Learn · Chinese astrology

How a BaZi chart is built

Eight characters, four pillars, and a birth moment written down as a position in a calendar that has been running without a break since the Shang dynasty. This is the derivation, start to finish, worked on one real birth.

A BaZi chart contains no planets. There are no houses in it, no aspects, nothing measured against the horizon. A practitioner who has been reading these charts for thirty years has never once needed to know where Mars was when you were born. Western and Vedic astrology model a sky. BaZi models a calendar.

What it records is where your birth moment fell in four nested counting cycles: which year, which solar month, which day, and which of the twelve two-hour blocks. Each of those four positions is written with a pair of characters, one drawn from a list of ten and one from a list of twelve. Four pairs, eight characters. That is the whole input. Everything a practitioner says afterwards is read off those eight characters and the relationships between them, which means the derivation has to be exactly right before any of it is worth anything. The Chinese name says as much: 八字 bāzì, “eight characters”. The other common name, 四柱 sìzhù, “four pillars”, describes the same thing by its shape: four columns, each two characters tall.

This page covers derivation and nothing else. It explains how the eight characters are computed and what the labels attached to them denote. It does not tell you what any of it means about a person, because that is a separate body of technique, it is disputed, and mixing the two is how a beginner ends up unable to tell an arithmetic fact from an opinion.

Stems, branches, elements

Two closed lists do all the work. The ten Heavenly Stems (天干 tiāngān) are five elements taken twice, once in a Yang form and once in a Yin form. The twelve Earthly Branches (地支 dìzhī) are the familiar zodiac animals, though the animals are a late decoration on what is really a twelve-part division of time and direction.

Yin and Yang here are not moral categories and not genders. They are a parity bit. A stem is either the outward form of its element or the inward one, and that single fact decides half of the ten-god lookup later on. Element and polarity between them are the only properties of a stem that the derivation uses.

The five elements (五行 wǔxíng, more literally “five phases” or “five movements”) relate to one another in two fixed cycles, and those two cycles are the engine of everything that follows. In the generating cycle, each element produces the next: wood feeds fire, fire leaves ash which is earth, earth yields metal, metal condenses water on its surface, water grows wood. In the controlling cycle, each element restrains the one two steps ahead of it: wood breaks up earth, earth dams water, water quenches fire, fire melts metal, metal cuts wood.

Neither cycle is a value judgement. Being controlled is not being harmed, and generating something is not always a gift. For the purposes of this page it is enough that both are symmetric, closed, and completely mechanical.

Why sixty

Pair a stem with a branch and step both forward together, one place each. The stems run out after ten and start again; the branches run out after twelve. The pairing does not return to its starting point until both lists finish at the same time, which happens after sixty steps, the lowest common multiple of ten and twelve. That sixty-term cycle is the sexagenary cycle, 六十甲子 liùshí jiǎzǐ, named after its first term 甲子 jiǎzǐ and ending on the sixtieth, 癸亥 guǐhài.

Sixty rather than a hundred and twenty, which is what ten times twelve would suggest, because stepping in lockstep keeps the parity aligned. Stem one is Yang and branch one is Yang; stem two is Yin and branch two is Yin; and since both lists have an even length, a Yang stem can never meet a Yin branch. Half the pairings are unreachable. This is not a rule anyone imposed on the system, it is a consequence of how the counting works, and it is why you will never see 甲丑 in a chart.

The same sixty terms are then used four times over, for years, for months, for days and for the twelve double-hours. A BaZi chart is four readings off the same ruler at four different scales.

Where it came from

The counting mechanism is far older than the divination built on it. Stem-branch pairs appear as day-dates on Shang-dynasty oracle bones from around 1250 BCE, and almost every surviving divination record from that period carries one. The day count has run from there to the present without a break, which makes it the longest continuous day count anywhere in the world and the reason the day pillar can be computed as plain arithmetic rather than looked up in a calendar. Using the same pairs for years came much later, somewhere around the middle of the third century BCE, and was settled into court calendar practice under the Han in the second century CE.

The cosmology arrived separately. Five-phase thinking was pulled together in the late Warring States and early Han period, traditionally crediting Zou Yan (邹衍) with turning the older idea of five materials into a doctrine of historical and cosmic cycles. It was worked out further in the Lüshi Chunqiu, in the “Yueling” chapter of the Liji, and in the Huainanzi, and then Dong Zhongshu (董仲舒, second century BCE) made a version of it central to Han state ideology. His correlative cosmology, in which heaven and human affairs resonate with one another, is the intellectual ancestor of the specific move BaZi makes: reading a birth moment through elemental relationships.

A loose tradition called 禄命 lù-mìng, “emolument and fate”, correlated birth data with stem-branch and five-phase reasoning across the Han and the Six Dynasties. Nobody has produced a single named method from it. It is better thought of as the soil than as a system.

Three pillars, then four

The first named figure is Li Xuzhong (李虚中), traditionally dated to roughly 761–813 CE, who is credited with a destiny method using three pillars, year, month and day, built on the 纳音 nàyīn element correspondences and centred on the year pillar. His contemporary Han Yu is supposed to have said he was right ninety-nine times in a hundred. Two cautions belong here. The surviving text that bears Li’s name carries a preface dated 806 CE, and scholars dispute both that date and whether the text has anything much to do with the historical man; it may be a later compilation attached to a famous name. Treat the man and the book as loosely linked at best.

The decisive change is credited to Xu Ziping (徐子平), placed in the Five Dynasties and Northern Song period. Two innovations, and the second matters more than the first: he added the hour pillar, giving four pillars and eight characters, and he moved the reference point from the year pillar to the day stem. From then on a chart is read as a set of relationships to the person’s own stem rather than to their ancestral year. The discipline is still called 子平术 Zǐpíng shù after him, and “Zi Ping” and “BaZi” are used almost interchangeably in classical writing.

How far it travelled

In Korea the system is 사주 saju (four pillars) or 팔자 palja (eight characters). It arrived during the Goryeo dynasty alongside the broader transmission of Tang and Song learning, and was localised heavily under the Joseon. Korean sources describe the Gwansanggam (관상감), the royal astronomy bureau, combining observation with saju analysis to advise on appointments and the timing of state business. That institutional claim comes from practitioner writing rather than from a Joseon-history monograph, so treat it as well attested but not independently checked. Saju consultation in Korea today is entirely mainstream.

Japan received it much later, as 四柱推命 shichū suimei. Japanese sources place the arrival in the mid-Edo period, with the earliest firm textual evidence being a translation of the Yuanhai Ziping published as Suimeisho by the Sendai Confucian scholar Sakurada Komon during the Bunsei era, 1818 to 1830. That dating comes from Japanese Wikipedia and popular divination writing rather than an academic study; it is consistent across sources but not verified against one.

In Vietnam it is Tứ Trụ, Bát Tự or Tử Bình. Beyond the names, we could not find scholarly sourcing for when or by what route it arrived, only practitioner sites asserting a general spread through Taiwan, Vietnam and Southeast Asia. That is a gap in our research, not a settled short answer.

On the mainland the twentieth century nearly ended it. During the Cultural Revolution, fortune-telling was named among the Four Olds: almanacs and divination materials were destroyed, practitioners were denounced, and the practice went underground entirely. It came back in the 1980s, after the 1978 reforms, in what the historian of the period calls a fortune-telling fever, and the returning practitioners often described their work in the language of “scientific research” and “reclaiming traditional culture” because that language bought legitimacy. Consultation has been continuously visible in Hong Kong, Taiwan, Singapore and Malaysia throughout. Since the early 2020s a wave of interest among younger people on the mainland has driven a sizeable market in metaphysical services, much of it now app-delivered; the numbers you will see quoted for it come from market research firms and the press rather than from peer-reviewed work, and should be read that way.

What it is actually used for

Four uses account for most consultations historically. Character and constitution, read off the balance of elements across the eight characters. Timing, read off the ten-year luck pillars. Marriage matching, 合婚 hé hūn, which was a formal stage of the traditional six rites: the groom’s family requested the bride’s eight characters in writing, a specialist compared the two sets, and an unfavourable comparison could end the match before betrothal. And naming a newborn, choosing given-name characters whose elemental associations supplement whatever the chart is short of.

The most striking modern use, and the one that shows how seriously the premise is still taken, is the scheduling of elective caesarean births. Chinese-language accounts describe families consulting a practitioner to choose a birth hour inside the window a doctor has already declared safe. The medical judgement sets the boundaries and the astrological one operates inside them. It is hard to think of a clearer demonstration that a culture believes the birth moment encodes something durable.

Alongside BaZi sits Zi Wei Dou Shu (紫微斗数), the other widely practised Chinese natal system. Practitioners generally treat the two as complementary rather than ranked, and many use both. You will often read that Zi Wei was restricted to the imperial court until the fall of the Qing; that claim circulates widely on practitioner sites and we could find no academic source for it, so it belongs in the category of things everybody repeats.

The year pillar

The year pillar changes at Lichun (立春 Lìchūn, “start of spring”), the first of the twenty-four solar terms, defined as the instant the sun’s apparent geocentric ecliptic longitude reaches 315°. That falls on or about 4 February. It does not change at Chinese New Year, and it does not change at the winter solstice. Within BaZi this is about as close to unanimous as the tradition gets; Chinese writers call it an iron rule.

Lichun is a solar event and Chinese New Year is a lunar one, so they drift past each other. Every year there is a window, sometimes a fortnight wide, in which the two boundaries disagree about which year it is.

Plenty of free online calculators roll the year at Lunar New Year. Our research treats that as a bug rather than a competing school, and the engine behind this page has no setting for it: there is no configuration field that selects the Lunar New Year boundary, because offering one would imply the question is open.

Given a Gregorian year y that the birth falls in or after Lichun of, the year’s own pair is pure arithmetic, anchored on 1984 CE being 甲子: stem index = 1 + (y + 6) mod 10, branch index = 1 + (y + 8) mod 12. For 1992: stem 1 + 1998 mod 10 = 9, which is 壬; branch 1 + 2000 mod 12 = 9, which is 申. The year pillar is 壬申, Yang Water Monkey.

The month pillar

The month pillar has nothing to do with lunar months. It changes at the twelve jié (节, the “sectional” solar terms), which are twelve of the twenty-four terms, spaced 30° of solar longitude apart. The other twelve, the zhōngqì (中气, mid-terms), sit between them and are used by the lunar calendar to decide where leap months go. BaZi ignores them completely.

This trips people up because the four terms everybody has heard of, the two equinoxes and the two solstices, are all zhōngqì. The spring equinox does not start a BaZi month. Lichun, a fortnight before the start of February’s zhōngqì, does.

Notice that the branches run 寅 卯 辰 and so on, starting at the Tiger rather than the Rat. Month one is the Tiger month because it begins at Lichun, and the year begins there too. The Rat month is month eleven, running from Daxue in early December across the Gregorian new year to Xiaohan in early January.

The branch is fixed for all time by which jié you are in. The stem is not; it depends on the year. The rule that connects them is the Five Tigers (五虎遁 Wǔhǔdùn), which gives the stem of the first month, the Tiger month, from the stem of the year. After that every later month in the year is one step forward through the sixty, one step per jié, and no further rule is needed.

The day pillar

The day pillar is the easiest of the four to compute and the one with the most interesting dispute attached. The day count is a single unbroken run of sixty, stepping once per calendar day, untouched by leap months, dynastic changes, the switch from the Julian to the Gregorian calendar, or anything else. Once you know the pair for any one day you know it for every day, forwards and backwards, by counting.

In practice an engine converts the corrected calendar date to a Julian Day Number and takes it modulo sixty against a known anchor. Two days that are independently attested as 甲子, the first of the sixty, make a good check on the arithmetic: 18 February 1912, and 1 October 1949, the founding day of the People’s Republic. They are 13,740 days apart, which divides by sixty exactly, and an implementation that does not land on both has a bug. There is a widely circulated hand formula for the day pillar, sometimes called Gao’s formula, which fails that second check. We tested it, it disagreed with itself, and we did not implement it.

Which day does a 23:30 birth belong to?

The 子 hour runs from 23:00 to 01:00, so it lies across midnight and therefore across the boundary between two day pillars. Schools disagree about what that means for someone born in the first half of it.

• Rollover (子初换日): the whole 23:00 to 01:00 window belongs to the next day. A birth at 23:30 already has tomorrow’s day pillar.
• Split (子正换日): 23:00 to midnight, the 夜子时, stays with the current day; only the hour after midnight rolls over.

The split school has a real textual foundation. The Ming-dynasty Sanming Tonghui says of the 子 hour that its first half, before midnight, belongs to yesterday, and its second half, after midnight, to today. It is a substantial minority position, not a fringe one. Our own research found the two initial reports contradicting each other on which side was the majority, which is how we ended up commissioning a third that concluded rollover leans majority and that named mainland practitioners have converged towards it over the past twenty years, while keeping the older vocabulary.

AstroDexa defaults to rollover and implements split in full. The mode used is printed with every chart. Both schools, incidentally, evaluate the question against the corrected time rather than the raw clock, which brings us to the last pillar.

The hour pillar

The day is divided into twelve 时辰 shíchén, double-hours, one per branch, and the first of them is centred on midnight rather than starting there.

The hour stem comes from the day stem by the Five Rats rule (五鼠遁 Wǔshǔdùn), which works exactly like Five Tigers one level down: it gives the stem of the 子 hour, and every later hour that day steps one place forward through the sixty.

Which clock, though?

Here is the part that beginners skip and that changes more charts than any other single correction. The time on a birth certificate is civil time, and civil time is an administrative convenience with two separate distortions built into it.

The first is longitude. China runs one legal time zone across roughly sixty degrees of longitude, and every clock in it is set to the meridian at 120°E. A clock in Chengdu says the same thing as a clock in Shanghai, but the sun over Chengdu is more than an hour behind. The correction is four minutes per degree: local mean time = clock time + 4 × (longitude − 120). East of the meridian you add, west of it you subtract.

The second is the equation of time (均时差 jūnshíchā). A clock ticks at a constant rate and the sun does not, because Earth’s orbit is elliptical and its axis is tilted. The gap between the two runs from about fourteen minutes behind to about sixteen minutes ahead over the course of a year. It depends on the date and not at all on where you are standing.

Apply both and you get apparent solar time, the time a sundial would show: the measurement the whole system was originally built on. This is the majority position among sources that take one, on two arguments. The historical argument is that the hour branches were defined by the sun’s position before mechanical clocks existed at all, so using clock time is the innovation. The practical argument is that at extreme longitudes the correction is simply decisive; in Xinjiang it can be over two hours, which is more than a whole double-hour.

There is a genuine minority that rejects the correction, either as a modern administrative overlay unknown to the classics, or as negligible for births near the reference meridian. The second half of that is fair: in Shanghai the longitude correction is about seven minutes and will rarely change anything. In Chengdu it is not.

Reading the skeleton

The eight characters are now fixed. What follows are the labels a chart carries, all of them derived by lookup from what we already have. None of this is interpretation yet.

The Day Master

The Day Master (日主 rìzhǔ, also 日元 or 日干) is the day pillar’s own stem. That one character is the chart’s fixed point: every other stem in the chart is described by its relationship to it. Xu Ziping’s shift of the reference point from the year to the day is what separates the method practised today from Li Xuzhong’s, and it is the reason a modern chart cannot simply be read off a Tang-dynasty manual.

The ten gods

The ten gods (十神 shíshén) sound mystical and are a lookup table. Take any stem in the chart, visible or hidden. Ask two questions. What is the five-element relationship between that stem’s element and the Day Master’s? Do the two share a polarity? Five relationships times two polarities gives ten answers, and each answer has a name.

Worth knowing: 印 is rendered “Resource” in our output and in a lot of modern practice, but many English books say “Seal” instead, and the two mean the same position. 七杀 is also written 偏官 piānguān, and 偏印 is sometimes 枭神 xiāoshén, the Owl. Different books, same cells of the same table.

To see how mechanical it is, work one by hand. The Day Master here is 癸, Yin Water. The hour stem is 己, Yin Earth. Earth controls water, so the relationship is “it controls the Day Master”. Both are Yin, so the polarities match. Row four, first column: 七杀, Seven Killings. The month stem 戊 is the same element in the other polarity, which lands one cell to the right: 正官, Direct Officer.

Hidden stems

Each branch contains one, two or three stems, which is where a large part of a chart’s substance lives. The main one is the 本气 běnqì, the primary qì; the others are secondary and residual. Each hidden stem gets its own ten-god label by the same lookup, so a four-pillar chart with only four visible stems can easily carry a dozen ten-god labels once the branches are opened. Which stems hide in which branch is not disputed. How heavily to weight them is: we use flat percentages, and a classical alternative weights them by how far into the month the birth fell.

Luck pillars

The luck pillars (大运 dàyùn) are ten-year blocks laid over the natal chart, and they are derived from the month pillar by stepping it through the sixty. Two things have to be decided: which way, and starting when.

Direction. Forward if the year stem is Yang and the person is male, or the year stem is Yin and the person is female. Reverse otherwise. Sex is a required input here and cannot be recovered from the chart, which is the one piece of information BaZi needs that the birth moment alone does not supply.

Start age, by the three-days rule. Measure from the birth to the next jié if the direction is forward, or back to the most recent jié if it is reverse. Any of the twelve; Lichun has no special standing here. Then convert at three days to the year: three days of elapsed time is one year of age, one day is four months, one double-hour is ten days. The result is the exact age at which the first block begins, and every subsequent block starts ten years after the one before.

The chart assembled

All four pillars, the hidden stems, the ten gods and the 纳音 labels, in the layout AstroDexa uses. Columns read year, month, day, hour from left to right; many Chinese charts print them in the other order, and nothing about the content changes either way.

• Wood
• Fire
• Earth
• Metal
• Water

壬申 戊申 癸亥 己未. Day Master 癸 guǐ, Yin Water. Every correction that produced those eight characters is on the record: input 1992-08-15T15:30 in Asia/Shanghai, zone offset 480 minutes with no daylight saving in force, longitude correction −63.73 minutes, equation of time −4.42 minutes, local apparent solar time 14:21:50, hour standard apparent, zi-hour mode rollover, solar terms compared as UTC instants, hidden-stem weights flat, no southern-hemisphere adjustment.

That list is the point of the page. A BaZi chart is not a single unambiguous fact about a birth; it is a fact plus a handful of documented choices, and a chart that does not tell you the choices has not really told you anything you can check.

Where schools disagree

The line we draw is between a dispute and a defect. The zi-hour boundary is a dispute: two positions, both with textual support, both held by working practitioners, so both are implemented and the choice is named on the output. Rolling the year at Lunar New Year is a defect: no school within BaZi holds it, and building it as an option would misrepresent the state of the question. Southern-hemisphere births sit awkwardly in between, with three incompatible answers and no way to choose among them that we found convincing, so we do the astronomically consistent thing and say plainly that it is unresolved.

Five things people get wrong

“My zodiac animal is my BaZi chart”

It is one character out of eight, and even that one is often wrong, because the animal year everybody knows is folk usage tied to Lunar New Year while the BaZi year pillar turns at Lichun. The animal-year tradition is also far older than the four-pillar method and developed separately. They share vocabulary, not machinery.

“The BaZi year changes at Chinese New Year”

Covered above, and worth repeating because it is the single most common error in free calculators. Lichun, roughly 4 February, computed from the sun’s position to the minute.

“BaZi, the Chinese zodiac and Western astrology are the same idea in different clothes”

Three different things. The zodiac animal is one label covering a whole year. Western astrology measures planets against houses and angles. BaZi takes a calendar position and evaluates it through five-element relationships, with no celestial geometry anywhere in the method. Sharing the words “element” and “sign” with another tradition does not make the mechanics the same.

“A BaZi reading tells you your fixed fate”

This page has computed a chart and named its parts. What a practitioner does with it afterwards is a separate discipline, it is argued over, and there are four classical texts above that disagree with each other about how to go about it. The derivation is arithmetic. The reading is not, and any tool that blurs the two is selling you a confidence it has not earned.

“There is one official answer from a BaZi calculator”

No. Run the same birth through three calculators and you can easily get three charts, because of the zi hour, because of true solar time, because one of them rolls the year at the wrong boundary. The difference between a tool you can trust and one you cannot is not that the first has no defaults. It is that the first tells you what they were.

Run your own

AstroDexa computes full BaZi charts from an exact birth moment: all four pillars, hidden stems, ten gods, na yin and the ten-year luck pillars, with true solar time applied by default, both zi-hour schools available, and every correction it applied listed underneath the chart. It sits alongside the Western and Vedic tools in the same app, on the same charts.

Open AstroDexa

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Chinese terms are given in simplified characters with Hanyu Pinyin. The worked example was computed with AstroDexa’s own calculation engine and is reproduced without alteration. Where sources conflict, particularly on the history, the conflict is stated rather than resolved. If something here is wrong, write to support@astrodexa.com and we will fix it.