Free synastry calculator
Enter the birth details of two people and see the real aspects between their planets: which are harmonious, which are tense, and each one's sun sign.
What synastry is
Synastry is the technique of overlaying two birth charts and measuring the angles formed between one person's planets and the other's. Each angle, or "aspect", describes a type of contact: tension, understanding, attraction or distance.
This calculator uses real ephemeris data to place the Sun, Moon and planets of each person at the moment of their birth, then measures the angular distance between each pair of planets. Nothing is invented: without an exact birth time, some aspects — the ones tied to the fast-moving Moon — lose precision.
Why it is not the same as comparing sun signs
"Aries and Taurus compatibility" compares only two signs out of twelve, a tiny fraction of a chart: the Sun is one planet among ten, and a sun sign says nothing about each person's Moon, Venus or Mars.
Synastry cross-checks the ten planets of one chart against the ten planets of the other: a hundred possible combinations, each with its own angle. Two people whose sun signs are labelled "incompatible" by generic lists can still have a Moon trine or a Sun-Venus conjunction, which carries far more weight than the sun sign alone.
Which aspects carry more weight, and why
Not every aspect counts the same. Contacts involving the Sun, Moon, Venus or Mars of one person touching the same planets of the other tend to shape a relationship more than, say, an aspect between the two Uranus placements.
The orb (the margin of degrees allowed for an aspect to count) matters too: this calculator uses a stricter orb than a single natal chart, around 80% of the usual margin, to keep only the clearest contacts between the two charts.
- Conjunctions and oppositions to the Sun or Moon: usually the aspects felt most in day-to-day life.
- Venus and Mars aspects: speak to attraction, desire and how affection gets expressed.
- Saturn aspects: mark commitment, structure, or, when tense, a sense of limitation.
Without a birth time, what you lose
The Moon travels through an entire sign in about 2.5 days and changes sign several times a month: without an exact birth time, it may still land in the right sign but at an approximate degree, which makes any aspects built on it less reliable.
The Ascendant and the astrological houses need a precise time and place to calculate; this calculator focuses on planet-to-planet aspects, which are more stable, and flags next to the result when a birth time is missing.
What the written reading adds
This page shows you the raw aspects: which planets touch, and at what angle. It deliberately does not turn that into a single joint verdict: doing that properly means weighing dozens of aspects together, seeing which reinforce or offset each other, and reading them against both full charts.
That is why you will not see a compatibility percentage here: making one up would be easy but false. The paid reading, written by AI from both full charts, does offer a reasoned assessment, aspect by aspect, explaining why each one matters.
Frequently asked questions
Does the synastry calculator give a compatibility score?
No. It shows the aspects between the two people's planets and how many are harmonious or tense, but it does not reduce that to a single number. A single score would oversimplify and sound more precise than it really is.
Do I need both people's exact birth time?
It helps a lot, especially for aspects involving the Moon. If you don't have it, you can still calculate aspects between the Sun and the slower-moving planets, which barely shift position within a single day.
What is the difference between a harmonious and a tense aspect?
Harmonious aspects (trine, sextile) describe an easier flow between two planetary energies. Tense ones (square, opposition) mark friction or contrast. Neither guarantees that a relationship will or won't work: they only describe dynamics.
Is this only for couples, or also for friendships?
It works for any relationship between two people: couples, friendships, business partners or family. The aspects are the same; what changes is how they play out depending on the kind of bond, which the written reading explains in more depth.