in synastry for marriage indicators. Vesta is associated with devotion and sacred service. The asteroid Eros (sometimes called the Lot of Eros in Hellenistic technique) carries the erotic chemistry signal, distinct from the Venus-Mars layer. These points are not part of every reading, but they are high-traffic among sophisticated synastry searchers and deepen the reading when included.

Chiron, the wounded healer, carries fate-flavored signal in a different register: Chiron contacts between two charts often describe a relationship that engages both partners' deepest wounds and offers a path of healing. The asteroid tool covers Chiron, Juno, Vesta, and Eros in the synastry context and is the natural next step after the main reading.

How to read your synastry chart

The reading sequence below is the one Astrid Moon uses in practice, distilled from Stephen Arroyo, Liz Greene, Robert Hand, and Howard Sasportas. Follow the five steps in order; do not jump ahead to specialty contacts before the foundation is set.

1. 1

   Generate both partners' natal charts

   Each partner needs birth date, birth time, and birth location. With both times confirmed, the Moon and the Ascendant land at exact degrees, which the synastry reading depends on. Without exact times, the Moon-Moon and angle contacts will be approximate, but planet-to-planet aspects between slower bodies remain reliable.

2. 2

   Overlay them: each partner's planets land in the other's chart

   Visualize Partner A's chart with Partner B's planets drawn on the outer ring, and vice versa. This is the bi-wheel view used in modern synastry software. Each planet from one chart lands at a specific degree in the other chart, in a sign and a house, and forms aspects (conjunctions, trines, squares, oppositions, sextiles) to the other partner's planets.

3. 3

   Note the Sun-Moon, Venus-Mars, and Moon-Moon contacts

   These three contact types are the foundation of the reading. Sun-Moon is the conscious-unconscious meeting, widely considered the strongest relational fit indicator. Venus-Mars is the romantic and sexual chemistry signature. Moon-Moon is the daily emotional rapport. Stephen Arroyo treats these as the synastry's structural backbone.

4. 4

   Identify Saturn and North Node connections for commitment and fate themes

   Saturn contacts from one partner to the other carry the bonding and lessons signal. Liz Greene calls Saturn the architect of commitment. North Node contacts carry the fated-growth signal. Together, Saturn and the Node describe what the relationship is asking of each partner and what is likely to keep them tied together over time.

5. 5

   Read house overlays: where each partner's planets land in the other's houses

   After planet-to-planet aspects, read which houses each partner's planets activate in the other chart. A Sun in the partner's 7th is partnership material. A Mars in the partner's 5th is the activator of creative and romantic life. House overlays describe where in life the contacts play out, completing the reading.

Crystal pairings for relationship astrology

Each major synastry contact pairs traditionally with one or two crystals that support its work. Crystal Cassandra, our crystal healing guide, has long noted that synastry-aware crystal pairings work better than generic relationship stones because they meet the chart's specific landscape. The grid below maps each priority contact to its classical stone, with care notes where the stone has handling considerations.

Crystal pairings by contact type

Sun-Moon contacts

Moonstone (lunar attunement, traveler's stone) paired with Sunstone (conscious warmth, joy). Together the two carry both halves of the luminary pair.

Venus-Mars contacts

Rose Quartz (the universal heart stone) paired with Carnelian (active desire, vital force). The Venus-Mars pairing in physical form.

Moon-Moon contacts

Moonstone for both partners, or Selenite (lunar clarity) when one Moon-Moon contact is challenging and asks for emotional translation.

Saturn contacts

Black Tourmaline (boundary, protection) and Smoky Quartz (grounded commitment, stabilizer). Saturn's gravity met by stones that hold structure.

North Node contacts

Labradorite (purpose, soul-recognition) and Selenite (clarity in the growth direction). For the fated-feeling layer of the reading.

Vertex contacts

Garnet (fated passion, commitment) and Obsidian (depth, truth). The two stones for fated encounters in classical practice.

General relationship harmony

Rose Quartz is the universal heart stone, recommended for any couple regardless of synastry specifics. Pair with Rhodonite for relationship healing work.

Difficult aspects

Rhodonite (relationship repair, compassion) and Amazonite (calm communication, truth-telling). For couples actively working through synastry challenges.

Care notes: Selenite must never get wet (it dissolves in water). Moonstone is safe with brief water exposure but benefits from being cleansed in moonlight, not sunlight. Rose Quartz can fade in prolonged direct sunlight, so keep it in indirect light when on display. The full crystal data is maintained in the crystal encyclopedia, with detailed care guidance per stone.

A brief history of synastry

Synastry is the oldest of the three modern relationship astrology techniques. Hellenistic astrologers practiced chart-overlay synastry as early as the first century BCE. Vettius Valens, the second-century author of the Anthology, includes synastry contacts in his case studies. Chris Brennan, in Hellenistic Astrology (2017), documents how classical practitioners would compare two charts for marriage prospects, political alliances, and business partnerships, with a particular focus on the Sun, Moon, and benefic-malefic placements.

Medieval Arabic astrologers refined the technique through the eighth to twelfth centuries, with practitioners like Abu Ma'shar and Al-Biruni systematizing the synastry reading. The technique survived into European Renaissance astrology, where William Lilly's seventeenth-century work includes synastry case studies for marriage questions.

The modern Western revival of synastry came in the 1970s, led by Stephen Arroyo, Liz Greene, Howard Sasportas, and Robert Hand. Arroyo's Relationships and Life Cycles (1979) and Greene's Relating (1976) are the two foundational modern texts. Hand's Planets in Composite (1975) introduced the composite chart technique and explicitly framed synastry as the necessary prior step before composite interpretation. Demetra George's later work, Astrology and the Authentic Self (2008), brought psychological depth to the synastry reading.

Today synastry is the default relationship-astrology technique in Western practice. Software tools render bi-wheel synastry charts instantly; the calculator above follows the same convention. The interpretive tradition remains rooted in the 1970s revival authors. For deeper study, the four sources to read first are Arroyo's 1979 book, Greene's 1976 book, Hand's 1975 book, and Sasportas and Greene's The Luminaries (1992).

Frequently asked questions about synastry

The most common questions readers send about synastry, with answers in the voice of Astrid Moon and the lineage of Stephen Arroyo, Liz Greene, Robert Hand, and Howard Sasportas.