How we calculate compatibility

Every score on this site comes out of the same inspectable pipeline, and this page documents all of it — the astronomy, the five components, the exact weights and orbs. Partly because I would want any site scoring my relationships to show its work, and partly because a number is far more useful once you know what it measures.

Step one: two real charts

Nothing here is looked up in a table of sun-sign verdicts. When you submit two sets of birth details, we geocode each birthplace, infer its timezone from the coordinates, convert the local birth time to universal time, and compute each chart with astronomy-engine, an ephemeris library that returns true planetary positions for any moment — the same math astronomers use, accurate to well under a degree. Planets are placed in the tropical zodiac, the standard frame of Western astrology.

If a birth time is missing we assume 12:00 noon. That leaves the Sun, Mercury, Venus, and Mars accurate to roughly a degree and introduces up to 6.5° of uncertainty in the Moon, which moves about 13° per day. Compatibility scoring uses planetary longitudes only — the Ascendant and house cusps never enter the math — which is why the calculator does not demand an exact time the way the birth chart generator does.

Step two: five weighted comparisons

The 0–100 score is a weighted average of five separate comparisons. These are the actual weights in the code, not approximations:

The Moon, Venus, and Mars components each work the same way: take the angle between the same planet in both charts, find the closest major aspect within orb, and map its character onto a 0–100 scale. Two Moons in trine score in the nineties; two Moons in a tight square drop toward the twenties; no aspect at all lands at a neutral 55. The cross-aspect component casts a wider net — every personal planet in chart A against every personal planet in chart B, twenty-five pairings in all — and averages what it finds, so a single lucky trine cannot carry two otherwise clashing charts.

The sun-sign blend is the one component that reads signs rather than angles. Element accounts for 65% of it and modality 35%, with a small bonus when both Suns share a sign. Complementary elements — fire with air, earth with water — score highest at 90 of 100; same element scores 80; the strained mixes (fire–water especially) fall to 40–55. On the modality side, cardinal–mutable is the easiest pairing at 80, while fixed–fixed bottoms out at 55: two anchors, nobody steering.

Aspects and orbs

Five major aspects are recognized, each with its own allowed orb — the distance from exact within which the aspect still counts:

Every aspect’s weight is then scaled by tightness: full strength at exact, fading to zero at the edge of orb. An exact square drags with its whole −0.6; a trine sitting 7.9° out barely registers. This matches how synastry feels in practice — the contacts people describe as unmistakable are nearly always the tight ones.

You may notice the conjunction is scored positive even though traditional astrology treats it as neutral — its character depends on the planets involved. Merging two people’s Venus energies is usually pleasant; merging two Mars placements can be a handful. We settled on mildly positive as the honest average, and it is the single judgement call in the aspect table I would most expect another astrologer to argue with.

Why the Moon outweighs the Sun

Giving Moon synastry 25% against the Sun’s 20% is a deliberate stance. The Moon governs the unperformed self — what you reach for when you are tired, hurt, or off guard — and long relationships are lived far more in that register than in the Sun’s. Having read a lot of charts for couples, the pattern is consistent: pairs with harmonious Moons describe each other as “easy to be home with” even when their Suns clash, and the reverse combination is where “great on paper, exhausting in the kitchen” comes from. Mars gets the smallest share (15%) because friction there, while real, is the kind couples most often convert into something workable.

What the score is not

It is not a verdict, and we have built the output to resist being read as one: any component under 50 is flagged as a growth area with its own explanation, because a low Mars score means something specific and addressable, not “doomed.” The score also has honest blind spots. It ignores houses and the Ascendant entirely. It excludes the outer planets from the synastry grid on purpose — Saturn sits in one sign for two and a half years and Pluto for over a decade, so those contacts describe generations more than couples. And the weights themselves are editorial choices. Defensible ones, I think, but a different astrologer would set them differently, which is exactly why they are published here rather than hidden behind the number. If we ever change them, this page changes with them.

Ready to see it applied? Run two charts through the calculator, or read more about who builds and writes Zodiac Match.