Saju Calculator: Korean Four Pillars (사주팔자)

Written by Orbitarot · Sources: our own engine, positions checked against NASA JPL Horizons (method); place data GeoNames (CC BY 4.0); interpretations our own · Updated October 17, 2026

What saju is

사주 (四柱) means "four pillars" and 팔자 (八字) "eight characters". Together, saju palja is the everyday Korean phrase for the pattern a person was born with, and the study of it is called 명리학 (命理學). Each pillar is one heavenly stem (천간) above one earthly branch (지지), taken from a cycle of sixty pairs that has counted years, months, days and hours in East Asia for well over two thousand years.

• Year pillar (연주): changes at 입춘 Ipchun, the start of spring, around 4 February.
• Month pillar (월주): changes at the twelve major solar terms (절기), about every 30 days.
• Day pillar (일주): one step of the sixty-day cycle per day. Its stem is your day master (일간), the character read as yourself.
• Hour pillar (시주): one of twelve two-hour periods, from 자시 (the rat hour) onward.

In Korea saju is part of everyday culture: people look up their chart at the new year, compare charts for fun, and read it in apps built on a 만세력. We present it the way it is meant here, as a traditional way of reflecting on a birth moment, not as a forecast.

Saju and Chinese BaZi

Saju and Chinese BaZi (八字, also "eight characters") are the same system. The pillars come from the same solar terms and the same sixty-cycle, and for the same moment and time setting this calculator gives exactly the same eight characters as our BaZi page, because it runs on the same engine. What changes is how the chart is read and set out in Korea:

• Names. The characters are read in Korean: 甲子 is gapja (갑자) in Korean and Jia Zi in Mandarin.
• The clock. Korean practice corrects the birth time by about 30 minutes, because Korean Standard Time runs ahead of the sun in Korea (see below).
• The rat hour. Korean schools argue openly about 야자시 and 조자시, the late and early rat hours around midnight, so the calculator offers the three rules in use.
• Lunar birthdays. Many Koreans know their birthday in the lunar calendar (음력), so you can enter it directly.
• Presentation. Korean 만세력 apps print the ten gods (십신) above and below every character and list the 대운 (luck pillars) with a start number, the 대운수. The result follows that layout.

The ten stems and twelve branches

The ten heavenly stems pair yang and yin for each of the five elements, and every stem has a traditional image that Korean readers use for the day master:

The twelve earthly branches carry the zodiac animals and the two-hour periods of the day (clock hours before any time correction). Each branch also holds one to three hidden stems, the 지장간:

Day master, five elements and ten gods

Reading starts with the day master: is it supported or drained by the season of the month pillar and by the other characters? The five elements (오행: 목 wood, 화 fire, 토 earth, 금 metal, 수 water) feed one another in a circle (wood feeds fire, fire makes earth, earth bears metal, metal carries water, water feeds wood) and control one another across it (wood parts earth, earth dams water, water puts out fire, fire melts metal, metal cuts wood). The calculator counts the elements in your eight characters and, if you tick the box, in the hidden stems too.

The ten gods (십신, also 십성) name every other character by its relation to the day master: the same element, the element you feed, the one you control, the one that controls you and the one that feeds you, each in a same-polarity and an opposite-polarity form. The names are old and sometimes dramatic; here is what each one covers in the reading:

Luck pillars (대운) and year pillars (세운)

After the natal chart, saju readers look at time in ten-year steps called 대운 (大運). The steps start from the month pillar and move one place along the sixty-cycle every ten years: forward (순행) when a yang year stem meets a male chart or a yin year stem meets a female chart (양남음녀), backward (역행) otherwise. This is the one place where the tradition asks for sex, so the calculator lets you choose it in the result.

The first step does not begin at birth. Count the calendar days from the birth date to the next major solar term (forward) or back to the previous one (backward) and divide by three, because three days stand for one year; a remainder of one day is dropped and a remainder of two rounds up. The result is the 대운수 printed in Korean 만세력, from 0 to 10. The calculator also gives the exact start date from the same count to the minute (one day for four months), since the rounded number can be almost a year out. Sources differ on whether the ages are international or Korean-style ages, so read the date column if you want to be precise. Alongside, each calendar year has its own pillar, the 세운 (歲運), which changes at Ipchun like the birth year pillar.

Korean time: why about 30 minutes

Korean Standard Time is UTC+9, the mean solar time of 135° east, a meridian that runs through western Japan. Korea lies further west: Seoul is at about 127° east, so the sun reaches its highest point there about 32 minutes after noon on the clock. Because the hour branches were defined by the sun, Korean saju practice usually moves the birth time back by about half an hour before choosing the hour pillar. With the correction, the 자시 rat hour runs from about 23:30 to 01:30 on the clock instead of 23:00 to 01:00.

Practice differs in the detail, so the calculator shows all four choices and which pillar each one gives:

• Korean standard, −30 minutes (the default): the mean time of 127.5° east, the middle of Korea and the meridian of UTC+8:30. This is the simple rule of thumb in Korean saju practice. Because it works from the real moment of birth, it also follows the old Korean rule of thumb for the years with other clocks: no correction under UTC+8:30, one hour in the summers of 1955 to 1960, an hour and a half in the summer-time years under UTC+9.
• Birthplace longitude: four minutes per degree from your exact longitude (Seoul −32 minutes, Busan −24 minutes). Use this for births outside Korea.
• True solar time: longitude plus the equation of time (up to about 16 minutes either way through the year), as on our BaZi page.
• Clock time: no correction at all.

Korea's clocks also changed over the years: UTC+8:30 in 1908 to 1911 and again in 1954 to 1961, and summer time (서머타임) in 1948 to 1951, 1955 to 1960 and for the Seoul Olympics in 1987 and 1988. The calculator reads these from the time zone database for the birthplace and works from the real moment of birth, so a summer-time clock reading is corrected automatically. The result shows the UTC offset it used; check it against your family's records if you were born in one of those years.

The 만세력, Korea's ten-thousand-year calendar

A 만세력 (萬歲曆), "ten-thousand-year calendar", is the reference book of saju: for every day it lists the solar date, the lunar date, the day pillar, and the exact times of the solar terms that set the year and month pillars. For generations, readers looked a birth up in the printed volume; today 만세력 apps and websites do the same job, and the word is searched almost 100,000 times a month in Korea.

This calculator is a 만세력 computed on the fly. The solar terms come from the Sun's position, to the minute. The day pillar comes from the unbroken sixty-day count. For the lunar calendar we checked our computation against the official tables of KASI, the Korea Astronomy and Space Science Institute: all 1,867 lunar months from 1900 to 2050, including 56 leap months, start on the same day and have the same length, and the day pillars on those days match KASI's 일진 as well. Full charts were also compared with two Korean 만세력 sites for 11 test births, from a birth minutes before Ipchun to a 1957 summer-time birth and a leap-month lunar birthday; every pillar and every 대운수 agreed, apart from three documented differences of method.

Lunar birthdays (음력 생일)

Many Koreans, especially older generations, celebrate their birthday by the lunar calendar, which moves against the solar date every year. Choose Lunar (음력) above the date field and enter the lunar year, month and day; the calculator converts it with the Korean lunar calendar and shows the solar date before you calculate. If your birthday fell in a leap month (윤달), a repeated month added about seven times in nineteen years, tick the leap month box: a lunar date can exist twice in such a year.

Use the Korean conversion even if a Chinese calendar gives a different day: the Korean calendar counts days in Korean time (UTC+9) and the Chinese one in Beijing time (UTC+8), so a new moon shortly after midnight in Korea can start a month one day apart. In 2012, for example, a new moon fell just after midnight in Korea and just before midnight in China, so the Korean calendar had its leap month after the third month and the Chinese one after the fourth. Before 1912 KASI's tables follow UTC+8, and the calculator does the same.

Questions

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