60Gapja / Saju calculator
How does a Korean Saju calculator turn a birth time into a chart?
Six steps from a birth certificate to eight characters, and the two steps most online calculators skip.
A Saju calculator converts a birth date, time and place into four pillars in six steps: correct the clock time to true solar time, find the year pillar from the Ipchun boundary, find the month pillar from the solar term, count the day pillar in the 60-day cycle, assign the hour pillar from the two-hour block, then derive the ten-year luck pillars. Online Saju calculators disagree with each other almost always at step one and step three, because summer time and longitude are ignored or the month is switched on the calendar date instead of the solar term.
Step 1: correct the clock time to true solar time
The hour pillar is defined by the sun, not by the clock. Noon in Saju is the moment the sun is highest, and that moment is only close to 12:00 when the birth city sits on the meridian the time zone is based on. Two corrections apply:
- Daylight saving time. Any summer-time hour is removed first. In the United States that affects two thirds of the year. Korea used summer time in 1948–1951, 1955–1960 and 1987–1988.
- Longitude. The clock is then shifted by four minutes for every degree between the birth city and the time zone's meridian. Seoul is about 32 minutes behind Korean Standard Time. Atlanta is about 38 minutes behind Eastern Time. New York is within a few minutes of Eastern Time.
The whole page Why birth time and place matter shows worked examples.
Step 2: the year pillar starts at Ipchun, not January 1
The Saju year begins at Ipchun (입춘), the solar term marking the start of spring, which falls on February 3, 4 or 5 depending on the year and is fixed to the minute by the sun's longitude. A person born on February 2 belongs to the previous Saju year. A calculator that switches the year on January 1 or on Lunar New Year builds the wrong year pillar for anyone born in January or early February.
Step 3: the month pillar changes on a solar term
Saju months are solar, not lunar. Each month begins at one of twelve solar terms (절기): Ipchun, Gyeongchip, Cheongmyeong, Ipha, Mangjong, Soseo, Ipchu, Baengno, Hallo, Ipdong, Daeseol and Sohan. The boundary is an exact instant, published by the Korea Astronomy and Space Science Institute, so a birth an hour before the term and a birth an hour after the term get different month pillars. This is the single most common error in free calculators, which often use calendar months or lunar months.
Step 4: the day pillar and the midnight rule
Days are counted continuously in the 60-pair cycle, so the day pillar is a lookup once the corrected date is known. The subtle part is where a day begins. The hour of the Rat (자시) runs from 23:00 to 01:00 in true solar time. Korean practice is split between switching the day pillar at 23:00 and switching it at midnight while keeping the Rat hour, known as the late-Rat rule (야자시). Jeolgi shows which rule was applied and lets the user switch.
Step 5: the hour pillar
The corrected time falls into one of twelve two-hour blocks, each named for an earthly branch. The stem of the hour pillar is then derived from the day stem by a fixed table. A birth at 13:20 on the clock in Atlanta in July is 11:42 in true solar time, which moves the hour pillar from the Goat block (13:00–15:00) to the Horse block (11:00–13:00).
Step 6: luck pillars
The ten-year luck pillars (대운) run forward through the sixty-pair cycle from the month pillar for men born in yang years and women born in yin years, and backward for the others. The starting age comes from the number of days between the birth and the nearest solar term, divided by three. Calculators differ on whether the remainder is rounded, and on whether the age is counted Korean-style or Western-style; both choices change the year in which each luck pillar begins.
Why Jeolgi shows the working
Jeolgi prints the daylight-saving and longitude corrections, the solar-term instants on either side of the birth, the midnight rule in force and the luck-pillar rounding, next to the chart. Two people can then see why their charts differ from another app's output, and a professional reader can confirm the pillars in seconds. The engine is tested against Cheoneulgwiin (천을귀인), the Korean reference almanac app used by many professional readers, with boundary cases fixed as regression tests.
Frequently asked questions
- Why do two Saju calculators give me different pillars?
- Almost always because of birth-time handling. One calculator removed summer time and corrected for longitude and the other did not, or one switched the month on the solar term and the other on the calendar date. Both differences move the hour pillar and sometimes the month or day pillar.
- Do I need my birth city for a Saju calculator?
- Yes, if the birth time is within about 40 minutes of a two-hour boundary. The city fixes the longitude correction. Away from a boundary, the city changes nothing and any correct calculator will agree.
- Is a BaZi calculator the same as a Saju calculator?
- For the four pillars, yes. Both compute the same stems and branches for the same true solar time. Korean calculators usually apply the 30-minute longitude correction for Korean births by default and offer the late-Rat (야자시) option, which Chinese BaZi calculators handle differently.
- Can a Saju calculator work for someone born outside Korea?
- Yes. The pillars are computed from the local birth time, corrected for that place's summer time and longitude. Converting a foreign birth time to Korean time is wrong and is no longer common practice.
- Is the Jeolgi Saju calculator free?
- Building the four pillars and seeing the corrections is free. Jeolgi charges for extended tables such as full luck-pillar timelines and exports.