Synastry Chart: How to Read Birth Chart Compatibility

A synastry chart places two people's birth charts side by side and reads the angles between their planets. Start with the contacts between the Sun, Moon, Venus and Mars, add the Ascendants, then see which of your houses your partner's planets land in. That is how astrologers read birth chart compatibility, and it is why two people with "incompatible" Sun signs can still have a strong connection.

This guide covers what synastry is, the aspects that carry the most weight, house overlays, how synastry differs from a composite chart and exactly how our free compatibility tool turns two charts into one score. It ends with a worked example of two fictional people, computed with the same engine the tool uses.

What is synastry?

Synastry is the astrology of relationships between two people. You take two complete birth charts and compare them planet by planet. When one person's Venus sits 120 degrees from the other person's Mars, for example, the two charts form a trine. Astrologers read that as easy attraction.

Sun-sign compatibility only compares two of those points (the two Suns) and only by sign. A full synastry reading compares every planet with every other planet by exact degree. That difference is large. In our study of 60,000 generated chart pairs, the two Sun signs explained only 24.3% of the full-chart score (the study is here). The rest came from everything else this guide covers.

What do you need to make a synastry chart?

For each person you need three things:

• Birth date. This places the Sun, Mercury, Venus, Mars and the slower planets.
• Birth time. This places the Moon precisely (it moves about 13 degrees a day) and sets the Ascendant and the houses.
• Birth place. Latitude and longitude, plus the local time zone at the time of birth, turn the clock time into a real moment in the sky.

Without a birth time you can still read most planet-to-planet contacts, but you lose the Ascendant, the house overlays and some certainty about the Moon. Our tool handles that case: tick "I don't know my birth time" and it reads the planets alone.

How to read a synastry chart in five steps

1. Cast both charts. Note each person's Sun, Moon, Venus, Mars and rising sign (what each planet means). If you do not know your own yet, start with a free birth chart reading or your big three.
2. List the cross-aspects. For every planet in chart A, check the angle to every planet in chart B. Keep the major aspects within a few degrees of exact.
3. Weight the personal contacts first. Sun-Moon, Venus-Mars and Moon-Moon tell you more about daily life together than any outer-planet contact.
4. Look at the house overlays. Ask which of your houses your partner's planets fall in, and the reverse.
5. Read tension and harmony together. A square is friction and engagement, not a verdict. Most lasting bonds have both.

The synastry aspects that matter most

These are the contacts astrologers look at first. The weight column shows how much each pairing counts in our scoring engine, read straight from its code: the higher the number, the more a contact moves your score.

Sun-Moon aspects

The classic marker of compatibility. When your Sun touches their Moon, who you are meets what they need, so each person tends to feel understood without explaining. A conjunction, trine or sextile reads as natural warmth. A square or opposition still shows a strong mutual read, with needs and nature that do not always line up.

Venus-Mars aspects

Venus is how you love and what you find beautiful. Mars is how you pursue and what you desire. A Venus-Mars contact between two charts is the textbook sign of chemistry. Harmonious angles feel easy and mutual. Tense angles bring heat and friction in equal measure. Read both directions: your Venus to their Mars and your Mars to their Venus can tell different stories.

Moon-Moon aspects

Your Moon sign is your emotional instinct. Two Moons in harmony means your emotional worlds rhyme and comfort comes naturally. Two Moons at a square means you feel things on different frequencies and need to name feelings out loud. Even without an exact aspect, Moons in the same or a complementary element tend to soothe each other.

Saturn contacts

Saturn in synastry is about commitment, responsibility and staying power. When one person's Saturn touches the other's Sun, Moon or Venus, the Saturn person often brings structure and seriousness. The other person can feel either steadied or held back. Astrologers often call Saturn the glue of long relationships, because it shows where both people are asked to show up consistently. Our tool does not score Saturn: it reads the five personal planets and the Ascendant. Saturn contacts are worth reading by hand, and the worked example below shows how.

Ascendant contacts

Your Ascendant is your rising sign, the face you show the world. When a partner's planet sits on or aspects your Ascendant, they register you strongly from the first meeting. Ascendant contacts need both birth times.

What the aspects mean

The orb is how far from exact an aspect can be and still count. Tighter aspects matter more, so our engine gives a contact at 1 degree more weight than the same contact at 6 degrees. The engine also counts tense aspects at 45% of a harmonious one rather than subtracting them, because squares and oppositions are engagement too.

House overlays: where your partner lands in your chart

A house overlay asks which of your twelve houses each of your partner's planets falls in. The house shows where in your life you feel that person. Our engine uses whole-sign houses, so the rule is simple: whatever sign your partner's planet is in, find which of your houses holds that sign.

Overlays into the 7th house are the ones people ask about most, because the 7th is the house of partnership. Our guide to the 7th house explains what it shows on its own, and the 12 houses covers the rest.

Composite chart vs synastry chart

The two are often confused, and they answer different questions.

A related method, the Davison chart, uses the midpoint in time and place between two births. Most astrologers start with synastry, because it describes the experience each person actually has of the other. Our tool reads synastry.

How our compatibility tool scores synastry

The compatibility tool casts both birth charts from a real astronomical ephemeris, then reads six points for each person: the Sun, Moon, Mercury, Venus, Mars and the Ascendant (when a birth time is given). It finds every major aspect between the two sets within the orbs above, then blends three signals into a score out of 100:

• Aspect balance, 50%. The weighted harmony of all cross-aspects, with tighter orbs counting more.
• Elemental ease, 30%. How the elements of the two Suns, two Moons and two Venuses mix: same element, complementary (fire with air, earth with water) or clashing.
• Density, 20%. The sheer amount of meaningful connection between the charts.

The result is mapped into a band from 28 to 99, with the top of the band reserved for rare, near-perfect synastry. The tool then splits the same signals into four axes:

The result also names the single strongest contact between you, the one you are likely to feel first and longest.

A worked example: Maya and Jordan (fictional)

Maya and Jordan are fictional people. Their birth details are invented for this example and the results come from our engine. We built them as an Aries and a Virgo on purpose, because Aries and Virgo is the lowest-ranked Sun-sign pairing in our study, at a mean of 76.0 out of 100.

• Maya: born March 24, 1995, at 11:15 pm in Manchester, England. Sun in Aries, Moon in Capricorn, Venus in Aquarius, Mars in Leo, rising sign Scorpio.
• Jordan: born September 15, 1992, at 6:40 am in Chicago. Sun in Virgo, Moon in Taurus, Venus in Libra, Mars in Cancer, rising sign Virgo.

Their score: 89 out of 100, "Effortless and electric". Heart 87, Spark 73, Mind 84, Rhythm 82.

We chose this pair because it scores high: across 900 fictional Aries and Virgo combinations we tried, scores ran from 56 to 89, with a median of 77. That is a point above the best Aries and Virgo chart pair in the random 60,000-pair sample, which topped out at 88. Same Sun signs, a 33-point range: that is the whole argument for reading full charts.

Step 1, the strongest contact. Maya's Moon in Capricorn trines Jordan's Sun in Virgo, 4.4 degrees from exact. It is a Sun-Moon contact, one of the two heaviest in the engine, between two earth signs. Jordan's steady, practical nature is exactly what Maya's Moon finds calming.

Step 2, the other personal contacts. The engine found ten contacts between the two sets of personal points, eight harmonious and two tense:

Two details are worth noticing. The Venus-Mars trine runs from Aquarius to Cancer, signs that do not normally trine. It counts because the engine measures the real angle between the planets (125.7 degrees, within 7 of a trine), not the signs. And the tightest contact of all is a tense one: Maya's Moon squares Jordan's Venus at 0.3 degrees, care that can feel like too much or too little until they calibrate.

Step 3, sign-level fit. Their Moons, in Capricorn and Taurus, make no exact aspect, but both are earth signs, so the tool still reads emotional ease: "you soothe each other without trying." Their Suns differ in element (fire and earth) and in mode (cardinal and mutable), so the tool reads different tempos. Rhythm still reaches 82 because the two Ascendants sextile each other within 0.1 degrees.

Step 4, what the tool does not score. Reading by hand, Maya's Mars in Leo opposes Jordan's Saturn in Aquarius, just 0.6 degrees from exact. Jordan can act as a brake on Maya's drive, which may feel steadying on a good day and restrictive on a bad one. Maya's Saturn also opposes Jordan's Sun (5.6 degrees). These are the commitment contacts: they ask both people to be patient with each other's pace.

Step 5, house overlays. Jordan's Moon falls in Maya's 7th house of partnership, so Maya is likely to experience Jordan's emotional nature as partner material. Jordan's Sun, Mercury and Ascendant fall in Maya's 11th house of friendship. Maya's Moon falls in Jordan's 5th house of romance. Maya's Mercury and Saturn fall in Jordan's 7th.

The read. On Sun signs alone this pairing sits last of 78. On full charts it is warm, talkative and grounded, with one tight emotional square to handle with care and a Saturn contact that rewards patience. That is synastry in one example.

What synastry cannot tell you

A synastry chart describes chart geometry. It does not describe whether two people are kind to each other, share values or want the same life. It cannot predict whether a relationship will last. A high score is an invitation to enjoy what comes easily. A low score points to where you will need to talk things through. Treat any result, ours included, as a mirror for a conversation, never a verdict.

Read your own synastry chart

Enter two birth dates, times and places in the free compatibility tool to get your full-chart score, the Heart, Spark, Mind and Rhythm breakdown and the single strongest contact between you, with no sign-up. To see how your Sun signs rank on average first, check the best and worst match for every sign or your own pair page, such as Leo and Scorpio. For the love placement that drives the Spark axis, read what your Venus sign means.

Synastry compares two charts. A composite chart blends them into one: see composite chart vs synastry for what each one tells you.