Synastry Chart Calculator
Compatibility by sign compares two labels. Synastry compares two actual charts — where your Moon falls against their Venus, what your Mars does to their Saturn. Enter both birth dates below and the aspects are calculated for real.
Calculate your synastry
What synastry actually is
Synastry lays one birth chart over another and looks at the angles between them. Not "you are a Leo and they are a Taurus" — that compares two Suns and ignores the other twenty points. Synastry asks what your Moon is doing to their Mercury, whether their Saturn sits on your Venus, how far apart your Ascendants are. Those angles are where the day-to-day friction and the day-to-day ease actually live, which is why two couples with identical Sun signs can have nothing in common.
What this calculator computes
Everything below is arithmetic on real planetary positions, not a personality quiz.
Two full charts
Swiss Ephemeris computes both charts for the exact moments given: Sun through Pluto, the lunar nodes, Lilith, Chiron, and — when birth time is known — the Ascendant and the house grid.
Cross-aspects between them
Every relationship point of chart one is measured against every relationship point of chart two: conjunction, sextile, square, trine, opposition. Orbs are 8° when the Sun or Moon is involved, 6° otherwise, 5° for sextiles.
Harmony and tension indices
Supportive contacts (Sun, Moon, Mercury, Venus, Jupiter, Ascendant) raise the first number; hard ones (Mars, Saturn, Uranus, Pluto, Lilith) raise the second. Venus–Mars raises both, because attraction and friction are the same aspect here.
Nothing about generations
Uranus, Neptune and Pluto contacts between two people of the same age exist for everyone born in those years. They say nothing about the two of you, so they stay out of the indices.
Why birth time matters here
Without a time we can still compute both charts — every planet keeps its sign, and most aspects between you are unchanged. Two things do disappear. The Ascendant, which is the "how we come across to each other" layer, needs the exact minute: it moves a full degree every four minutes. And the Moon travels roughly 13° a day, so on some dates it changes sign between midnight and midnight — when that happens we say so and show both options rather than picking one. Everything else stands.
How to read the aspect table
Five angles, and what each one usually does in a pair.
Conjunction (0°)
Two planets in the same place. The strongest contact there is, and the most double-edged: it amplifies whatever the two bodies already do.
Trine (120°) and sextile (60°)
The easy angles. Things work without either of you trying, which is pleasant and occasionally means nothing gets pushed.
Square (90°)
The productive irritation. Two functions pulling in different directions — the aspect most couples describe as "we keep having the same argument".
Opposition (180°)
Facing each other across the chart. Attraction and stalemate share this angle: each of you holds what the other refuses to.
Orb
How far the aspect is from exact. 0.4° is a contact you feel constantly; 7.8° is real but quiet. Smaller orb, louder aspect.
Questions about synastry
Is this synastry calculator really free?
Yes, and there is no account, no email and no trial. The calculation is pure arithmetic on planetary positions, so it costs us processor time and nothing else. The paid part of the product is the written interpretation, which is a separate step you can take or skip.
Do I need the birth time for both people?
No. Without a time you still get both charts and almost every aspect between them. What you lose is the Ascendant — it needs the exact minute — and certainty about the Moon on days when it changes sign. The result marks both cases explicitly instead of quietly guessing.
What is the difference between synastry and sign compatibility?
Sign compatibility compares two Sun signs, which is two data points out of roughly forty. Synastry compares whole charts: Moon to Venus, Mars to Saturn, Ascendant to Sun. It is why "we are a terrible match on paper" and "we have been together twelve years" are not a contradiction.
What do the harmony and tension percentages mean?
They are a weighted count of the aspects listed underneath them, not a verdict. Supportive contacts feed one number, hard contacts feed the other. Both are capped at 15–85 on purpose: a pair with no friction at all does not exist, and neither does one with no common ground.
Is a high tension score bad?
It is not a warning label. Squares and oppositions describe where two people keep meeting resistance — which is also where most of the charge in a relationship comes from. Long relationships tend to have plenty of both indices; flat pairs are the ones with neither.
Can I calculate compatibility for friends or business partners?
The aspects are the same either way — the geometry does not know what kind of relationship it is describing. The written reading does: it comes in three modes, one of which evaluates the pair strictly as colleagues and co-founders, with no romance in it.
Which astrological system do you use?
Tropical zodiac with Placidus houses, computed by Swiss Ephemeris — the same library professional astrology software uses. Above the polar circles, where Placidus is undefined, the calculation falls back to Porphyry.
Now the part the numbers cannot say
A table of aspects tells you where the contacts are. The written reading takes both charts and says what they actually do to each other — in romantic, psychological or business mode. First reading is free.
Get the written readingAstraBrain is for entertainment and self-reflection. Nothing here is medical, psychological, financial or legal advice, and no chart should decide who you stay with or leave.