Birth chart compatibility calculator

What is birth chart compatibility?

Birth chart compatibility is the practice of comparing two natal charts to see how the people behind them are likely to connect, clash and grow together. Astrologers call it synastry. Where a Sun-sign horoscope compares one placement (Leo, Scorpio), a birth chart comparison looks at the whole sky at each person's birth: the Moon for emotional needs, Venus for how each of you loves, Mars for desire and conflict, Mercury for communication, Saturn for commitment and the lunar nodes for the feeling of direction. Then it maps each person's planets onto the other person's houses to see which areas of life the relationship lights up.

This calculator is the free, instant version of that comparison. It uses birth dates only, so it works with the placements that can be computed without a time: Sun, Venus, Mars, Mercury and, within a few degrees, the Moon. It applies the same element and modality logic that astrologers use as a first filter, then scores the major contacts between the two charts. The app's compatibility feature runs the full synastry calculation with times and places, which is why the two can differ; more on that below.

What does a compatibility percentage mean?

The percentage answers one question: how much of this relationship runs on ease rather than effort? It is built from weighted contacts. Harmonious aspects (conjunctions, trines and sextiles between the Sun, Moon and Venus, Venus–Mars links, matching elements) add points. Challenging aspects (squares and oppositions from Mars or Saturn to the other person's Sun, Moon or Venus, clashing modalities) subtract or, more precisely, move points from the ease column into the effort column.

Here is how to read the number, based on how synastry charts typically distribute:

• 85–100%: rare and usually a sign of several exact contacts, such as Sun conjunct Moon or Venus trine Mars within two degrees. Very easy chemistry. The risk is complacency, not conflict.
• 70–84%: strong. Most happy long-term couples we have seen fall here: enough ease to enjoy each other, enough friction to keep growing.
• 55–69%: workable, and often more passionate than the top band. This is where most fixed-sign squares land, including a classic like Leo and Scorpio. The relationship needs explicit communication about the specific tension the reasons list names.
• 40–54%: significant effort. Not a reason to leave, but a reason to look at the chart together and decide, with eyes open, whether the specific friction is one you can live with.
• Under 40%: unusual with real birth data, and usually means a stack of Saturn or Pluto hard aspects. It can still be a meaningful relationship; it is rarely a restful one.

A birth chart compatibility percentage is not a probability of staying together. Two people at 62% who name their pattern and work with it outlast two people at 88% who assume the stars will do the work.

What this free calculator checks

Sun to Sun. Elements and modalities between the two Sun signs. Same element or complementary element (fire–air, earth–water) scores high; same modality in different signs (the squares and oppositions) flags a push-pull.

Sun to Moon, both directions. The classic marriage contact. Your Sun in harmony with their Moon means they feel seen by who you are, and vice versa.

Moon to Moon. How you each decompress. Trine or same-element Moons want the same kind of evening after a hard day; square Moons want opposite things and both feel unsupported until they learn to say so.

Venus to Venus. Whether you show love in a language the other person reads as love. Venus in Cancer cooks; Venus in Aquarius gives space. Both are love. Whether they land is what this contact measures. See Venus in astrology for the full logic.

Venus to Mars. Attraction and desire. Conjunctions and trines feel effortless; squares are hot and argumentative; no contact at all often means a friendship that stays a friendship.

Mars to Mars. How you fight and how you pursue. Harmonious Mars contacts mean you argue the same way; squares mean one of you goes loud while the other goes cold.

Each contact is listed with the aspect, the orb (how exact it is) and a one-line reason, so the score explains itself.

What the app adds: exact synastry with birth times

A birth date gives you the planets. A birth time gives you the houses and the Ascendant, and in synastry those change the reading more than most people expect.

• House overlays. Where a partner's Sun, Moon or Venus falls in your chart shows what the relationship is about for you. Their planets in your 7th house feel like partnership; in your 5th like romance and play; in your 8th like intensity, merging and shared money; in your 12th like something you cannot quite see clearly. The 7th house guide explains the partnership axis.
• Ascendant contacts. Their Venus on your Ascendant is instant physical liking. Their Saturn on your Ascendant is respect that can tip into feeling judged.
• Exact Moon degrees. The Moon moves about 13 degrees a day, so without a time it can be off by up to 6–7 degrees. That is the difference between a Moon–Moon trine and no aspect at all.
• Saturn, the outer planets and the nodes. The app scores Saturn contacts (the glue), Pluto, Uranus and Neptune (the intensity, surprises and blur) and the lunar nodes (direction), which the browser version leaves out.
• Four-dimension scoring. Instead of one number, you get love, communication, friction and growth scored separately, plus an AI synastry reading in plain English.

The compatibility hub explains each dimension, and the synastry guide lists every aspect the engine weighs. Cosmic Dating in the app uses the same calculation to match you with people whose charts fit yours; join early access to be in the first wave.

Compatibility calculator vs synastry calculator vs relationship quiz

People search for these three things and usually want the same thing, so here is the difference. A compatibility calculator (this page) gives a score from birth data. A synastry calculator draws the actual bi-wheel chart with every aspect line, which is what the app does once you add birth times. A relationship compatibility quiz asks you questions about yourselves; it measures what you already believe. Astrology's advantage is that it does not ask. It reads the same two charts whether you are in love, angry or unsure.

If you want a playful second opinion, the biorhythm compatibility calculator compares your physical, emotional and intellectual cycles; if you want your own chart first, start with the free birth chart.

Why your score differs between calculators

If you have run the same two birth dates through three sites and got 58%, 71% and 89%, nothing is wrong with your relationship. Calculators differ in three ways: which planets they include (Sun-only tools inflate or deflate wildly), which orbs they allow (a 10° orb counts far more aspects than a 6° one), and how they weight hard aspects (some treat a Venus–Saturn square as a minus, others as commitment). We show every contact and its weight so you can see where the number comes from, and we use the same orbs for everyone. Compare reasons, not percentages, and the picture becomes consistent.

Methodology

We use the tropical zodiac, Placidus houses in the app, ephemeris-grade planetary positions, and standard synastry orbs (up to 8° for Sun and Moon conjunctions, oppositions and trines; up to 6° for Venus and Mars; 3° for sextiles). Scores are weighted by aspect type and orb: an exact trine counts more than a wide one. The browser calculator uses noon as the default time when none is entered, which is why we call the Moon approximate. Nothing here is a prediction; synastry describes tendencies, and two people decide what to do with them.