The Sixty-Year Cycle and Hwangap, Korea’s 60th Birthday

Every pillar in a Saju chart is one of sixty possible stem-and-branch pairs. Together they form the sexagenary cycle (육십갑자, 六十甲子, yuksip gapja), one of the oldest continuously used counting systems in the world. It names years, months, days and hours, and it is the reason a sixtieth birthday carries special weight in Korea.

Why sixty, not one hundred and twenty

There are ten stems and twelve branches, which might suggest 120 combinations. But the two lists advance together, one step each at a time: stem 1 with branch 1, stem 2 with branch 2, and so on. After the tenth stem the stems start over while the branches continue. A Yang stem therefore only ever meets a Yang branch, and Yin only meets Yin, so half the combinations never occur. The cycle returns to its starting pair after sixty steps, the least common multiple of ten and twelve.

The first pair: Gapja

The cycle begins with 甲子, Gapja: the first stem, Yang Wood, on the first branch, the Rat. The year 1984 was a Gapja year, and the next will be 2044. Other years are named the same way. 2024 was 甲辰 (Gapjin), the Wood Dragon, and 2025 was 乙巳 (Eulsa), the Wood Snake. This is where descriptions such as “Year of the Wood Dragon” come from: the stem gives the element and the branch gives the animal.

Days cycle too

The same sixty names are applied to days, in an unbroken sequence. January 1, 2024 happened to be a Gapja day, and so was every sixtieth day after it. Your day pillar is simply the name of the day you were born, and its stem is your day master. There are sixty possible day pillars, which is why Saju describes more types than a twelve-sign zodiac.

Hwangap: the cycle comes home

Because the year names repeat every sixty years, the year you turn sixty carries the same name as the year you were born. Koreans call this hwangap (환갑, 還甲), “the return of the cycle”. It was traditionally celebrated with a large family feast, at a time when living through a full cycle was an achievement. Today it is usually marked more quietly, often with a family trip, and the bigger party has moved to the seventieth birthday.

How many charts are there?

A chart has four pillars, but they are not independent: the month stem follows from the year stem, and the hour stem follows from the day stem. Counting the real possibilities gives 60 years × 12 months × 60 days × 12 hours, or 518,400 distinct charts. People born in the same two-hour block of the same day share one, anywhere in the world.

Curious which of the sixty pairs is yours? Enter your birthday on the home page. For why the year pillar changes in early February rather than on January 1, see the Saju year and Ipchun.