How to Calculate Your BaZi

八字怎么算 · 四柱的推导

A BaZi chart is eight characters: a Heavenly Stem and an Earthly Branch for each of four pillars — year, month, day and hour. All eight come from one birth moment. This page shows where each pillar comes from, and is honest about the two steps that are convention rather than arithmetic. If you would rather just see yours, cast it free.

What do you need before you start?

A date of birth, a time, and a place. The date and time should be the ordinary civil clock reading recorded at birth — the number on the wall clock or the certificate, in the local time in force that day. The place matters because the next step needs the longitude. You do not need a lunar-calendar birthday: BaZi is a solar system throughout.

Step 1 — Correct the clock to true solar time

Clock time is a civil convention: a whole time zone shares one reading, and that reading is not what the sun over your birthplace was doing. Two corrections recover the sun’s own time — the longitude gap between your birthplace and the meridian your zone is pegged to, and the equation of time, the seasonal wobble that runs roughly ±16 minutes across the year. Together they can move the birth moment by close to an hour, which is more than enough to change the hour pillar. The full arithmetic, worked to two decimals, is on the true solar time page.

Why does the BaZi year start at Lichun instead of Chinese New Year?

This is the single most common error in casual BaZi calculators. The BaZi year begins at Lichun (立春), the solar term that opens spring, falling around 4 February. It does not begin on 1 January, and — despite how often the two are confused — it does not begin at Chinese New Year, which is a lunar date drifting across several weeks. Anyone born in January or the first days of February belongs to the previous BaZi year, and a tool that switches at New Year will hand them the wrong year pillar.

Step 3 — The month pillar follows the solar terms

The month pillar does not follow calendar months, and it does not follow lunar months either. It follows the twelve sectional terms (节) — Lichun, Jingzhe, Qingming, and so on — each of which is a position of the sun along the ecliptic. The distinction that catches people out is that the twenty-four solar terms alternate between 节 (sectional) and 气 (mid-terms): the month turns on the 节 only. Using the nearest term of either kind puts the boundary half a month out. The month branch also carries extra weight when the chart’s strength is scored, because season dominates how strong an element is.

Step 4 — The day pillar, and your Day Master

Days cycle through the sixty stem-branch combinations one per day, in an unbroken sequence that has been counted continuously for a very long time — which is why the day pillar is looked up rather than derived from a formula. The stem of this pillar is your Day Master (日主): the single character that stands for you, and the reference point the rest of the chart is read against. Because it needs only the date, it is the one part of the chart available even when the birth time is unknown.

Step 5 — The hour pillar, and the midnight problem

The day divides into twelve double-hours of two hours each, one per branch, and the stem of the hour is derived from the day stem by a fixed rule, so the hour pillar follows once the correct double-hour is known. The complication is 子: it runs from 23:00 to 01:00 and therefore straddles midnight. Whether the hour between 23:00 and 24:00 belongs to the day just ending or the one beginning is a school convention, not a fact. We keep the day pillar with the calendar day you were born on, and we say so on the method page rather than leaving you to discover it by comparing charts.

Which steps are convention rather than fact?

Two, and knowing them explains most of the disagreement between calculators. The late 子 hour is the first, above. The second is how a month is divided among the hidden stems of its branch: the tables disagree with one another — the 寅 month alone is split 7/7/16 days by some and 5/9/16 by others — and the choice changes which Ten God is judged to dominate. We give you the day count from the sectional term and let you apply whichever table you hold to, rather than silently picking one. Everything else on this page is arithmetic, and two correct engines will agree on it.

Can I calculate BaZi without my birth time?

Three pillars out of four, yes. Year, month and day need only the date, so your Day Master and most of the element distribution survive. What you lose is the hour pillar — a quarter of the chart, and the pillar traditionally read for later life. The wrong response is to guess a time and present eight characters as though all eight were known; we cast what the date supports and mark the rest unknown. If you are trying to narrow a half-remembered time, the calculator conformance test shows how much a shifted hour actually moves.

Why do two BaZi calculators give me different charts?

Usually one of three places, and they are quick to check. Whether true solar time was applied at all — many tools skip it. How the late 子 hour was assigned, if you were born between 23:00 and midnight. Or a birth moment falling within a few hours of a sectional term or of Lichun, where a small difference in the term time moves an entire pillar. Comparing the two charts pillar by pillar normally identifies which of the three it is; if you want to test a tool deliberately, three worked birth times will tell you what it gets right.

For what the eight characters mean once you have them, start with what BaZi is and the Five Elements. For the exact conventions and scoring weights this engine uses, see the charting method. Or cast your chart — free, no account, every step above applied.