How this saju calculator works and who runs it

Who runs Plain Saju?

Plain Saju is run by the SajuWiki (운세위키) team, a Korean saju service based in Korea.

You can reach us at luckylovemesaju@gmail.com. Use that address for questions about the method, for corrections to a page, or to report a chart you think is wrong.

What rules does the calculator follow?

The saju calculator takes four inputs: a birth date in the Gregorian calendar, a clock time to the minute (or "Birth time unknown"), a birthplace chosen from a list, and a gender. It turns them into a chart with a fixed algorithm. No AI model is involved, so the same input always returns the same chart. Nothing you enter is stored, as the privacy page explains.

In the calculation, gender is used for one thing only: the direction of the 10-year luck cycles. The ChatGPT saju prompt page prepares text that you paste into an assistant yourself; this site does not call an AI service.

The table lists the conventions that decide which characters appear in your chart. The sections after it give the detail behind each row.

When do the year and month pillars change?

The saju year starts at Ipchun, and each saju month starts at one of the 12 jeol. Of the 24 solar terms, only these 12 are boundaries. The 12 mid-month terms between them (junggi, 中氣) change nothing in the chart. The instant of each term comes from the open-source lunar-typescript library.

The calculator first converts the birth to an absolute instant, using the UTC offset of the birthplace on that date. It then compares that instant with the instant of the term. A birth time is entered to the minute and is treated as second :00 of that minute, while the library gives each term to the second. A birth entered in the very minute of a term therefore falls before the term.

Ipchun in 1995 shows the effect. The library puts it at 07:12:51 UTC on February 4, 1995, which is 16:12:51 in Seoul and 02:12:51 in New York. A birth entered as 16:12 in Seoul on that date gets the year pillar 甲戌 and the month pillar 丁丑. One minute later, at 16:13, it gets 乙亥 and 戊寅. A birth at 02:13 in New York on the same date is the same moment as 16:13 in Seoul, and it gets the same year and month pillars.

The next table shows the 12 jeol of the saju year 丙午, from Ipchun 2026 to Sohan in January 2027, as the calculator uses them. Times are in UTC with the seconds dropped. Korean time is nine hours ahead.

How is clock time converted to true solar time?

The day and hour pillars use true solar time at the birthplace, which is the time a sundial there would show. The calculator gets it in two steps.

First it finds the UTC offset that was in force at the birthplace at that date and time. The offsets come from the IANA Time Zone Database as built into the server's JavaScript runtime. They include past changes of standard time and daylight saving time.

Then it adds a correction, in minutes, to the clock time:

correction = 4 × longitude + equation of time − UTC offset in minutes

Longitude is in degrees, positive east of Greenwich. The factor of 4 is there because the Earth turns one degree every four minutes. The equation of time is the seasonal gap between sundial time and mean clock time. The calculator uses the approximation 9.87 sin 2B − 7.53 cos B − 1.5 sin B minutes, where B is 360° × (day of the year − 81) / 365. The PVEducation reference on solar time by Honsberg and Bowden gives the same formula. In this formula the value runs from −14.6 minutes in mid-February to +16.5 minutes at the end of October. The correction is rounded to a whole minute.

The table shows three births in Seoul (longitude 126.98° E) as the calculator converts them.

The last row is the example chart on the home page. Seoul lies 8.02 degrees west of the 135° E meridian that UTC+9 is based on, which accounts for 32 of the 42 minutes. The equation of time on March 15 accounts for the other 10.

Enter the time the clock showed at birth, and do not adjust it for daylight saving time yourself. If the time you enter fell in the hour that was skipped when clocks went forward, it is read with the offset in force before the change, so 02:30 becomes 03:30. If it fell in the hour that was repeated when clocks went back, the first of the two is used. The result states each of these cases, and the size of the correction, under "How this chart was calculated".

Other calculators make different choices at several of these steps. The page on how saju and BaZi conventions differ covers the alternatives.

When do the day and hour pillars change?

The day pillar counts days in the 60-pillar cycle. The calculator takes the calendar date of the birth in true solar time, converts it to a Julian Day Number, and reads the pillar from that number: the number plus 49, modulo 60, is the position in the cycle, with 甲子 at 0.

The day turns over at 23:00 true solar time, when 子 hour begins. A birth from 23:00 to 23:59 solar time gets the day pillar of the next calendar day. The calculator does not split 子 hour into a late-night half that stays with the old day and an early-morning half that belongs to the new one, as some practitioners do.

A birth in Seoul at 23:50 clock time on March 15, 1995 is 23:08 in true solar time. It gets the day pillar 丙午 and the hour pillar 戊子. A birth at 14:30 on the same date, the home-page example, has the day pillar 乙巳.

The hour branch comes from the solar-time hour, as in the table. The hour stem follows from the day stem by the standard rule, so that 子 hour on a 甲 or 己 day is 甲子. From 23:00 on, the day stem used is that of the new day.

How are the 10-year luck cycles (daeun) calculated?

Daeun (대운, 大運) are the ten-year periods in the luck-cycle section of the result. Three rules produce them.

Direction: the cycles run Forward when the year stem is yang and the gender is male, or when the year stem is yin and the gender is female. In the other two cases they run in Reverse.

Starting age: for a Forward chart the calculator measures the time from the birth to the next jeol. For a Reverse chart it measures the time from the previous jeol to the birth. Three days count as one year. The interval is measured in minutes, so 4,320 minutes make one year, and the age is shown to one decimal place. The first cycle is labeled with the calendar year reached by adding that age to the birth date. Each later cycle starts ten years after the one before.

Pillars: the first cycle takes the pillar that follows the month pillar in the 60-pillar cycle when the direction is Forward, or the one before it when the direction is Reverse. Each later cycle moves one more step the same way. Ten cycles are shown.

In the home-page example the year stem 乙 is yin and the gender is female, so the cycles run Forward. The next jeol, Cheongmyeong, came 21.03 days after the birth, which gives 7.0 years. The result reads "The first cycle starts at age 7, in 2002", and the first pillar is 庚辰, the one after the month pillar 己卯.

When the birth time is unknown, the interval is measured from noon and the result says that the starting age is approximate.

What rules produce the rest of the result?

Every other section of the result is derived from the four pillars with fixed tables. The guide to reading a saju chart explains what each section means. This table records only how each one is computed.

The ten gods section also names a type, such as "The independent type", from the group that holds the most characters. When groups tie, the first one in the order Companions, Output, Wealth, Officer, Resource is used.

One rule can change the ten gods of the year stem and the month stem. When those two stems form a stem combination and the rest of the chart lets the pair transform, both are read as the element the pair combines into. For that, the month branch has to favor the combined element, and the two pillars have to be free of conflicts with other pillars and of roots that hold a stem to its own element. The result says so when the rule applies. Combinations that involve the day pillar never change an element.

Day Master strength is rated on seven levels, from Very weak through Balanced to Very strong. The rating comes from three yes-or-no checks and a score. A character supports the Day Master when it has the same element or the element that produces it. In season (deukryeong) asks whether the month branch supports the Day Master. Rooted (deukji) asks the same of the day branch. Supported (deukse) asks whether the other stems and branches mostly help the Day Master. The score adds up the support found in each pillar, with the month pillar counting most, and a branch counts through its hidden stems, the main one most. The strong levels need both a high score and checks passed.

The home-page example is in season and supported but not rooted. Its rating is Slightly strong, level 5 of 7.

Dohwa (Peach Blossom), Yeokma (Traveling Horse) and Hwagae (Canopy Star) use the three-harmony groups (samhap, 三合). The calculator takes the group that the year branch belongs to and looks for that group's star branch in the other pillars. It then does the same from the day branch.

The saju compatibility calculator computes two charts by the rules on this page and then compares them. Gunghap (궁합, 宮合) is the saju word for that comparison. It looks at the combination of the two day stems, the relation between the two day branches, the cross relations between all the branches, and how the elements of each chart complement the other. It returns a score from 50 to 100 with the reasons.

What data does the calculator use?

The birthplace list holds 450 cities in 188 countries. For each city the calculator uses the GeoNames time zone and the GeoNames longitude rounded to two decimal places. Place data © GeoNames (geonames.org), CC BY 4.0.

The day pillar, the hour pillar, the solar-time correction, the luck cycles and every rule in the previous section are computed by this site's own code.

What are the known limits?

A chart is a calculation. The descriptions attached to it say what the saju tradition reads into each feature. They are not predictions and not professional advice.

How are pages written and corrected?

The guides on this site are published and maintained by the Plain Saju team. Each guide names its sources in the text and lists them at the end of the page.

Corrections go to luckylovemesaju@gmail.com. Say which page or which birth details are involved, so that we can reproduce the problem. Confirmed errors are corrected.

Every page shows the date it was published. A second date, "Updated", appears only after the content of the page has changed.

This page is the reference for the calculator's rules. When a rule changes, this page is changed with it.