Saju and BaZi: The Same System, Two Names

Search for Chinese astrology and you get BaZi. Search in Korean and you get 사주 — Saju. The two words turn up different sites, different books and different vocabularies, which makes it look as though there are two systems to choose between.

There aren't. It is one system with two names, and the names are almost literally the same words.

八字 is Chinese for eight characters. 四柱 is four pillars, and the Korean 사주 is that same compound read in Korean. Korean sources often use the longer 사주팔자 — four pillars, eight characters — which is both names at once. Chinese sources say 四柱八字 for exactly the same reason.

Two languages naming the same object by two of its properties. Four columns, eight characters. Same object.

What is identical

The whole computation.

A Korean 만세력 and a Chinese BaZi calculator, given the same birth moment and place, produce the same eight characters. Not equivalent-in-spirit — the same. The stems, the branches, the sixty-term cycle, the solar terms that fix the month boundaries, 立春 opening the year rather than the lunar new year: all of it is shared, and none of it is a translation.

The interpretive layer is shared too. 십신 is 十神, the Ten Gods. 지장간 is 地藏干, the hidden stems. 대운 is 大運, the luck pillars. 용신 is 用神, the favourable element. These are not Korean concepts with Chinese parallels. They are the same terms in the same characters, pronounced in Korean.

So a Korean reader learning from Chinese material and a Chinese reader learning from Korean material are learning the same thing in different phonetics. That is worth saying plainly, because the two web ecosystems barely link to each other and it is easy to assume a divide that isn't there.

Where they actually differ

Vocabulary drift. Korean practice favours some terms that Chinese practice uses less, and the reverse. 신강 and 신약 for a strong and weak day master are standard in Korean; Chinese sources say 身強 and 身弱, the same characters, but often reach for 得令 and 失令 where a Korean reader expects the 신강신약 framing. The concepts line up. The habitual phrasing doesn't always.

The day boundary: 야자시. The 子 hour spans 23:00 to 01:00, straddling midnight, and the two halves raise a question the tradition never settled — does the day pillar advance at midnight, or at 23:00 when 子 begins? Korean practice names the two halves: 야자시 for 23:00–24:00, 조자시 for 00:00–01:00.

This site takes the 야자시 convention. A birth at 23:30 keeps the calendar day's day pillar, and the hour stem derives from that day's stem, so 28 February 1990 at 23:30 gives a day pillar of 甲子 and an hour pillar of 甲子 as well. Cross midnight and both advance together: at 00:00 the day pillar becomes 乙丑 and the same 子 hour becomes 丙子.

Under the other convention that 23:30 birth would take 乙丑 — which is what this engine returns for a birth thirty minutes later. And that is not a cosmetic difference. The day master moves from 甲 Yang Wood to 乙 Yin Wood, and because the Ten Gods are keyed on polarity, every named relationship in the chart changes with it.

We chose one and pinned it with a test, so a future change to the code cannot switch schools silently. Other implementations choose differently, and that is one of the genuine forks rather than a bug in either.

Why it looks like two systems

Mostly because of how each one reached the public.

Saju has never been marginal in Korea. It sits in ordinary life — a 사주카페 in a university district, a 만세력 app on a phone, compatibility checked before a marriage. Bookstores carry it. People who describe themselves as sceptics still know their day master. The vocabulary is common enough that it turns up in conversation without explanation.

BaZi reached English-speaking readers by a different route: largely through practitioners writing for a Western audience, often alongside feng shui, and often for people encountering the whole framework for the first time. So English-language BaZi material tends to teach from zero, while Korean material tends to assume you already know what a 일간 is.

That difference in register makes them feel like separate traditions. It is a difference in audience, not in method.

Japan and Vietnam

The same system runs in both, again under local names.

Japan calls it 四柱推命 — shichū suimei, four pillars fate-divination. Vietnam calls it Tứ Trụ, the Vietnamese reading of 四柱. Both compute the same eight characters from the same calendar, and both carry local emphases in which schools took root.

Four countries, four names, one computation.

What this means practically

If you have read your Saju in Korean, you can read BaZi material in English and you already know the content. The terms map one to one — day master is 일간, luck pillar is 대운, hidden stem is 지장간. Nothing needs relearning.

And if a Korean and an English source disagree about your chart, the disagreement is almost never Korean-versus-Chinese method. It is one of the ordinary implementation forks — the day boundary, longitude correction, how a solar term is rounded — that split any two calculators regardless of language.

This site leads in English, with hanja and the Korean term alongside — 身強身弱 · Day Master Strength, 用神 for what a chart needs. The Korean never stands alone, because bare Hangul is unreadable to most of this journal's readers in a way hanja is not.