Birth chart compatibility calculator: one real reading, worked through

Most compatibility charts online match Sun signs. Pisces and Cancer: a perfect match! It’s fun, but it compares one point in each chart and ignores the other twelve. A birth chart compatibility calculator does something different. It sets two whole charts side by side and measures every angle between them.

This post shows exactly what that looks like on two real charts, Robin and Sam, a couple from our example circle. You’ll see the five scores our compatibility calculator gives them, every contact behind each score, and how to read your own result without getting hung up on the numbers.

What does a compatibility calculator measure?

The technique is called synastry. Take every planet and angle in one chart and check its angle to every planet and angle in the other. When two points make one of the five major aspects, within a few degrees, that’s a contact.

Our engine is strict about which contacts count:

• Anything involving a Sun or Moon counts within 6°. Other pairs count within 5°, and sextiles within 3°.
• Uranus, Neptune, Pluto and the node only count within 3° when they touch a personal point.
• Two outer planets touching each other don’t count at all. Everyone born within a few years of each other shares those.

Each contact then feeds one or more of five themes. Every score starts in the middle, at 50 (Friction starts at 20), and each contact moves it by up to 14 points, more when it’s tight, less when it’s loose. That’s the whole method. There’s no hidden formula and no star rating.

The two charts

Robin was born on 14 March 1990 at 8:30am in Chicago. Sam was born on 2 July 1992 at 9:10pm in London. Here are the points that matter most for relationships.

On a Sun-sign chart this is a dream match: Pisces and Cancer, two water signs, a trine by sign. But look at the degrees. Robin’s Sun is at 23°44′ Pisces and Sam’s at 11°09′ Cancer. They’re 107.4° apart, which is 12.6° away from a real trine. Too wide to count. In the actual synastry, their Suns don’t touch at all.

That’s the first lesson of birth chart compatibility. Sign-to-sign matches are a rough guess. Degrees decide. Between Robin’s and Sam’s charts, the engine finds 26 real contacts, and only 1 of them involve either Sun.

The five scores, and what moved them

This is the real result, rendered by the same component you see on a comparison page:

Spark: 61

Above the middle. The lift comes mainly from Robin’s Pluto trine Sam’s Venus, a slow, magnetic pull. There’s a looser Venus–Mars square too, which adds a little push and pull.

Warmth: 47

Just under the middle, and from a single contact: Robin’s Venus opposite Sam’s Moon, 5.27° wide. An opposition is a hard aspect, so it nudges the score down slightly. In practice that reads as affection that has to be negotiated: Robin shows love one way, Sam needs it another.

Talk: 46

Two contacts pulling in opposite directions. Robin’s Mercury trine Sam’s Venus helps; Robin’s Venus opposite Sam’s Mercury, tighter at 1.38°, works against it. Easy to chat, easy to misread each other’s tone.

Staying power: 58

Two node contacts lift this one. The lunar nodes are traditionally read as a sense of being fated or familiar. A Saturn square to Robin’s ascendant pulls it back a little: commitment, with some weight to carry.

Friction: 45

Well above its starting point of 20. The biggest single contact in the whole reading is here: Robin's Rising opposite Sam's Pluto (0.85°). Sam’s Pluto sits right on Robin’s descendant, the point of partnership. It’s intense, not bad. Higher friction often means a couple who never bore each other.

Where each lands in the other’s life

Because both birth times are known, the engine can also place each person’s planets in the other’s houses. These house overlays say which part of your life the other person switches on.

• Sam’s Sun and Venus fall in Robin’s 3rd house: everyday talk, siblings, neighbours and short trips.
• Sam’s Moon and Mercury fall in Robin’s 4th house: home, family and where you come from.
• Sam’s Mars fall in Robin’s 12th house: rest, solitude, the hidden and the unconscious.
• Robin’s Venus and Mars fall in Sam’s 1st house: you: your body, your style and how you begin things.
• Robin’s Sun and Mercury fall in Sam’s 2nd house: money, belongings and what you feel you're worth.
• Robin’s Moon fall in Sam’s 9th house: travel, study, beliefs and the bigger picture.

So Sam tends to light up Robin’s everyday conversations and home life, while Robin lands right on Sam’s sense of self. That fits the rest of the reading: lots of pull, and some work to understand each other.

A second pair, for contrast

Compatibility isn’t only for couples. Maya and Jo, a sister and a best friend from the same example circle, score very differently: Spark 74, Warmth 50, Talk 30, Staying power 53 and Friction 52.

A high Spark between friends is usually energy and competitiveness, not romance. A low Talk score of 30 means their Mercury contacts are tense. They may need to say things twice. Same five themes, very different shape. That’s the point of scoring themes instead of handing out one percentage.

How to read your own result

1. Read the contacts, not the number. Under every score is the list of contacts behind it. That list is the reading.
2. Start with the tightest. Anything under 1° is the loudest thing between you. For Robin and Sam it’s the ascendant–Pluto opposition.
3. Check both birth times. Without them you lose the rising-sign contacts and the house overlays. If one time is shaky, see how much it can move in our birth time study.
4. Know your own chart first. Synastry makes more sense when you already know your big three. If you don’t yet, start with how to read your birth chart.
5. Learn the five angles. Conjunctions blend, sextiles and trines flow, squares and oppositions rub. Our aspects guide goes deeper.

Common mistakes

• Treating a low score as a red flag. Robin and Sam’s Warmth of 47 comes from one contact. It describes a style, not a shortage of love.
• Trusting sign-to-sign matches. Their “perfect” water-sign Suns don’t even make an aspect.
• Counting generational contacts. Uranus and Neptune in the same sign for both of you is a birth-year thing, not a relationship thing. That’s why we leave them out.
• Using it to decide whether to stay together. It’s a conversation starter. Your actual relationship is better evidence than any chart.

Positions come from astronomy-engine, an open-source ephemeris tested against NASA data (source and tests on GitHub). Put in your own two birth dates and you’ll get the same breakdown for you.