BaZi luck pillars: how the starting age is calculated
Luck pillars (Da Yun, 大運) are ten-year stem-branch pairs laid over the natal chart in sequence from the month pillar. Which direction the sequence runs, and at what age the first decade opens, are both fixed by rule — worked below on a real chart, and checked against the engine on every build.
The starting-age formula (Da Yun 大運)
- Direction. Forward for a yang year stem read as male or a yin year stem read as female; backward for the opposite pairing. Each decade steps one place through the sixty-pair cycle from the month pillar.
- Bounding term. The onset is the distance from birth to the month-opening solar term (節) that bounds the birth month — the next one running forward, the previous one running backward. Only the twelve opening terms count, not the twelve mid-month terms.
- The count. Whole calendar days between the two dates, plus the difference in two-hour periods (時辰) between the two clock readings, both in the solar-term frame (China Standard Time).
- Conversion. One day counts as four months; one two-hour period as ten days. Twelve months make a year.
- Ages. Nominal (虛歲): the calendar year of birth is age 1, anchored to the year on the birth certificate.
The full statement, including what this engine does when it disagrees with another practitioner, is §7 on the methodology page. That the starting point and direction are calculated from the month pillar and differ by gender and year-stem polarity is the standard account — see Four Pillars of Destiny: luck cycles.
Worked example: the sample birth, read as male
The public sample chart — born 8 October 1985, 15:20 · Singapore — year pillar 乙丑, month pillar 乙酉. Every value below is the engine's; the count in the middle rows is re-derived independently on this page and must equal it for the site to build.
| BIRTH, CST | 8 October 1985, 15:20 — two-hour period 8 |
|---|---|
| DIRECTION | Year stem 乙 is yin (Yin Wood); read as male → backward |
| BOUNDING TERM | Bai Lu, 8 September 1985, 00:53 CST — the previous month-opening term; period 0 |
| THE COUNT | 30 calendar days and 8 two-hour periods |
| CONVERTED | 30 × 4 months + 8 × 10 days = 10 years, 2 months, 20 days after birth |
| FIRST DECADE OPENS | Age 11 (虛歲), 1995 — 甲申 Yang Wood · Monkey |
THE TEN DECADES
| Age (虛歲) | Years | Luck pillar | Reading |
|---|---|---|---|
| 11–20 | 1995–2004 | 甲申 | Yang Wood · Monkey |
| 21–30 | 2005–2014 | 癸未 | Yin Water · Goat |
| 31–40 | 2015–2024 | 壬午 | Yang Water · Horse |
| 41–50 | 2025–2034 | 辛巳 | Yin Metal · Snake |
| 51–60 | 2035–2044 | 庚辰 | Yang Metal · Dragon |
| 61–70 | 2045–2054 | 己卯 | Yin Earth · Rabbit |
| 71–80 | 2055–2064 | 戊寅 | Yang Earth · Tiger |
| 81–90 | 2065–2074 | 丁丑 | Yin Fire · Ox |
| 91–100 | 2075–2084 | 丙子 | Yang Fire · Rat |
| 101–110 | 2085–2094 | 乙亥 | Yin Wood · Pig |
The same birth, read as female
Nothing about the natal chart changes. The year stem's polarity meets the other gender, so the sequence runs the other way and the onset is counted against the other bounding term — here a term that falls the same afternoon, which is why the first decade opens at once.
| BIRTH, CST | 8 October 1985, 15:20 — two-hour period 8 |
|---|---|
| DIRECTION | Year stem 乙 is yin (Yin Wood); read as female → forward |
| BOUNDING TERM | Han Lu, 8 October 1985, 16:24 CST — the next month-opening term; period 8 |
| THE COUNT | 0 calendar days and 0 two-hour periods |
| CONVERTED | 0 × 4 months + 0 × 10 days = 0 years, 0 months, 0 days after birth |
| FIRST DECADE OPENS | Age 1 (虛歲), 1985 — 丙戌 Yang Fire · Dog |
THE FIRST FOUR DECADES
| Age (虛歲) | Years | Luck pillar | Reading |
|---|---|---|---|
| 1–10 | 1985–1994 | 丙戌 | Yang Fire · Dog |
| 11–20 | 1995–2004 | 丁亥 | Yin Fire · Pig |
| 21–30 | 2005–2014 | 戊子 | Yang Earth · Rat |
| 31–40 | 2015–2024 | 己丑 | Yin Earth · Ox |
If the birth hour is unknown
The sequence and its direction do not involve the hour, so they are still derived. The onset does — it is a count to the bounding term in two-hour periods — so no onset, and no ages or years, are produced. Inventing them from a placeholder time would be the same error as guessing an hour pillar.
| Age (虛歲) | Years | Luck pillar | Reading |
|---|---|---|---|
| — | — | 甲申 | Yang Wood · Monkey |
| — | — | 癸未 | Yin Water · Goat |
| — | — | 壬午 | Yang Water · Horse |
Where practitioners differ
The traditional method is the count worked on this page: whole days and two-hour periods between birth and the bounding month-opening term, with every three days counting as one year of onset. A second method proportions the exact number of minutes to the bounding term instead; across the charts this engine has been measured on, the two differ by up to about twelve days of onset — usually invisible, but when those days straddle a New Year every decade shifts by one year of age. Roughly one chart in a hundred. A practitioner who places you a year earlier or later near a boundary is not necessarily disagreeing with your chart; they may be using the other count. Details under where schools differ.
Cast your chart with a gender and the calculator prints your own sequence. The natal report reads all ten decades against your day master — what each luck pillar is to you, not just what it is.