Beyond Sun-sign matching
Synastry calculator.
Two charts, one conversation.
Compare the planetary patterns between two birth charts. You will see the strongest conjunctions, sextiles, squares, trines and oppositions—calculated locally in your browser and explained without turning them into a relationship verdict.
Two birth charts · calculated in your browser
Explore your synastry
Enter birth details for two people. We calculate planetary positions and the strongest major aspects between the charts. Nothing is uploaded or saved.
Your chart comparison
First chart
Second chart
Strongest major aspects
Ordered by personal-planet relevance and closeness. An orb is the distance from the exact aspect.
Read patterns, not verdicts.
Synastry cannot measure trust, communication, safety, shared values or whether a relationship will last. Use an interpretation as a conversation prompt—not permission to ignore lived experience.
What this birth chart compatibility calculator includes
The calculator finds the zodiac positions of the Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto for each person. It then measures the angular distance between the two charts and shows up to twelve major aspects, prioritizing close contacts involving personal planets.
Conjunctions can amplify a shared theme. Trines and sextiles are traditionally read as easier exchanges. Squares and oppositions may describe tensions or polarities that ask for more awareness. None of those labels tells you whether two people treat each other well.
How unknown birth times are handled
If you mark a birth time as unknown, the calculation uses local noon for the slower-moving planets and omits that person’s Moon. This avoids presenting a time-sensitive Moon placement as certain. This version does not calculate Ascendants, house overlays or chart angles, even when both times are known.
Your details remain private. Dates, times, cities and optional names are processed on your device. They are not uploaded, saved or placed in the page URL.
How to use the reading
Begin with one or two aspects that resemble something you both recognize. Ask for a real example. If an interpretation does not fit, leave it. A chart is most useful here as a prompt for a clearer conversation—not as an authority over either person.
Questions, answered
What is a synastry calculator?
A synastry calculator compares planetary positions in two birth charts and identifies geometric relationships called aspects. Astrologers interpret those contacts as themes to explore, not as proof that a relationship will succeed or fail.
Do I need an exact birth time for synastry?
A recorded birth time is important for the Ascendant, houses and precise angles. This calculator compares planetary aspects and omits the Moon when a birth time is unknown because the Moon can change sign during a day. House overlays and angles are not included in this version.
Is synastry the same as birth chart compatibility?
Birth chart compatibility is a common plain-language name for synastry. Both describe comparing two natal charts, although different tools may include different planets, aspects, angles or houses.
Can synastry predict whether a relationship will last?
No. Synastry cannot measure behavior, trust, safety, communication or shared values. It can offer symbolic prompts for reflection, but it is not a scientifically validated relationship forecast.