Synastry Chart Calculator
Enter both birth dates, times and places to compare the two charts: every aspect between your planets, what it means for the couple, and a compatibility score you can check. Free, no sign-up, birth time optional.
How to read a synastry chart
Conjunction · 0°
Two planets merge, and both people feel that theme strongly. Soft with the Moon, Venus or Jupiter; intense with Saturn, the outer planets or two Mars.
Trine · 120°
The easiest aspect: the two sides of the relationship support each other without effort.
Sextile · 60°
A friendly contact that works when you use it: shared interests, easy talk, small chances to grow together.
Square · 90°
Friction and pressure to change. Uncomfortable, and often what keeps a couple moving.
Opposition · 180°
The pull of opposites: strong attraction and a need to balance two different styles.
The orb is how far an aspect is from exact: the smaller it is, the stronger the aspect. The Sun, Moon, Venus, Mars and the Ascendant weigh the most, so their aspects come first in the list.
Questions about synastry
What is a synastry chart?
Synastry compares two birth charts. It sets the planets of one person against the planets of the other and measures the angles between them, called aspects. Each aspect describes one side of the relationship: how you talk, what attracts you, where you comfort each other and where you clash. This calculator finds every aspect between the two charts, lists them strongest first and explains each one.
Can I do synastry without a birth time?
Yes. Tick “I don't know the birth time” and that chart is cast for noon. The Sun and the planets barely move in a day, so their aspects hold. Two things change: there is no Ascendant, and the Moon can be up to about 7° off, so its aspects are marked as approximate. The full written reading needs both birth times.
Which aspects matter most in synastry?
Those between the personal points: the Sun, Moon, Venus, Mars and the Ascendant, then Mercury. Sun–Moon contacts show how naturally you understand each other, Venus–Mars shows attraction, Moon–Venus tenderness, and Saturn on a personal planet shows commitment and its weight. The smaller the orb, the stronger the aspect. Contacts between two slow planets are left out, because everyone born in the same years shares them.
How is the compatibility score calculated?
Every aspect gets a strength from 0 to 1: the weight of its two planets times how close it is to exact. Trines, sextiles and soft conjunctions add to harmony; squares, oppositions and conjunctions with Saturn, Uranus, Neptune, Pluto or between two Mars add to tension. The score is 50 + 50 × (harmony − tension) ÷ (harmony + tension + 2), so it stays near 50% when both sides weigh the same. Romance reads the aspects of Venus and Mars, emotions the Moon, communication Mercury, chemistry Mars and the Ascendant, and shared growth Jupiter and Neptune.
What is the difference between synastry and a composite chart?
Synastry keeps the two charts apart and reads how they act on each other, such as your Venus on their Mars. A composite chart merges them into one chart of the relationship, using the midpoint of each pair of planets. This calculator does synastry; the composite chart is among the advanced charts in Premium.
What is free and what is in the full reading?
The charts, every aspect with its meaning, both people's Sun, Moon and rising signs and all the scores are free and need no account. The full reading is part of Premium: a written analysis of the couple built on these aspects, with whether it is worth investing time, the potential for love, intimacy and the long term, strengths, areas for growth and advice. It needs both birth times.