BaZi & Zi Wei computation rules — the deterministic rules our engine applies, with worked examples
1. The four pillars (year / month / day / hour)
BaZi (八字) is a stem–branch pair written for four calendar positions. Which calendar boundary applies is not universal: the same birth data produces different pillars under different conventions. So we state ours explicitly, and each row below carries a measured example.
| Pillar | Boundary we use | Measured example |
|---|---|---|
| Year | Lichun (立春), the traditional start of spring — not the first day of the lunar year | 2026-01-15 → year pillar 乙巳 (Yi-Si), before Lichun · 2026-02-05 → 丙午 (Bing-Wu), after Lichun |
| Month | Solar terms (节气, jieqi) — not lunar month beginnings | Within early February the month pillar changes from 己丑 (Ji-Chou) to 庚寅 (Geng-Yin) |
| Day | Local 00:00; the late-Zi hour (晚子时, 23:00–24:00) rolls the day pillar to the next day (mainstream 子平 convention) | Late-Zi on 2026-01-01 → day pillar 丙子 (Bing-Zi), identical to early-Zi on 2026-01-02 |
| Hour | Stem derived from the day stem by the Five Rats Escape rule (五鼠遁); branch taken from the local clock time; 13 selectable slots | Early-Zi 00:00–01:00 · 丑 01:00–03:00 · 寅 03:00–05:00 … 亥 21:00–23:00 · late-Zi 23:00–24:00 |
Why publish this: because the year, month and day boundaries are conventions, a reader who cannot see the convention cannot judge whether a chart is computed correctly. Writing the convention down is what makes a result checkable rather than merely believable.
2. How the Ten Gods (十神) are decided
The Ten Gods are relationship labels, not deities. Taking the day stem as the Day Master (日主), every other stem is classified by two questions: which of the five phases (五行) does it relate to the Day Master's phase by, and is its polarity (阴阳) the same as, or opposite to, the Day Master's?
| Relation to the Day Master | Same polarity | Opposite polarity |
|---|---|---|
| Same phase as me — peers (比劫) | 比肩 Peer | 劫财 Rival |
| The phase I produce — output (食伤) | 食神 Eating God | 伤官 Hurting Officer |
| The phase I control — wealth (财) | 偏财 Indirect Wealth | 正财 Direct Wealth |
| The phase that controls me — officers (官杀) | 七杀 Seven Killings | 正官 Direct Officer |
| The phase that produces me — resources (印) | 偏印 Indirect Resource | 正印 Direct Resource |
Worked examples, exported from the engine — the same ten stems read from two different Day Masters:
- 乙 (Yi, yin wood) Day Master: 甲 = Rival (劫财) · 乙 = Peer (比肩) · 丙 = Hurting Officer (伤官) · 丁 = Eating God (食神) · 戊 = Direct Wealth (正财) · 己 = Indirect Wealth (偏财) · 庚 = Direct Officer (正官) · 辛 = Seven Killings (七杀) · 壬 = Direct Resource (正印) · 癸 = Indirect Resource (偏印)
- 甲 (Jia, yang wood) Day Master: 甲 = Peer (比肩) · 乙 = Rival (劫财) · 丙 = Eating God (食神) · 丁 = Hurting Officer (伤官) · 戊 = Indirect Wealth (偏财) · 己 = Direct Wealth (正财) · 庚 = Seven Killings (七杀) · 辛 = Direct Officer (正官) · 壬 = Indirect Resource (偏印) · 癸 = Direct Resource (正印)
Two families are easy to mix up — Eating God / Hurting Officer and Direct Resource / Indirect Resource — because same and opposite polarity swap them. The two rows above can be cross-checked against each other. Note also that a change of polarity does not change the relation family: 食神 and 伤官 are both output, 正财 and 偏财 are both wealth.
3. Hidden stems (藏干) in the earthly branches
Each earthly branch carries one to three stems (藏干, hidden stems). Our Ten God derivation reads the main hidden stem of a branch alongside the visible stems, so this table is part of the calculation and not decoration. Order below is main stem first, matching the traditional table used in the engine.
| Branch | Hidden stems | Branch | Hidden stems |
|---|---|---|---|
| 子 Zi | 癸 Gui | 午 Wu | 丁 Ding · 己 Ji |
| 丑 Chou | 己 Ji · 癸 Gui · 辛 Xin | 未 Wei | 己 Ji · 丁 Ding · 乙 Yi |
| 寅 Yin | 甲 Jia · 丙 Bing · 戊 Wu | 申 Shen | 庚 Geng · 壬 Ren · 戊 Wu |
| 卯 Mao | 乙 Yi | 酉 You | 辛 Xin |
| 辰 Chen | 戊 Wu · 乙 Yi · 癸 Gui | 戌 Xu | 戊 Wu · 辛 Xin · 丁 Ding |
| 巳 Si | 丙 Bing · 庚 Geng · 戊 Wu | 亥 Hai | 壬 Ren · 甲 Jia |
This is the traditional table, the same choice recorded under "disputed points" on the method page. Modern references sometimes differ in the secondary stems; if you compare two tables, expect a few disagreements there.
4. Luck pillars (大运): direction and starting age
- Direction (顺逆): yang year stem + male, or yin year stem + female → forward; yin year stem + male, or yang year stem + female → backward. It is fixed by the year-stem polarity and the gender entered in the form.
- Starting point: the sequence is laid out from the month pillar, stepping forward or backward one stem–branch pair at a time.
- Starting age: the number of days from the birth date to the adjacent solar term ÷ 3 — three days count as one year, one day as four months.
- Season of birth matters twice: the same month pillar is the source of the whole sequence, which is why the month boundary rule above is not a technicality.
Naming matters here, and the two systems are often conflated. BaZi uses luck pillars (大运, also rendered "decade pillars"), arranged from the month pillar and timed by days to a solar term. Zi Wei Dou Shu uses major decades (大限), arranged from the life palace and timed by the five-element class. The algorithms and the year ranges differ; one cannot stand in for the other, so the site shows them separately and never mixes them.
5. Zi Wei Dou Shu: palaces, element class, decade limits and the four transformations (四化)
| Item | What the engine does |
|---|---|
| Year boundary | Lichun (立春), the same boundary as BaZi — one boundary for both systems, so a chart cannot display two conflicting years |
| Life palace (命宫) / body palace (身宫) | Derived from birth month and birth hour: counting from 寅 (Yin) as the first month to the birth month, then from 子 (Zi) to the birth hour; the body palace counts forward |
| Five-element class (五行局) | Read from the life palace's stem–branch (纳音): Water-2 / Wood-3 / Metal-4 / Earth-5 / Fire-6 |
| Starting age of the first decade | Equals the class number: Water-2 starts at 2, Wood-3 at 3, Metal-4 at 4, Earth-5 at 5, Fire-6 at 6 |
| Decade ranges | Ten nominal years per decade; backward for yin-year males and yang-year females, forward for yang-year males and yin-year females |
| Calendar year of a decade | Birth year + the decade's starting nominal age − 1 (a Metal-4 chart opens with nominal ages 4–13, then 14–23, and so on) |
The four transformations (四化) are read from the year stem, in the order 化禄 (Lu) · 化权 (Quan) · 化科 (Ke) · 化忌 (Ji). This is our table, exported from the engine:
| Year stem | Lu (禄) | Quan (权) | Ke (科) | Ji (忌) |
|---|---|---|---|---|
| 甲 Jia | 廉贞 Lianzhen | 破军 Pojun | 武曲 Wuqu | 太阳 Taiyang |
| 乙 Yi | 天机 Tianji | 天梁 Tianliang | 紫微 Ziwei | 太阴 Taiyin |
| 丙 Bing | 天同 Tiantong | 天机 Tianji | 文昌 Wenchang | 廉贞 Lianzhen |
| 丁 Ding | 太阴 Taiyin | 天同 Tiantong | 天机 Tianji | 巨门 Jumen |
| 戊 Wu | 贪狼 Tanlang | 太阴 Taiyin | 右弼 Youbi | 天机 Tianji |
| 己 Ji | 武曲 Wuqu | 贪狼 Tanlang | 天梁 Tianliang | 文曲 Wenqu |
| 庚 Geng | 太阳 Taiyang | 武曲 Wuqu | 太阴 Taiyin | 天同 Tiantong |
| 辛 Xin | 巨门 Jumen | 太阳 Taiyang | 文曲 Wenqu | 文昌 Wenchang |
| 壬 Ren | 天梁 Tianliang | 紫微 Ziwei | 左辅 Zuofu | 武曲 Wuqu |
| 癸 Gui | 破军 Pojun | 巨门 Jumen | 太阴 Taiyin | 贪狼 Tanlang |
On school differences: published four-transformation tables do not fully agree. 化禄 and 化权 are consistent across the schools we have seen; 化科 and 化忌 diverge for some stems (壬 Ke is written 左辅 in some tables and 天府 in others; 庚 also varies). We use the table above, and it is worth naming the version whenever a Ke or Ji placement is cited. 己 (武曲 · 贪狼 · 天梁 · 文曲) is not disputed.
6. Not implemented and boundaries — stated plainly
| Item | Status | Effect on a chart |
|---|---|---|
| True solar time (真太阳时) | Not implemented | No longitude and no equation-of-time correction: the entered clock time is used as-is, and the form has no longitude field. Births close to a double-hour boundary (around 01:00, 03:00 and so on) may sit in the neighbouring slot — check the birth record if that matters to you |
| Unknown birth hour | Not supported | A double-hour slot must be selected. The hour pillar is read for later life and the household, and it also drives the Zi Wei life palace and body palace, so there is no degraded "hour unknown" chart; comparing the neighbouring slots is the honest alternative |
| Overseas time zones | No conversion | Enter the local clock time at the place of birth; the tool does not shift between zones |
| Switching convention (流派) | Not offered | One documented convention at a time — Lichun year, solar-term months, late-Zi rollover, traditional hidden-stem table, the four-transformation table above |
Chart calculation is deterministic: the same birth data under these rules always yields the same chart. This page is documentation for anyone who wants to check our output rather than take it on trust.
Scope: the site computes charts and translates fixed rules into plain language. It is offered for entertainment and cultural study only. It makes no claims and no predictions about your future, your health, your money or your relationships, and nothing here is medical, financial, legal or life-decision advice.
Frequently asked questions
Is the year boundary Lichun (立春) or the lunar new year?
Lichun. A birth on 2026-01-15 still carries the year pillar 乙巳 (Yi-Si); by 2026-02-05 the year pillar is 丙午 (Bing-Wu). This is the mainstream Zi Ping (子平) convention, and we apply it to Zi Wei Dou Shu as well so one chart cannot show two different years.
I was born after 23:00 — is the day pillar that day or the next?
The next day. We use the mainstream late-Zi rule: a birth between 23:00 and 24:00 rolls the day pillar forward. Measured example: late-Zi on 2026-01-01 gives day pillar 丙子 (Bing-Zi), the same as early-Zi on 2026-01-02. Some lineages count the same day instead; the convention is written down so you can recompute and compare.
Does the tool use true solar time (真太阳时)?
No — it is not implemented. Charts are computed from the clock time you enter, with no longitude correction and no equation-of-time adjustment, and the form has no longitude input. Births near a double-hour boundary should be checked against the birth record.
At what age does the first Zi Wei decade (大限) start?
The starting age equals the five-element class number: Water-2 starts at 2, Wood-3 at 3, Metal-4 at 4, Earth-5 at 5, Fire-6 at 6. Decades are listed as nominal-age windows of ten years, and the calendar year of a decade equals birth year + starting nominal age − 1.
Does this site predict anything?
No. The chart is deterministic calendar and stem–branch arithmetic, and the reading is a plain-language rendering of fixed rules. The site is for entertainment and cultural study only: it makes no predictions and no claims about your future, sells no remedies, and gives no medical, financial, legal or life-decision advice.
Related entries
Want to check these rules against your own birth data?
See also：Computation method · Computation rules · Pricing