BaZi compatibility (八字合婚) reads two birth charts against each other rather than scoring one person. Enter two birth dates, times and longitudes and the engine returns five dimensions: the relation between the two day masters, whether the day branches clash or combine, the year branches, the nayin of the two year pillars, and how the two charts' elemental balances fit together.
It is a structural reading, not a verdict. The output says which parts of two charts pull the same way and which pull against each other, and names the classical relation behind each — 合 and 冲, 三合 and 六害 — rather than returning a compatibility percentage with nothing behind it.
The five dimensions are: Day-Stem Pairing (日干相配), how the two day masters stand to one another in the generating and controlling cycles; the Marriage Palace (日支), the day branch, which classical method weights most heavily because it is the seat of the spouse; the Root Branch (年支), read for family background; the Sound Element (年命纳音) of the two year pillars; and Useful-God Support (用神互补), whether what one chart needs is what the other supplies.
One thing worth saying plainly: the classical texts have no percentage scale. The affinity number this tool shows is a modern presentation of a weighted sum, and it is the qualitative bands underneath it — and the named relations in each dimension — that follow the classical method. Read those, not the number.
The calculator loads on this page and needs JavaScript. The reference pages linked below do not — they are plain server-rendered HTML.