How a Saju Chart Is Calculated (Step by Step)
By Saju Unni · Updated August 10, 2026 · 5 min read
A saju chart is calculated, not intuited. One birth moment converts into eight characters through fixed calendar rules — four pillars of two characters each, from the 60-step stem-and-branch cycle. The Encyclopedia of Korean Culture describes saju in exactly these terms: birth year, month, day, and hour written as ganji (간지, 干支) pairs.
What information a saju reading needs
Three inputs build a chart: birth date, birth time, and birth place. Each unlocks a different piece.
- Date → year, month, and day pillars — six of the eight characters, including the day master the whole reading pivots on.
- Time → the hour pillar, the remaining two characters.
- Place → the timezone the clock was set to, plus the longitude correction below — what turns "7am" into a solar moment.
A reading without a birth time still works: three of four pillars stand, so 75% of the chart is intact — narrower, not empty.
Step by step: how the four pillars are derived
Four derivations run in order, and each has its own clock.
- Year pillar — from the solar year. The saju year turns at ipchun (입춘, 立春, Start of Spring, around February 3–5), not January 1 and not Lunar New Year. A January 20 birth carries the previous year's pillar.
- Month pillar — from solar terms. Month boundaries fall on the twelve sectional solar terms (절기, 節氣), roughly the 4th–8th of each Gregorian month. The branch follows the term-to-term window; the stem keys off the year stem.
- Day pillar — from the continuous 60-day cycle. The stem-branch day count has run unbroken for centuries with no annual reset, so the day pillar is a straight count from a reference day. January 1, 2000 is 戊午 — a standard verification anchor.
- Hour pillar — from the 12 two-hour branches. A day is cut into twelve branches of two hours each, starting with 子 at 23:00. The stem derives from the day stem by a fixed rule.
Why the month boundary is a solar term, not the 1st
Saju months track the sun's position, not the civil calendar. The 24 solar terms divide the ecliptic into 15° arcs, and the twelve sectional terms among them mark month boundaries — about 15 days apart, at an hour and minute that shifts each year.
Practical consequence: a February 2 birth sits before ipchun, so it belongs to the previous month and previous year pillar. A lifelong "February baby" can carry a January-side chart. Near a boundary the exact minute decides, which is why real engines carry a solar-term timing table.
True solar time (진태양시) and why longitude matters
True solar time is the sun's actual time at the birth longitude, not the administrative clock time. Standard time zones are political conveniences, and the gap is measurable.
- Longitude offset: 1° of longitude equals 4 minutes of solar time. Korea keeps standard time on the 135°E meridian while Seoul sits near 127°E — roughly 30 minutes of gap.
- Equation of time: Earth's elliptical orbit swings true solar time about ±16 minutes across the year.
Combined, the correction can exceed 45 minutes — and at the western edge of a wide zone like UTC+8, past two hours. Each hour branch is only two hours wide, so a birth recorded near an even hour can land in a neighboring branch once corrected, which shifts the balance of the whole chart.
The 23:00 day boundary rule
The saju day begins at 23:00, not midnight. The first branch 子 (ja) spans 23:00–00:59, so a 23:30 birth already sits in the next day's first hour branch.
Schools differ on whether the day pillar also advances at 23:00 or waits for midnight — the yaja-si versus joja-si (야자시 · 조자시) split. Stakes are high because the day stem is the day master, so any honest calculator states which convention it applies.
Does saju use the lunar calendar?
No — saju pillars come from the solar-term calendar, not the lunar one. This is the most common misconception, likely because Korean families often record birthdays as lunar dates.
Lunar dates enter only as an input conversion: a birthday remembered as "the 15th of the sixth lunar month" converts to its solar equivalent first, and pillar derivation starts from there. Moon phases play no role in the four pillars.
Why hand-calculating (or asking a chatbot) often goes wrong
Manual derivation fails at boundaries, and boundaries are common. Four things have to be right at once — the solar term to the minute, the true-solar-time correction, the 23:00 rule, the stem-branch tables — and one miss moves a pillar.
Chatbots are especially shaky: a language model generates plausible next words rather than running a calendar, and can't show which solar term it used. A birth at 11:20pm on a solar-term day can flip three of four pillars on these details. Can ChatGPT read your saju breaks down where the slips happen.
Get your chart calculated properly
Skip the arithmetic. Our free chart runs the whole sequence — solar terms, true solar time, day boundary, stem-branch tables — in a couple of seconds. From there, your day master is the character to read first.
FAQ
- Can I get a saju reading without my birth time?
- Yes. The birth date alone fixes the year, month, and day pillars — six of the eight characters, including the day master that the whole reading pivots on. Only the hour pillar is missing, so the reading is narrower rather than empty.
- Does saju use the lunar calendar?
- No. The four pillars come from the solar-term calendar, which tracks the sun's position, not moon phases. A birthday remembered as a lunar date simply gets converted to its solar equivalent first, as a separate input step before any pillar math begins.
- Why does my saju year start in February, not January?
- The saju year turns at ipchun (입춘, Start of Spring), which falls around February 3–5, rather than on January 1 or Lunar New Year. A birth on January 20 therefore carries the previous year's pillar, which surprises a lot of people the first time they see their chart.
- Why does my birth city change the chart?
- Birth place supplies both the timezone the clock was set to and the longitude used for the true-solar-time correction. One degree of longitude equals four minutes of solar time — Korea keeps standard time on the 135°E meridian while Seoul sits near 127°E, roughly a 30-minute gap — and each hour branch is only two hours wide, so the correction can move the hour pillar.
- What happens if I was born close to midnight?
- The saju day begins at 23:00, so the 子 (ja) branch spans 23:00–00:59 and a 23:30 birth already sits in the next day's first hour branch. Schools differ on whether the day pillar also advances at 23:00 or waits for midnight — the yaja-si versus joja-si split — so a calculator should state which convention it uses.