A bazi calculator, called a saju calculator in Korean, takes your birth date, time and place and turns them into four pillars: year, month, day and hour. Each pillar has two characters, a heavenly stem on top and an earthly branch below, so you end up with eight characters. Bazi (八字) literally means “eight characters,” and saju (사주, 四柱) means “four pillars.” Same chart, two names.
Most of the work is calendar math, and that’s where calculators quietly disagree. Two sites can show different pillars for the same birthday because they handle time zones, the start of the year or the late-night hour differently. This guide walks through each step, so you can tell whether a result makes sense, including the one from SajuBunny’s free saju calculator.
What you need to enter
| Input | Why it matters |
|---|---|
| Birth date (solar or lunar) | Sets the year, month and day pillars. Lunar dates are converted to solar first, and a leap month has to be marked. |
| Birth time | Sets the hour pillar, and near midnight it can move the day pillar. Without it you get six characters instead of eight. |
| Birth city | Gives the time zone, including daylight saving time, and the longitude used for local solar time. |
| Gender | Traditionally decides whether your 10-year luck cycles run forward or backward. It doesn’t change the eight characters. |
Step 1: the year pillar changes at Ipchun
The year pillar doesn’t turn over on January 1 or on the lunar new year. It changes at Ipchun (立春, start of spring), a solar term that falls around February 4. A good calculator uses the exact moment, not just the date. In 2027, for example, Ipchun is at about 10:46 on February 4, Korea time. A baby born at 10:30 that morning still has the 2026 year pillar, 丙午.
If a calculator gives you the “wrong” animal for a January birthday, this is usually why. The zodiac years chart lists Ipchun for every recent year.
Step 2: the month pillar follows solar terms
Months in bazi are solar months, not calendar months and not lunar months. Each one starts at one of twelve solar terms, roughly on the 4th to 8th of each Western month. The month around February 4 to March 5 is the Tiger month, the one starting around March 6 is the Rabbit month, and so on. The month’s stem follows from the year’s stem by a fixed rule.
That means a birth on March 3 and one on March 8 can have different month pillars even though both are “March.” Calculators that skip solar terms get this wrong.
Step 3: the day pillar comes from a 60-day count
The day pillar is the simplest step and the one you can’t do in your head. Days have been counted in an unbroken cycle of sixty stem-branch pairs for well over two thousand years, so every date maps to exactly one of the sixty. Calculators read it from a reference date.
The stem of your day pillar is your day master, the character that stands for you. Most readings start there, which is why the day pillar matters more than your zodiac animal. The day master guide explains the ten types.
Step 4: the hour pillar uses two-hour blocks
A day has twelve double hours, one for each branch. The Rat hour (子時) runs from 23:00 to 01:00, the Ox hour from 01:00 to 03:00, and so on to the Pig hour from 21:00 to 23:00. The hour’s stem is set by your day stem.
The Rat hour is where schools split. In the most common rule, a new day starts at 23:00, so someone born at 23:30 gets the next day’s pillar. Another rule, called yajasi (야자시) in Korean, keeps the same day until midnight. If you were born between 23:00 and 01:00, check which rule your calculator uses. SajuBunny uses the 23:00 rule by default and lets you switch to yajasi under More options.
Step 5: time zones and local solar time
Clock time isn’t sun time. The hour pillar was designed around the sun’s position, so many calculators adjust your birth time to local solar time, based on your city’s longitude. The correction is four minutes per degree of longitude away from your time zone’s reference meridian.
Korea is a good example. Korean clocks run on UTC+9, which is set for 135°E, but Seoul sits near 127°E. Sun time in Seoul is about half an hour behind the clock, so a 13:15 birth in Seoul is closer to 12:45 by the sun. That can move the hour pillar when a birth falls near a boundary.
Daylight saving time matters too. Korea used it in 1987 and 1988, and much of North America and Europe use it every summer. A calculator that knows your city’s time zone history handles this for you. One that asks only for a UTC offset may not.
What a good calculator shows you
Beyond the eight characters, a useful bazi calculator usually shows:
- Day master: your day stem, with its element and yin or yang.
- Five-element balance: how many characters fall in wood, fire, earth, metal and water.
- Hidden stems: the stems stored inside each branch, which shift the balance.
- Ten gods: how each character relates to your day master, such as output, wealth or resource.
- Luck pillars: your 10-year luck cycles and the age each one starts.
SajuBunny’s free saju card covers the first two at a glance. The full chart adds hidden stems, life stages, special stars, clashes and luck by decade, year, month and day.
How SajuBunny calculates
The saju calculator runs on your device. It converts lunar dates with the Korean or Chinese lunar calendar, uses the exact Ipchun and solar-term moments, reads your city’s time zone history, applies local solar time by default (you can turn it off), and offers both Rat-hour rules. If you don’t know your birth time, it shows six characters and tells you when the month could change during your birth day.
The calculation is the same for everyone. The reading on top, from the free card to the longer written readings that cost Tteok, is where interpretation comes in, and we treat it as a tool for reflection rather than a forecast.
FAQ
Is a saju calculator the same as a bazi calculator?
Yes. Saju is the Korean name and bazi the Chinese name for the same four-pillar chart. The calendar math is identical; Korean and Chinese readers differ more in how they interpret the chart than in how they calculate it.
Why do two bazi calculators give me different pillars?
Usually because of one of four settings: the year change at Ipchun, solar-term months, the Rat-hour rule between 23:00 and 01:00, or local solar time and time zone history. Check those first.
Do I need my exact birth time?
It helps, but you can skip it. Without a time you get the year, month and day pillars, six characters. The day master, which most readings start from, doesn’t depend on the hour unless you were born near midnight.
Is the free saju calculator really free?
Yes. The saju card with your eight characters, day master and five elements is free and needs no account. Longer written readings open with Tteok, the site’s credits, and each shows a free preview first.