Birth Chart Compatibility Calculator — Synastry & Relationship Astrology

What synastry is

Synastry is the astrological comparison of two charts. It is the oldest technique we have for asking what astrology can say about a specific relationship, going back at least to the Hellenistic practitioners of the second century CE who were already noting which inter-chart contacts predicted lasting bonds. The procedure is simple in principle: you cast both birth charts, lay them on top of each other, and read the angular relationships between Person A's planets and Person B's planets — and the house in Person B's chart where Person A's planets land, and vice versa.

The aspects you look for in synastry are the same five major aspects you look for in a single chart — conjunction, sextile, square, trine, opposition — applied between charts rather than within one. A conjunction between Person A's Mars and Person B's Venus reads similarly to a Mars-Venus conjunction inside one chart: heat, attraction, mutual ignition. The difference is that in synastry the two planets belong to two different people, and the result is something each of them experiences when they are together rather than something one of them experiences alone.

How inter-chart aspects work

The orbs (allowable distance from exact angle) used for synastry are usually tighter than for natal charts, because the effect of a synastry aspect tends to fade quickly as the angle widens. We use 8° for conjunctions and oppositions involving the Sun or Moon, 6° for those involving other personal planets, and 4° for trines and squares. Aspects involving outer planets (Uranus, Neptune, Pluto) are kept very tight — within 2° — because outer-planet aspects describe generational rather than personal contact unless they are exact.

The most important inter-chart aspects are between personal planets: Sun, Moon, Mercury, Venus, Mars, and the angles (ascendant and midheaven). Outer-planet contacts add depth and difficulty but rarely make or break a connection on their own. The exception is when an outer planet of one person's chart hits a personal planet of the other's: Person A's Pluto conjunct Person B's Sun is a fateful, transformative contact that can be either electrifying or punishing depending on what else is in play.

The most powerful synastry aspects

Sun-Moon contacts. When one person's Sun conjuncts, trines, or opposes the other's Moon, there is an instinctive, almost familial recognition. The Sun person feels seen; the Moon person feels nourished. These are the contacts of long marriages and lifelong friendships.

Venus-Mars contacts. Venus is what we love and Mars is what we want. When they meet across charts — particularly conjunction, opposition, square, trine — there is sexual chemistry. The square and opposition produce the kind of attraction that doesn't quite die down; the trine and conjunction produce the kind that's easy to act on.

Ascendant-Descendant axis contacts. The ascendant is the persona; the descendant (its opposite) is the projected partner-image. When one person's planets — especially Sun, Moon, Venus, or Saturn — land on the other's ascendant or descendant, the encounter often feels destined. These are the meetings that re-route lives.

Saturn contacts. Saturn is time, weight, and obligation. Saturn aspecting a partner's personal planet — particularly conjunction or square — describes commitment. Often the relationship feels heavier than its surface suggests; sometimes it feels restrictive. But Saturn contacts are what make relationships endure, even when easier pairings drift apart.

Composite charts vs synastry

Synastry compares two charts. A composite chart creates a third one by taking the midpoints of every planet — Person A's Sun at 5° Leo and Person B's Sun at 25° Aquarius produce a composite Sun at 15° Taurus. The composite chart is read as the chart of the relationship itself, an entity with its own ascendant, its own Sun sign, its own characteristic patterns. Synastry tells you what the two of you do to each other; the composite tells you what the two of you, together, are. Many practitioners use both: synastry for the chemistry, composite for the long-term identity of the partnership.

Saturn and the long view

One of the most reliable findings in synastry research is that long-married couples tend to have strong Saturn contacts between their charts, even when those contacts are uncomfortable. Saturn slows things down, imposes structure, and asks for commitment in a way that the airier planets do not. A pairing rich in Venus-Jupiter trines but bereft of Saturn often describes a wonderful affair that ends well; a pairing with Sun-Saturn or Moon-Saturn aspects (even squares) is much more likely to describe a marriage. This is the case even when those Saturn contacts feel restrictive in the early years — they are what holds the structure during the inevitable difficult passages.

Juno and other compatibility points

Beyond the major planets, several minor points are worth checking. The asteroid Juno describes the kind of partner you instinctively seek for marriage; Juno aspects to a partner's personal planets are classic marriage indicators. The vertex, sometimes called the "third angle," is associated with fated encounters; planets conjunct another person's vertex often mark life-routing meetings. The Part of Fortune in synastry indicates where the pairing brings each partner ease and good fortune.

Reading the compatibility score

The 0-100 score the calculator returns is a weighted summary, not a verdict. It rewards aspects between personal planets, contacts to the angles, supportive house overlays, and presence of Saturn (because Saturn predicts duration). It does not punish hard aspects — squares and oppositions are often what drives growth — but it does favour density of contact over scattered single hits. Relationships scoring above 70 typically have many points of resonance and feel obvious. Relationships scoring 50-70 often have the most material to work with; the sparkling and the difficult both register and the partnership has texture. Relationships scoring below 40 may still be meaningful — astrology does not exhaust the explanation of why two people fit — but the chart support is lighter, and the relationship is more likely to depend on circumstance, choice, and effort than on the gravitational pull of stars.

Beyond Sun-sign compatibility

Newspaper compatibility — "Scorpio and Capricorn are compatible, Scorpio and Aquarius are not" — is too coarse to be useful. It compares one twelfth of one chart to one twelfth of another, and it ignores everything that actually drives relational chemistry: the Moon signs that describe what each person needs to feel safe, the Venus signs that describe how each person loves, the Mars signs that describe how each person wants, the rising signs that describe how the two first meet, and Saturn's role in deciding whether the bond can hold over time. Two Scorpios with incompatible Moons and clashing Venuses will struggle; a Scorpio and an Aquarius with a tight Sun-Moon trine and Venus-Mars sextile will find each other effortlessly. Sun-sign compatibility is fortune-cookie astrology; chart-to-chart synastry is the real conversation.

Frequently asked questions

Composite charts: the relationship as a third entity

Synastry compares two charts. A composite chart creates a third one from the midpoints of every corresponding planet. Person A's Sun at 10° Aries and Person B's Sun at 20° Libra produce a composite Sun at 0° Cancer — the midpoint. The composite chart is read as the chart of the relationship itself, with its own rising sign, its own Sun sign, its own characteristic strengths and frictions. It answers a different question from synastry: not "how do we affect each other?" but "what are we, together, as a unit?"

Composite charts tend to be most revealing in long-term relationships. The composite rising sign describes how the couple is perceived by outsiders. The composite Sun shows the relationship's core purpose — what it is fundamentally for. A composite Saturn in the seventh house is the hallmark of a serious, durable partnership; a composite Pluto in the fifth describes a relationship that is passionate, transformative, and difficult to step back from once entered.

Juno: the asteroid of commitment

Juno is a minor asteroid associated with marriage, long-term partnership, and the qualities we need in a committed partner. It is not the same as Venus (which describes what we love and find beautiful) or the seventh house (which describes the projected partner-image). Juno is specifically about the terms of the bond — what we are willing to commit to and what we require in return.

In synastry, Juno contacts to personal planets — especially Sun, Moon, Venus, Mars, and the ascendant — carry real weight. Person A's Juno conjunct Person B's Sun is a classic marker of marriage potential: the Sun person embodies exactly what the Juno person seeks in a long-term partner. Juno-Moon contacts describe emotional security and domestic compatibility; Juno-Mars contacts describe desire within commitment rather than the restless desire that seeks novelty.

Venus signs in synastry: love styles meeting

Venus describes what you find beautiful, how you express affection, and what you need from a relationship to feel loved. When two people's Venus signs are in compatible elements — both fire, both earth, or fire and air, or earth and water — the love styles tend to feel natural to each other. When they are in incompatible elements — one person's Venus in Scorpio meeting another's in Gemini — the love styles can feel foreign: one person communicates love through intensity and loyalty, the other through lightness and variety.

The actual Venus-to-Venus synastry aspects matter more than the signs alone. Venus trine Venus between charts is a hallmark of two people who enjoy similar pleasures and express affection in ways the other recognises without translation. Venus square Venus creates friction around values and pleasure — not necessarily incompatibility, but a requirement for genuine adjustment and, over time, mutual learning about what love actually looks like to someone different from you.

Mars signs: desire and drive in relationship

Mars in synastry describes how two people's energy and desire interact when they share space. The classic contact is Venus trine Mars or Venus conjunct Mars between charts — Venus finds what Mars offers appealing, Mars is energised by what Venus represents. The square and opposition produce sharper versions of the same attraction, often with more friction and staying power.

Mars-Mars aspects tell a different story: how two people's drives interact when they act in parallel. A Mars-Mars trine between charts describes two people who work naturally in the same direction — they do not compete, they reinforce each other's momentum. A Mars-Mars square describes a pairing where each person feels perpetually challenged by the other's pace and angle of attack. In long partnerships this can be galvanising or exhausting, and usually both at different points in the relationship.

Beyond the score: reading the chart pair as a story

The 0-100 compatibility score is useful as a starting orientation — it reflects the weighted density of supportive inter-chart aspects against tension-creating ones. But it has a ceiling on its usefulness. Some of the most significant relationships in a person's life have been "difficult" by aspect counting while containing one or two extraordinarily tight contacts that make the whole thing feel unavoidable.

The right way to use the score: treat a high score as confirmation that a relationship has natural resonance, and a low score as a prompt to look more carefully at what the specific aspects actually describe. A chart pair with a score of 42 and a Sun conjunct Moon within 1° is more significant than a score of 78 with no tight contacts and no axis connections. Numbers describe volume; closeness of aspect describes intensity. Both matter, and neither alone tells the full story.

House overlays: whose chart contains whom

When you overlay two charts, Person A's planets fall into Person B's houses, and vice versa. The house where Person A's Sun lands in Person B's chart describes what area of Person B's life Person A illuminates — and often what area of life Person B associates with Person A. Person A's Sun in Person B's seventh house makes Person A feel like a natural partner or significant relationship figure to Person B. Person A's Sun in Person B's twelfth house puts their relationship somewhat outside ordinary social awareness — often private, significant, and hard to fully explain to outsiders.

House overlays are typically more positive than their aspect equivalents because they describe influence rather than tension. The same Sun that squares Person B's Moon may still land in Person B's fourth house — where it activates home and family themes, however challenging the aspect may be. Practitioners who read house overlays alongside aspects get a more complete picture: the aspects describe the chemistry between the people; the overlays describe the territory of life where they most affect each other.

The Moon's Nodes in synastry

The North and South Nodes of the Moon are not planets but points — the two places where the Moon's orbit intersects the plane of the ecliptic. In natal astrology they describe the soul's direction (North Node) and the habitual past (South Node). In synastry, Node contacts carry a sense of fate or familiarity that is hard to explain rationally. When one person's Sun or Moon falls on the other's North Node, the Nodal person often experiences the planetary person as a teacher or guide — someone who pushes them toward their growth edge. South Node contacts feel eerily familiar, as if the two people have been in each other's lives before.

Tight Node conjunctions (within 3°) to a partner's Sun, Moon, Venus, or ascendant are among the aspects that produce the "have we met before?" feeling characteristic of significant partnerships. They do not guarantee harmony — Node contacts can be uncomfortable precisely because they push each party toward unfamiliar territory — but they rarely produce indifference.

Timing and relationship astrology

Synastry describes the natal structure of a relationship — its permanent, built-in qualities. But relationships also have timing. The techniques used for timing in individual charts — transits, progressions — apply to relationship charts as well. When Saturn transits the composite midheaven, the relationship faces a test of public status or shared career direction. When Jupiter transits the composite Venus, there is likely a period of generosity and warmth between the two people. When the Saturn return coincides with major synastry transits between charts, the relationship faces the same defining evaluation that the Saturn return poses to individuals: is what we have built real enough to carry the next chapter?

Relationship astrology at its most serious tracks both the natal synastry and the current transits simultaneously. The calculator gives you the natal picture; return to it at major life moments and compare where today's slow-moving planets land against both your natal chart and your partner's. The interaction between the two levels — natal synastry and current transits — is where the timing becomes clear.

The vertex and anti-vertex in relationship astrology

The vertex is a sensitive point in the chart — the intersection of the prime vertical and the ecliptic in the western hemisphere — associated with fated encounters and significant others. It is not a planet and carries no rulership, but when activated by transit or synastry contact it is associated with the sense of "this was meant to happen." When one person's planet — particularly their Sun, Moon, Venus, or Mars — lands on the other person's vertex within a tight orb of 2-3°, the vertex person often describes the planetary person as someone who arrived at exactly the right moment, someone who re-routed their life simply by appearing. The anti-vertex, the opposite point, tends to attract people who bring the themes of one's own chart back to one in acute form — relationships where self-recognition through the other is the central experience. Both are worth checking in any chart pairing that feels significant beyond ordinary friendship or attraction.

Mercury and communication styles in synastry

Mercury describes how a person thinks, speaks, and processes information. In synastry, Mercury-to-Mercury aspects and Mercury-to-personal-planet aspects describe communication compatibility — whether two people can actually talk to each other easily, or whether every conversation feels like a translation exercise. Mercury trine Mercury between charts describes people who share a basic intellectual wavelength; their humour lands, their explanations track, their written messages feel right. Mercury square Mercury is not fatal — it often describes stimulating conversations — but it means one person's natural communication style will regularly feel oblique, too fast, too slow, or simply differently formatted to the other.

Mercury-Saturn aspects in synastry are worth noting separately. Saturn-to-Mercury contacts (one person's Saturn on the other's Mercury) can produce a silencing effect: the Mercury person may find their thoughts dismissed or held to too high a standard, leading over time to self-censorship. The Saturn person often does not intend this — but the effect accumulates. Close Mercury aspects between charts are one of the underrated relationship indicators, because the ability to actually talk to someone — to explain yourself and be understood — is foundational in a way that Venus and Mars attraction alone is not.

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Use the main calculator to inspect either chart on its own, learn the underlying interpretive framework on the reading guide, or read about the zodiac signs for sign-by-sign compatibility notes.