Free synastry chart
Synastry Chart Calculator
Enter the birth date, time and place of two people. The chart lists where the Sun, Moon and planets stood for each of you, the aspects between your planets and your partner’s, and, with times and places, the houses your planets fall in.
What a synastry chart shows
Synastry is the part of astrology that compares two birth charts. Instead of reading one person’s planets on their own, it looks at the angles between one person’s planets and the other’s: when your Venus falls close to your partner’s Mars, astrologers speak of a Venus–Mars conjunction in synastry.
This calculator finds those angles. For each of you it works out where the Sun, the Moon and the eight planets stood at the moment of birth, then checks every planet of yours against every planet of your partner’s and lists the aspects it finds, tightest first.
How the aspects are found
Five classic aspects are used: the conjunction (0°), the sextile (60°), the square (90°), the trine (120°) and the opposition (180°). An aspect counts when the real angle is within its orb: 8° for conjunctions and oppositions, 7° for squares and trines, 5° for sextiles. The smaller the orb, the stronger the contact is traditionally read.
Aspects between two slow outer planets (Uranus, Neptune, Pluto) are left out, because almost everyone born within a few years of each other shares them. Positions are tropical and geocentric, calculated with the open-source astronomy-engine library; they agree with the Swiss Ephemeris to within a few arcseconds.
Why the birth time matters
The Moon moves about 13° a day, so a birth time changes its position a lot; the other planets move far less. If you do not know a time, the chart uses noon, and every aspect that involves the Moon is marked as approximate.
The place of birth sets the time zone, so that the local birth time is converted to universal time correctly, including summer time, and it gives the houses. With a time and a place the chart adds the Ascendant, the Midheaven and the house overlays: in which of your houses your partner’s planets fall, and the other way round. The houses are Placidus, the usual choice in synastry.
How to read the result
Astrologers usually start with the tightest contacts between the personal planets: the Sun, the Moon, Mercury, Venus and Mars. Harmonious aspects (trines and sextiles) are traditionally read as ease, challenging ones (squares and oppositions) as friction that asks for attention, and conjunctions as a blending of two energies.
No single aspect describes a whole relationship, and none of them decides whether two people will be happy together. Use the chart as a map of themes to talk about; where an aspect has its own page on AstroSynth, the link leads to a longer explanation.
Frequently asked questions
Is this synastry chart free?
Yes. Enter two birth dates and times and the chart is calculated in your browser; there is no sign-up, and nothing you enter is sent or stored.
Do I need an exact birth time?
It helps, mostly for the Moon, which moves about 13° a day. Without a time the chart uses noon and marks aspects of the Moon as approximate.
How are the houses calculated?
With the Placidus system, from the exact birth time and the coordinates of the city. Without a birth time or a place the houses are left out; above the polar circles, where Placidus does not work, the quadrants are divided equally (Porphyry).
Which orbs are used?
Conjunction and opposition 8°, square and trine 7°, sextile 5°. These are the usual synastry orbs, a little tighter than for a single natal chart.
Can a synastry chart predict whether a relationship will last?
No. Synastry describes symbolic themes in astrological tradition. It can start a useful conversation, but it does not measure love, trust or the future of a couple.