How a BaZi Chart Is Calculated: The Four Pillars and Eight Characters

Four pillars, eight characters

The chart is organised as four columns, read conventionally from right to left, one each for the year, the month, the day, and the hour of birth. These are the four pillars (四柱). Each pillar has two rows: an upper character, which is one of the ten Heavenly Stems, and a lower character, which is one of the twelve Earthly Branches. Four pillars multiplied by two rows gives the eight characters — the bazi — from which the system takes its name. Practitioners often speak of the upper row as the “heavenly stems” line and the lower as the “earthly branches” line, and much of the later analysis concerns how these eight relate to one another.

The calendar used is not the civil one

The first and most important point to grasp is that the calendar used is not the ordinary Gregorian civil calendar, nor is it simply the familiar lunar calendar of Chinese festivals. BaZi uses the solar terms — the twenty-four divisions of the tropical year (二十四节气) that mark the sun‘s position along the ecliptic. This matters most for the year and month pillars. The astrological year does not begin at the lunar New Year but at the solar term known as the “beginning of spring” (立春), which falls around the fourth of February. A child born in late January, before that term, belongs to the previous year’s pillar, not the new one — a subtlety that trips up the unwary and that distinguishes a properly cast chart from a naive one.

The year pillar

The year pillar is found by taking the Stem and Branch assigned to the astrological year of birth within the sixty-year cycle. The month pillar is determined by which of the twelve solar-term months the birth falls in, each month being bounded by specific solar terms rather than by the civil calendar‘s dates. The month’s Branch is fixed for each solar month, while the month‘s Stem is derived from the year’s Stem by a standard rule that classical texts set out in a short table or verse. In practice, casters look these up rather than deriving them afresh each time.

The month pillar

The month pillar is where the calendar question stops being a technicality and starts to matter, and it is also the pillar that carries the most weight after the Day Master itself.

Its branch is fixed entirely by the solar terms, not by the lunar month. The year is divided into twelve segments, each beginning at one of the twelve节 or sectional terms, and each segment is assigned a branch: the segment beginning at the start of spring in early February is the tiger month, the one beginning at the waking of insects in early March is the rabbit month, and so on around the cycle to the ox month beginning at minor cold in early January. Because these terms are astronomical points in the sun‘s apparent path, they fall on almost the same civil dates every year, varying by a day either way. A birth on the third of February and one on the sixth belong to different months and, if the year turns between them, to different years as well.

The stem of the month pillar is not free either; it is derived from the year stem by a fixed rule, so that once you know the year stem and the month branch, the month stem follows. The traditional mnemonic gives the tiger-month stem for each of the five possible year stems and the rest of the months follow in sequence. This is why only twelve month pillars are possible for any given year rather than sixty, and it is part of why the total number of charts is so much smaller than a naive calculation would suggest.

What makes this pillar important is that its branch is the yueling (月令), the command of the month, understood as the season into which the person was born. Since the five phases wax and wane seasonally, the month branch is the primary indicator of which phases are strong and which are weak in the chart as a whole, and therefore of whether the Day Master is supported or isolated. Almost every classical method for judging a chart begins by looking at the Day Master against the month branch. If you learn to read one thing in a chart beyond the Day Master, this is the one.

The day pillar and the Day Master

The day pillar is the anchor of the entire chart, because its Stem becomes the Day Master, the figure standing for the person whose chart it is. The day‘s Stem and Branch are found by continuous count within the sixty-day sequence that has, in principle, been running without interruption for millennia. Because there is no simple formula relating a calendar date to its day-pillar in the way there is for years, the day pillar is almost always obtained from a perpetual calendar (万年历) or from software. This unbroken day count is one of the more remarkable features of the Chinese calendrical tradition, and its continuity is what allows the day pillar to be assigned at all.

The hour pillar

The hour pillar completes the set. The day is divided into twelve double-hours (时辰), each two ordinary hours long and each governed by one of the twelve Branches; the first, the hour of the Rat (子时), straddles midnight. The hour‘s Branch follows directly from the time of birth, which is why an accurate birth time matters so much and why an uncertain time weakens a reading. The hour’s Stem, like the month‘s, is derived from the day’s Stem by a fixed rule. Because the hour depends on true local time, careful casters adjust for the difference between clock time and solar time at the place of birth, and for any daylight-saving offset in force — corrections that can move a birth from one double-hour into the next.

Hidden Stems inside the Branches

Once the four pillars are set, the caster adds a layer that the eight visible characters do not show on their surface: the “hidden stems” (藏干) concealed within each Earthly Branch. Every Branch is understood to contain one or more Stems in latent form, representing the Five-Phase energies stored within that segment of the year or day. These hidden stems are essential to interpretation, because they reveal elemental influences — and, through them, the relationships discussed on later pages — that would otherwise be invisible. A chart is therefore richer than its eight surface characters suggest.

A schematic example

To make this concrete without tying it to any real person, consider a schematic example. Suppose a birth falls in the solar month following the beginning of spring, in a given year of the cycle, on a day whose Stem is the yang form of Wood, at an afternoon double-hour. The year, month, day, and hour each receive their Stem-and-Branch pair; the day‘s yang-Wood Stem becomes the Day Master; and the hidden stems within the four Branches are noted. From this point the chart is complete as a structure, and everything that follows is interpretation rather than calculation. The worked logic of that interpretation is the subject of the next pages.

Where the input data goes wrong

Everything above assumes an exact birth moment, and in practice that assumption is shakier than the precision of the method suggests. This is the most consequential technical problem in the art, and popular writing tends to pass over it.

The first difficulty is that clock time is not solar time. China has used a single time zone since 1949 while spanning some sixty degrees of longitude, so in the far west of the country the clock can differ from local solar noon by well over an hour. Since the hour branch changes every two hours, a westward birth can fall in a different hour branch depending on whether the correction is applied. Practitioners divide on whether to correct to true solar time, and both positions have respectable defenders. The same issue arises anywhere with wide time zones, and to it must be added daylight saving, which China operated between 1986 and 1991 and which is routinely forgotten when converting births from those years.

The second and more serious difficulty is the midnight boundary. The hour branches are two-hour blocks, and the rat hour straddles midnight, running from eleven at night to one in the morning. The question is whether a birth at half past eleven belongs to the current day or the next one. One position holds that the day changes at eleven, so such a birth takes the following day‘s pillar; another holds that the day changes at midnight and distinguishes an early rat hour from a late one. The two produce different day pillars, which means a different Day Master, which means an entirely different reading. This is not a marginal disagreement about emphasis but a fork that changes the answer, and it has never been resolved.

Third, a great many people simply do not know their birth time. Records may give the hour to the nearest half hour, or a rounded time entered by a clerk, or nothing at all. Traditional practice had techniques for inferring a missing hour from a person‘s characteristics, which is plainly circular: the chart is being fitted to the life it is then used to explain. A practitioner who reconstructs a birth time this way and then reads the resulting chart as independent evidence has closed a loop, and it is fair to ask about it.

The honest summary is that the casting stage is arithmetically exact but only as good as its inputs, and the inputs are frequently uncertain in ways that change the result. A system that distinguishes two-hour blocks and treats the day stem as the centre of everything is unusually sensitive to exactly the data most likely to be wrong.

Nothing controversial so far

It is worth stressing that up to this point nothing controversial has occurred. Casting the chart is an exercise in calendar conversion, no more speculative than translating a date from one calendar to another, and modern software performs it flawlessly. The interpretive claims — that these eight characters describe a temperament or foretell a life — belong to a different order entirely and are matters of traditional belief, not of the arithmetic that produces the chart. Keeping the two apart, the reliable calculation and the contested reading, is the clearest way to understand what a BaZi chart actually is.

Key points

• A BaZi chart is a birth moment rewritten in Stems and Branches — nothing more.

• It has four columns for year, month, day and hour, each with a Stem above a Branch: eight characters.

• The calendar used is the traditional solar-term calendar, not the civil one, so boundary dates differ.

• The year pillar comes from the astrological year within the sixty-term cycle.

• The day pillar anchors the chart, since its Stem becomes the Day Master representing the person.

• The hour pillar uses twelve double-hours of two ordinary hours each.

• Each Branch also contains hidden Stems that do not appear on the surface of the chart.

• Casting the chart is pure calendrical calculation; interpretation is a separate step.

Sources drawn on for this article include: Ho Peng Yoke, Chinese Mathematical Astrology: Reaching Out to the Stars; Richard J. Smith, Fortune-tellers and Philosophers: Divination in Traditional Chinese Society; Joseph Needham, Science and Civilisation in China (Vol. 3, on the calendar and astronomy); and standard reference accounts of the twenty-four solar terms and the sexagenary day-count in the Chinese calendrical tradition.