How the Four Pillars Are Cast

BaZi means "eight characters": four pillars of two characters each — one heavenly stem, one earthly branch. Casting them holds no mystery. It is a fixed calendar conversion: the same moment of birth gives the same chart no matter who does the arithmetic.

The four steps below are what this site actually runs, written out so you can check the result yourself.

The year turns at Lichun, not New Year

Many people assume the animal year changes at Chinese New Year, and more assume it changes on 1 January. In BaZi it does neither — the year pillar turns over at Lichun, around 4 February.

So anyone born in January belongs to the previous year, and anyone born in early February needs the exact time of that year's Lichun. This is the single most commonly miscounted step, and when it is wrong the year pillar and the zodiac animal are both wrong.

The arithmetic itself is simple: take (year - 4), then the remainder modulo 10 gives the stem and modulo 12 gives the branch. 1984 minus 4 is 1980; 1980 mod 10 is 0 (Jia) and mod 12 is 0 (Zi) — a Jia-Zi year, which checks out.

The month follows solar terms, its stem by the Five Tigers

Month branches do not follow calendar months. They follow the solar terms: Lichun opens the Yin month, Jingzhe the Mao month, Qingming the Chen month, and so on. Each handover falls somewhere between the 4th and the 8th of a Gregorian month, never on the 1st.

The month stem comes from the year stem by a rule called the Five Tigers:

Jia and Ji years — the first month (Yin) starts at Bing-Yin
Yi and Geng years — start at Wu-Yin
Bing and Xin years — start at Geng-Yin
Ding and Ren years — start at Ren-Yin
Wu and Gui years — start at Jia-Yin

Once the Yin month is fixed, the rest follow the sexagenary cycle in order.

The day pillar is counted from an anchor

The day pillar is the easiest to verify because it is nothing but a division. Pick a day whose pillar is known, count the days to your target date, then take the remainders modulo 10 and 12.

This site anchors on 1 January 1900 being a Jia-Xu day. To check it: 1 January 2024 is 45,290 days later; 45,290 mod 10 is 0 (Jia), and stepping 45,290 branches on from Xu lands on Zi — a Jia-Zi day. Any almanac will confirm that 1 January 2024 was indeed Jia-Zi.

The day pillar is unaffected by solar terms, which makes it the least error-prone of the four.

The hour changes every two hours, its stem by the Five Rats

A day holds twelve double-hours: Zi runs 23:00–00:59, Chou 01:00–02:59, and so on.

Note that anything from 23:00 belongs to the next day's Zi hour, and the day pillar advances with it. Someone born at 23:30 has the following day's day pillar — a detail many online calculators get wrong.

The hour stem comes from the day stem by the Five Rats rule: Jia and Ji days start at Jia-Zi, Yi and Geng days at Bing-Zi, Bing and Xin at Wu-Zi, Ding and Ren at Geng-Zi, Wu and Gui at Ren-Zi.

If you do not know your birth hour

A missing hour is common. This site casts the chart at noon and states plainly in the result that the hour pillar is left out of the element count. The six characters of the first three pillars still read; the detail is simply coarser.

Saying which pillar is missing and what it affects matters more than pretending it does not.

Once the chart is cast, the judgement is made by the Ten Gods and by the strength of the five elements. To skip straight to the result, enter your birth details.