Free synastry chart

What a synastry chart shows

A synastry chart compares two birth charts to see how they interact. Instead of asking whether two Sun signs get along, synastry looks at every planet in one person's chart against every planet in the other's: where your Moon falls relative to their Venus, whether their Saturn squares your Sun, and so on. It's the method astrologers actually use when they read a relationship chart, and it applies to partners, friends, family or business partners alike.

This free tool calculates both charts, draws them together on a single bi-wheel and lists the strongest contacts between them. The positions are pure astronomy and fully reproducible. What those contacts mean is interpretation, and we try to keep the two clearly apart.

How the comparison is calculated

Each chart is calculated exactly like our free birth chart: the birth time is converted to universal time with historical time zone and daylight saving rules, and the planet positions come from NASA JPL's DE440 ephemeris in the tropical zodiac. The details are on our methodology page.

Then we compare the two sets of positions. For each pair of points, one from each chart, we check whether the angle between them forms one of the five major aspects: conjunction (0°), sextile (60°), square (90°), trine (120°) or opposition (180°). An aspect counts only if it's close to exact. In synastry we use tighter orbs than in a single chart, 6° for conjunctions and oppositions, 5° for trines and squares and 4° for sextiles, because with twenty-odd points on each side, wide orbs would make nearly everything look connected.

Reading the bi-wheel

The bi-wheel places the first person's planets on the outer ring and the second person's on the inner ring, around the same zodiac. Lines between the rings show the aspects. If you see one person's planets clustering in a particular part of the other's chart, that's often a theme worth noticing.

The contacts astrologers look at first

Not all aspects carry the same weight. In a relationship chart, these usually get attention first:

• Moon contacts. The Moon describes emotional needs and comfort. Harmonious Moon–Moon or Sun–Moon aspects are traditionally read as a sense of feeling understood without much effort.
• Venus and Mars. Venus is affection and taste, Mars is desire and drive. Contacts between one person's Venus and the other's Mars are the classic signature of attraction.
• Mercury. How easily two people talk, argue and make plans together.
• Saturn. Saturn contacts are often read as commitment, responsibility and sometimes restriction: the "glue" that holds people together and the weight they may feel.
• Jupiter. Generosity, encouragement and the sense that the other person helps you grow.

If both birth times are known, the Ascendants and houses come into play too: which of your houses the other person's planets fall into says something about the area of life the relationship tends to touch.

How to read your synastry results

• Look at both directions. Your Venus trine their Moon is a different story from their Venus square your Moon. Synastry is rarely symmetrical.
• Don't count harmonious versus tense aspects. A square often brings energy and growth, and lots of trines can feel pleasant but undemanding. Long relationships usually have both.
• Start with the tightest aspects. The smaller the orb, the more prominent the contact tends to be.
• Look for repetition. If several contacts point to the same theme, such as communication or security, that theme probably matters more than any single aspect.

Our guide on how to read a synastry chart walks through a full example.

Why birth times matter here too

Without a birth time, the Moon's position can be off by several degrees, enough to make or break a tight aspect, and the Ascendant and houses can't be calculated at all. You can still run a synastry chart with unknown times: the planet-to-planet contacts between the slower planets remain reliable. Just give less weight to aspects involving the Moon and ignore anything that depends on houses.

Synastry, sign compatibility and the full report

If you only know your two Sun signs, start with zodiac compatibility: it describes the general dynamic between the signs. Synastry is the more personal comparison and usually the more revealing one.

The free tool shows the bi-wheel and the most significant contacts. The love compatibility report interprets every significant contact, grouped by theme (attraction, emotional bond, communication, commitment, shared growth and identity), in a web report and PDF. See all reports for current prices.

Whatever you read, synastry describes tendencies and dynamics, not outcomes. It can't tell you whether a relationship will work or whether someone is "the one", and it shouldn't replace talking to the actual person.