How We Calculate Your Four Pillars: A Worked Example

A Four Pillars chart (四柱, also called 八字 / bāzì, "eight characters") is four stem-branch pairs, one each for the year, month, day, and hour of birth. The popular Chinese zodiac only ever shows you the year branch, the animal. The other three pillars are where the actual reading lives, and they are exactly the parts a cheap calculator gets wrong, because each is anchored to the real position of the sun rather than the civil calendar.

To make this concrete, we will compute one full chart by hand-rules and show our engine's output for the same birth. The example: 7 November 1990, 14:30, Prague (longitude 14.42° E, time zone UTC+1).

The engine returns:

Pillar	Stem-branch	Reading
Year	庚午 (gēng wǔ)	yang Metal on a Horse (Fire) branch
Month	丙戌 (bǐng xū)	yang Fire on a Dog (Earth) branch
Day	丙子 (bǐng zǐ)	yang Fire on a Rat (Water) branch
Hour	乙未 (yǐ wèi)	yin Wood on a Goat (Earth) branch

The Day Master (the day stem, the symbol the whole chart is read around) is 丙, yang Fire. Here is where each pillar comes from.

The year pillar turns at 立春, not January 1

The single most common error in free calculators is using the wrong year boundary. The stem-branch year does not begin on January 1, and it does not begin at Lunar New Year. It begins at 立春 (Lìchūn, "the beginning of spring"), the instant the sun reaches 315° of ecliptic longitude, around February 4.

For 1990, our engine places 立春 at 1990-02-04 03:12 local time. Our birth is in November, well after that, so it belongs to the 1990 stem-branch year. The year stem-branch for 1990 is 庚午, the Metal Horse.

Why this matters: a baby born on, say, 1 February 1990 would be before that year's 立春 and would belong to the previous stem-branch year (1989, 己巳, the Earth Snake), even though the Western calendar already says 1990. Tools that switch the animal on January 1 (and tools that switch it at Lunar New Year) both misclassify births in the late-January-to-early-February window. We switch it at 315° of solar longitude, computed to the minute.

The month pillar follows the solar terms (節), then the 五虎遁

The month is not the calendar month and not the lunar month. It is fixed by the twelve "sectional" solar terms (節, jié), each a point 30° apart on the sun's annual path. The month that begins at 立春 (315°) is always the 寅 (Tiger) month; each following 30° of solar longitude advances the branch.

At our birth the sun is at 224.9°. Counting 30°-sectors from 315° places this in the 戌 (Dog) month, the branch for the sector that began at 立冬 (the start of winter), the eighth month after 寅.

The month stem then comes from a fixed rule called the 五虎遁 ("Five Tigers"), which derives the 寅-month stem from the year stem and counts forward. With a 庚 year stem, the rule yields a 戊 stem for the 寅 month; advancing to the 戌 month lands on 丙. So the month pillar is 丙戌.

The day pillar is an unbroken 60-count

The day pillar is the simplest in principle and the most remarkable in fact: it is a continuous count of days through the sixty stem-branch combinations, running unbroken for millennia. Our engine anchors the count to a known reference, 15 April 1924 = 甲子 (the first pair, index 0), and counts the days between that anchor and the birth date, modulo 60.

For 7 November 1990 that count lands on 丙子, yang Fire on the Rat branch. The day stem, 丙, is the Day Master.

One subtlety most calculators ignore: when the birth time (after the true-solar correction below) crosses midnight, the day itself rolls forward. A birth at 23:55 corrected to 00:11 belongs to the next day's pillar, and because the hour stem is derived from the day stem, getting the day wrong silently corrupts the hour too. Our engine rolls the day correctly across that boundary.

The hour pillar uses true solar time and the 五鼠遁

The hour is a double-hour (時辰): twelve two-hour branches, with 子 spanning 23:00–00:59. But the clock on the wall is not the sun. Two corrections turn civil time into the true solar time the tradition assumes:

1. Longitude within the time zone. Prague sits at 14.42° E, west of the UTC+1 standard meridian (15° E), so the real sun runs slightly behind the clock here.
2. The equation of time. The sun is "fast" or "slow" against clock time by up to ~16 minutes depending on the date, because the earth's orbit is elliptical and tilted.

For this birth the two corrections sum to about +14 minutes: clock 14:30 becomes solar 14:44. That still falls in the 未 (Goat) double-hour (13:00–14:59), so the hour branch is 未.

The hour stem comes from the 五鼠遁 ("Five Rats") rule, which derives the 子-hour stem from the day stem and counts forward to the birth hour. With a 丙 day stem, the rule places 乙 on the 未 hour. So the hour pillar is 乙未.

Putting it together

Four pillars, eight characters: 庚午 · 丙戌 · 丙子 · 乙未, read around a 丙 (yang Fire) Day Master, with an elemental balance of three Fire, two Earth, and one each of Wood, Metal, and Water. None of that is visible from the year animal alone. By the popular shorthand this person is "a Horse," and three-quarters of the chart never appears.

Every step above is computed, not looked up: solar longitude from an astronomical model, the 立春 and solar-term boundaries solved to the minute, the day count from a fixed anchor, and the true-solar correction from longitude and the equation of time. You can check the year boundary yourself on our methodology page, and read why the year doesn't start on January 1 in the history of the system.