What the stems and branches are
Ten stems: jia, yi, bing, ding, wu, ji, geng, xin, ren, gui. Twelve branches: zi, chou, yin, mao, chen, si, wu, wei, shen, you, xu, hai. Both began as ordinals - stem like a trunk, branch like a twig. Pair them step by step, jia with zi, yi with chou, down to gui-hai: exactly sixty pairs, and odd-even mismatched pairs like jia-chou never occur. That is the sexagenary cycle.
Why sixty years
The least common multiple of ten and twelve is sixty. As both sequences advance together, a pair repeats only after sixty steps - 1984 and 2044 are both jiazi years, which is why a sixty-year lifespan is called "one jiazi". Day-counting has run continuously for millennia, decoupled from months and years, so the name of any day can be looked up directly.
The four pillars: mapping a birthday
Express the year, month, day and hour of birth each as a stem-branch pair - four pillars, eight characters. The year pillar turns at the lunar year (or Lichun); the month pillar follows solar terms (yin month runs from Lichun to Jingzhe); the day pillar is the continuous day count; the hour pillar derives from the day stem - the hour branch is fixed (two hours each from 23:00) while the hour stem depends on the day stem, the wuzidun rule: a jia or ji day starts with a zi hour, a yi or geng day with a bing hour. To compute directly, enter the birthday in our sexagenary tool for the four pillars and an element count.
Useful shortcuts
The animal comes from the year branch: zi-rat, chou-ox, yin-tiger... take the Gregorian year minus 4 modulo 12 (born before New Year, use the previous year). Same branch means same animal: any zi year is a rat year - jiazi, bingzi, wuzi differ only in the stem. Day names have no relation to weekdays. And lunar month branches are fixed: month 11 is always zi, month 1 always yin, since the year anchors at the winter solstice.
Related references and tools
查任意日期的四柱、纳音与五行计数，用 六十甲子对照；查农历日期与生肖，用 农历万年历 与 生肖年份对照表；五行与阴阳的对应关系见 五行速查表；农历的运行机制看 农历入门指南。
FAQ
Why is there no jia-chou year?
The pairing advances both sequences one step at a time. Jia is 1st (odd), chou is 2nd (even); odd and even positions never meet. The sixty pairs are the unique result of advancing together - a complete set, not a pick of sixty.
Does the year pillar change at New Year or Lichun?
Both conventions exist: lunar years turn at New Year, the solar-term calendar (four-pillar tradition) turns at Lichun. Births between the two dates get different year pillars - the same issue as the two zodiac conventions. Our tool lists both.
What can the four pillars tell you?
Calendrically, the four pillars are just a stem-branch coordinate of the birth moment - objectively computable and identical for everyone born at that time. Traditional interpretations of the coordinates belong to folk culture; this site provides objective conversion and counting only.