What is Saju? A complete guide to Korean astrology

Saju in one paragraph

Saju (사주, literally “four pillars”) is Korean astrology. It converts the year, month, day, and hour of a person's birth into four pillars, each made of two characters: a heavenly stem on top and an earthly branch below. The eight characters together are the chart, which is why the full Korean name is saju palja (사주팔자, “four pillars, eight characters”). A reading looks at how the five elements (Wood, Fire, Earth, Metal, Water) are balanced across those eight characters and how they relate to the Day Master, the character that stands for the person.

The same chart is called BaZi (“eight characters”) in Chinese and the Four Pillars of Destiny in English. Saju is the Korean name and the Korean tradition of reading it.

Where Saju comes from

Saju grew out of Chinese cosmology: yin and yang, the five elements, and the sexagenary calendar that has counted years, months, days, and hours in cycles of sixty for more than two thousand years. The method of reading a chart around the day of birth is traditionally credited to Xu Ziping, a scholar of the early Song dynasty in the 10th century, which is why Chinese texts also call it Ziping astrology.

The system reached Korea centuries ago and became part of everyday life. Today Koreans read Saju at saju cafés, in New Year fortune readings, before weddings, and increasingly in apps. Korean practice uses Korean readings of the characters and its own vocabulary, but the chart itself is the same one used in BaZi.

The four pillars

Each pillar is one unit of time, written as a stem over a branch. Traditional readings also link each pillar to an area of life:

• Year pillar: family background, ancestry, and the social world you were born into.
• Month pillar: parents, upbringing, and the season you were born in. It carries the most weight when judging which elements are strong.
• Day pillar: you. The day stem is your Day Master, and the day branch below it is read as your inner self and your closest partner.
• Hour pillar: children, later life, and private ambitions. It needs a birth time.

Heavenly stems and earthly branches

There are ten heavenly stems (천간, cheongan). Each is one of the five elements in its yang (strong, outward) or yin (soft, inward) form:

• 甲 Gap · Yang Wood, and 乙 Eul · Yin Wood
• 丙 Byeong · Yang Fire, and 丁 Jeong · Yin Fire
• 戊 Mu · Yang Earth, and 己 Gi · Yin Earth
• 庚 Gyeong · Yang Metal, and 辛 Sin · Yin Metal
• 壬 Im · Yang Water, and 癸 Gye · Yin Water

The cycle of sixty

Stems and branches pair up in order: 甲子 Wood Rat, 乙丑 Wood Ox, 丙寅 Fire Tiger, and so on. Because ten and twelve only line up again after sixty steps, the calendar runs in a cycle of sixty combinations (육십갑자, yuksip gapja). The same cycle labels every year, month, day, and two-hour block.

That is also where the familiar zodiac animal comes from: it is just the branch of the year pillar. 2024 was 甲辰, a Wood Dragon year, and 2026 is 丙午, a Fire Horse year. Your animal is one character out of eight, so it is only a small part of a Saju chart.

The five elements

Every stem and branch belongs to one of five elements (오행, ohaeng). The elements relate to each other in two cycles.

In the generating cycle, each element feeds the next: Wood feeds Fire, Fire makes Earth, Earth bears Metal, Metal carries Water, and Water nourishes Wood. In the controlling cycle, each element keeps another in check: Wood parts Earth, Earth dams Water, Water quenches Fire, Fire melts Metal, and Metal cuts Wood.

A first look at any chart counts the elements across the eight characters. A chart heavy in one element tends to express that element's qualities strongly; a missing element is often read as something a person seeks out in work, places, or relationships. Readings are about balance, not about any element being good or bad.

The Day Master: the center of the chart

The heavenly stem of the day pillar is the Day Master (일간, ilgan). It stands for the core self, and every other character is read by how it relates to it. There are ten Day Masters, from Yang Wood to Yin Water, so about one person in ten shares yours.

Those relationships are called the Ten Gods (십성, sipseong) and fall into five groups, each in a yin and a yang form:

• Peers: the same element as the Day Master. Friends, rivals, independence.
• Output: the element the Day Master generates. Expression, creativity, talent.
• Wealth: the element the Day Master controls. Money, resources, what you manage.
• Authority: the element that controls the Day Master. Discipline, status, pressure.
• Resource: the element that generates the Day Master. Learning, support, care.

Why the solar calendar and birth time matter

Saju does not follow January 1 or the Lunar New Year. It follows the 24 solar terms (절기, jeolgi), which are set by the sun's position. The year begins at Ipchun (입춘, “start of spring”), around February 4, and each month begins at one of twelve solar terms. Someone born on January 20 therefore belongs to the previous year's pillar, and their zodiac animal can differ from the one a Lunar New Year chart gives.

The hour pillar changes every two hours; the 子 (Rat) hour runs from 11 pm to 1 am. Without a birth time a chart has three pillars (six characters). That is still enough for the Day Master and most of the element balance, but the hour pillar is missing.

Clock time is not always solar time. Korea Standard Time is set for 135°E, while Seoul sits near 127°E, so the sun in Seoul runs about 32 minutes behind the clock. Many practitioners correct for this. OtterPrism converts a known birth time to true solar time for the birthplace, including historical daylight saving, before building the chart.

Timing: luck pillars and yearly energy

A chart is not only a portrait. Saju also reads timing through ten-year luck pillars (대운, daeun) and the pillar of each year (세운, seun). When a period's elements support or clash with the birth chart, readings describe it as a time to push forward, rest, or change direction. This is why Korean New Year readings (신년운세, sinnyeon unse) are so popular.

How Saju is used in Korea today

For many Koreans, Saju is a familiar cultural habit rather than a belief system. Common uses include:

• Self-understanding: temperament, strengths, and blind spots.
• Compatibility (궁합, gunghap): traditionally checked before marriage, now also for dating, friends, and coworkers.
• Timing: New Year readings and choosing auspicious dates (택일, taegil) for weddings, moves, or openings.
• Fun with friends and fandoms: reading the charts of K-pop idols and comparing members' energies.

Is Saju scientific?

No. There is no scientific evidence that a birth chart predicts personality or events. Saju is best understood as a long-lived cultural tradition and a vocabulary for reflection: it gives you a structured way to think about temperament, relationships, and timing. OtterPrism presents Saju for entertainment and self-reflection.

How OtterPrism reads Saju

OtterPrism builds the four pillars from a birth date, optional birth time, and birthplace (using true solar time), identifies the Day Master, counts the five elements, and writes a reading in plain English. Compatibility between two charts is scored the same way every time, from the rules described in the compatibility guide.

You can explore the charts of K-pop idols, see how members of a group match, and compare any two idols' chemistry.

How Saju compatibility works →Browse idol birth charts →Explore group chemistry →Compare idol pairs →