How to calculate the BaZi month pillar
The month branch changes at a solar term, not on the first day of a calendar or lunar month. The month stem then follows a fixed sequence determined by the year stem.
1. Find the solar year and month branch
First locate the birth instant relative to Li Chun (Start of Spring). This calculator changes the year pillar at the exact Li Chun boundary. The Yin month begins at Li Chun; Mao begins at Jing Zhe; Chen at Qing Ming; Si at Li Xia; Wu at Mang Zhong; Wei at Xiao Shu; Shen at Li Qiu; You at Bai Lu; Xu at Han Lu; Hai at Li Dong; Zi at Da Xue; and Chou at Xiao Han. Each interval ends when the next monthly Jie begins.
Consequently, a birth near a solar term can belong to a different month pillar even when both dates fall in the same Gregorian month. The lunar first day does not set the BaZi month boundary.
2. Use the year stem to start the Yin month
The traditional Five Tigers rule (五虎遁) sets the stem of the first, Yin, month. Pair the year stem with its starting stem:
- Jia 甲 or Ji 己 year → Bing-Yin 丙寅.
- Yi 乙 or Geng 庚 year → Wu-Yin 戊寅.
- Bing 丙 or Xin 辛 year → Geng-Yin 庚寅.
- Ding 丁 or Ren 壬 year → Ren-Yin 壬寅.
- Wu 戊 or Gui 癸 year → Jia-Yin 甲寅.
From there, advance both stem and branch one step for every new monthly Jie. For example, a Ding-year Yin month is Ren-Yin 壬寅; the following Mao month is Gui-Mao 癸卯. The year stem is determined first; the month stem is not chosen from the birth month's Gregorian number alone.
3. Check the boundary before comparing websites
Use the actual birth place and its historical time zone to identify one absolute birth instant. Compare that instant with the solar-term boundary in the same time standard. A displayed local solar-term time cannot be subtracted directly from a birth time displayed in another time zone.
If two calculators disagree, ask for their Li Chun and monthly Jie instants, birth-place time-zone treatment, and year-stem rule. Small differences in solar-term data matter only when the birth lies near a boundary; otherwise, check the input and time-zone interpretation first.
What this formula does not decide
This procedure calculates the month pillar. It does not, by itself, decide the Day Master strength, a chart pattern, a useful element, or a prediction. Those are separate interpretive questions. The site's chart and AI prompt should preserve the calculated pillar even when interpretations differ.