Free · No signup · Two charts

Free Synastry Chart Calculator

Compare two birth charts and see every connection between you: the aspects that spark, soothe and challenge, and where each of you lands in the other’s chart.

Sample synastry chart

Here’s what you’ll get

Sam’s Mars opposite Riley’s Sun: a magnetic pull from opposite sides. You complete each other and drive each other a little crazy.

Both charts on one wheel, with the strongest connections drawn between you
Your strongest connections in plain language, ranked
Every aspect by theme: chemistry, emotional bond, staying power and more, each linked to our guide
House overlays both ways: where each of you lands in the other’s life

Sample: two invented people, Sam (March 8, 1990, New York) and Riley (July 21, 1992, Chicago).

The basics

What is synastry?

Synastry is the astrology of relationships. It compares two birth charts to show how two people affect each other: where the attraction comes from, where you feel at home together, and where you rub each other the wrong way.

It works for any relationship, not just romance. The same contacts that make a couple click explain why a friend feels like family or why a coworker gets under your skin. For a deeper walkthrough, read how to read your synastry chart.

Reading it

How to read a synastry chart

1Start with the personal planetsSun, Moon, Mercury, Venus and Mars describe day-to-day chemistry. Contacts between them matter most.
2Look at the tightest orbsAn aspect within 1 or 2 degrees is louder than one at 6. The calculator ranks your strongest connections first.
3Read it both waysYour Venus to their Mars is a different story from their Venus to your Mars. Notice who brings what.
4Check the house overlaysWhere their planets land in your chart shows which part of your life they light up.

The aspects

What each aspect means between two people

Aspects are the angles between one person’s planets and the other’s. Start with our guide to synastry aspects.

☌︎Conjunction · 0°Two planets fuse. The most noticeable bond, for better or worse.
⚹︎Sextile · 60°Friendly and easy. It grows when you both put in effort.
□︎Square · 90°Friction and spark. It pushes you both and rarely feels boring.
△︎Trine · 120°Natural flow. Things between you just work.
☍︎Opposition · 180°A magnetic pull from opposite sides that teaches you balance.

Compatibility

Signs of a strong match

No single aspect makes or breaks a relationship, but these contacts show up again and again in lasting couples. See the top five synastry aspects and marriage aspects in synastry.

☉︎Sun and MoonOne person’s identity meets the other’s emotional needs. A classic marker of feeling understood.
♀︎Venus and MarsThe chemistry contact. Attraction, desire and the push-pull of romance.
☽︎Moon and MoonMatching emotional rhythms. Being together feels like home.
♄︎Saturn tiesSaturn to the Sun, Moon or Venus adds commitment and staying power, sometimes with a side of pressure.

Beyond synastry

Synastry vs. composite charts

Synastry compares two separate charts to show how you affect each other. A composite chart merges them into a single chart for the relationship itself. Explore the best composite chart aspects, karmic and spiritual synastry and progressed synastry when you are ready to go deeper.

Go deeper

Know what’s happening between you right now

Your synastry never changes, but the planets moving today keep setting it off. SASS Insider sends a horoscope written for your exact chart every morning, plus SASS Chat, an AI astrologer you can ask about any connection on this page. It’s $50 a year.

Join SASS Insider Get your AI relationship reading

Every aspect, explained

The SASS synastry library

Every synastry aspect and house overlay we’ve written about, in one place. Your calculated chart links straight to the ones in it.

Planets in synastry

What each planet means between two people

House overlays

Every synastry aspect

☉︎Sun aspects

☿︎Mercury aspects

♅︎Uranus aspects

♆︎Neptune aspects

♇︎Pluto aspects

Guides

Reading synastry, compatibility and beyond

Questions

Synastry FAQ

What is a synastry chart?

A synastry chart lays two birth charts on top of each other to show how two people interact. Astrologers read the angles between one person’s planets and the other’s (the aspects), and which houses each person’s planets fall into in the other chart.

Is this synastry calculator free?

Yes. There is no signup and no email required. Enter both birth dates, times and cities and the full comparison appears on the page.

Do I need both birth times?

No, but they help. Without a birth time you still get every aspect between the planets, with the Moon marked as approximate. House overlays need the birth time of the person whose houses are being used.

What are the best synastry aspects for a relationship?

Contacts between personal planets matter most: Sun and Moon, Venus and Mars, Moon and Moon, and Venus to the Sun. Saturn contacts add staying power. Squares and oppositions are not bad; they bring the friction that keeps a relationship alive, as long as there is enough ease elsewhere.

Why do my aspects look different from my partner’s?

Synastry runs both ways. Your Venus trine their Mars is a different contact from their Venus to your Mars, so the calculator lists each person’s planets separately and names whose planet is whose.

What is the difference between synastry and a composite chart?

Synastry compares two separate charts. A composite chart blends them into one chart for the relationship itself, built from the midpoints between your planets. Synastry shows how you affect each other; the composite shows what you become together.