Birth Chart Compatibility (Synastry)

What is birth chart compatibility?

Birth chart compatibility compares two whole birth charts rather than two sun signs, and reads the angles between one person's planets and the other's. Every angle that lands close enough to a classic shape is kept, weighted, and added into one score out of 100, with each kept angle listed underneath.

Astrologers call the method synastry, and this page treats the two names as one thing. A birth chart is a map of the sky at a single birth, so the word chart here means neither the 12 by 12 zodiac grid nor the numerology grid. Synastry is the branch of Western astrology that compares two charts, and what is synastry takes the idea on its own.

How do you use the birth chart compatibility calculator?

Enter a birth date for each person and press the button. The time and the city are optional.

1. Enter Person 1's birth date.
2. Add a birth time if you know it, in local clock time.
3. Pick the birth city from the list, which is what turns that clock time into a real moment. The list holds the 300 largest cities, and the nearest one sets the time zone.
4. Do the same for Person 2.
5. Press "Compare the two charts", then read the score and the aspect table under it.

The calculator is free, it needs no account, and it runs in your browser, so nothing you enter is sent anywhere.

Do you need a birth time for birth chart compatibility?

No. Without a time the chart is set for noon, and the seven planets are still positioned for both people, so both charts and every aspect between them come out as usual. A time and a city add the rising sign, and they are what makes a house comparison possible at all.

What does the no-birth-time mode leave out?

Three things go missing when a birth time is not given.

• The rising sign, which is never guessed at from a date.
• The house overlays, because houses need a time and a place before they mean anything.
• Precision on the Moon, which moves about 13 degrees a day, so a noon position can sit some way from the real one. The tool widens the Moon's orb by 3 degrees to allow for that rather than claim a precision it doesn't have.

Everything else, including the Sun, Venus, Mars and the aspects between them, is computed the same way with or without a time, and the result lists whatever the missing details cost you.

Which birth chart placements matter most for love?

The key placements for relationship dynamics are five, and this calculator weights them in that order when it scores two charts.

Everything else counts once, Mercury, Jupiter and the ascendant included, and the tool positions seven bodies plus the ascendant so that those quieter contacts still appear in the list.

Each of those placements gets a page of its own as the birth chart set lands. The Moon in a chart is covered under Moon Sign, the attraction pair under Venus and Mars, and the ascendant under Rising Sign. The method that holds them together is Synastry.

What are the most common synastry aspects?

The common synastry aspects are five, and an aspect is simply the angle between one person's planet and the other's. Each one has an orb, which is how far from exact that angle may sit and still count.

• Conjunction, 0 degrees, orb 8.
• Opposition, 180 degrees, orb 8.
• Trine, 120 degrees, orb 6.
• Square, 90 degrees, orb 6.
• Sextile, 60 degrees, orb 4.

Conjunctions, trines and sextiles are counted as flowing. Squares and oppositions are counted as friction when one of the two bodies is a personal planet, meaning the Sun, Moon, Mercury, Venus or Mars, and as neutral between two slow ones. Saturn contacts are not counted against a pair: the tradition reads them as stabilising rather than warm, which is why Saturn carries a weight of 2 here instead of a minus.

All of that begins with two signs at the simplest level, and the sun-sign layer on its own is zodiac sign compatibility.

How does the birth chart compatibility calculator score the aspects?

The score is the weighted balance of every kept contact, scaled to a number from 0 to 100. Each contact takes its weight from the table above, a direction from the list above, and a tightness, which is how close to exact it landed inside its orb. The Sun and the Moon are allowed 2 extra degrees of orb, and a Moon with no birth time behind it gets 3 more. If no contact falls inside an orb at all, there's no balance to scale, so the score is set to the midpoint of 50 and the result says why.

Two halves of that rule stand on very different footing, and the difference is worth stating plainly.

The measured half is the positions. They come from astronomy-engine 2.1.19, on the tropical zodiac that Western astrology uses, and they were checked on 22 September 2026 against the NASA JPL Horizons ephemeris. All 28 values agreed to within 0.003 degrees.

The convention half is the scoring. The orbs, the weights in the table above and the extra Moon orb without a birth time are this site's own reading of how astrologers treat those contacts. Every number is published on the methodology page, each one is marked there as proposed, and we are looking for an astrologer to review them.

What do the 5th and 7th house overlays show in birth chart compatibility?

A house overlay is what you get when one person's chart is laid over the other's and you see which of their houses the first person's planets land in. The tradition reads the 5th house overlay as romance and play, and the 7th as partnership and commitment.

Houses need a birth time and a birth city for both people, so an overlay appears only when both charts have one. When they do not, the result says so among the things it left out.

What is the difference between synastry and a composite chart?

They are two different charts. Synastry keeps both charts whole and reads the angles between them, which is what this tool does. A composite chart averages the two charts into a single third chart that stands for the relationship rather than for either person in it.

This site doesn't draw composite charts, and says so rather than letting the word sit there unexplained. Synastry is the method behind every other name this tool goes by, which the next three questions take in turn.

Is a synastry chart the same as birth chart compatibility?

Yes. Synastry is the astrologer's word for the same comparison, and the phrase synastry birth chart usually means the two charts drawn together on one wheel. Ask for one and you are asking for what this page computes.

Is natal chart compatibility the same as birth chart compatibility?

Yes. Natal chart compatibility, known as synastry, compares two birth charts, because a natal chart and a birth chart are the same document under two names. Astrology natal chart compatibility and love compatibility birth chart describe that one comparison too.

Can you get a free birth chart compatibility report?

Yes. The report on this page is a list rather than a document. It holds the placements for both charts with their signs and degrees, every aspect between them with its orb and its weight, the score, and anything the missing inputs left out. No account, no email, no payment.

If you only know the two sun signs, the quick version is on the 12 x 12 zodiac chart. The hub lists every compatibility test we run with the label each one carries.