How Our Calculator Works: Chinese Gender Calendar Rules and Tests
Written by Sukie Chinese | Last Updated: October 30, 2026
How our calculator works, in four steps: it takes the date you enter, converts it to a Chinese lunar month using the Hong Kong Observatory's calendar tables, works out the mother's lunar age on that date, and reads one cell of the traditional chart. This page documents every rule it uses, where the data comes from, the cases we test it against, and every correction we have made, so you can check any result by hand.
What a result does and does not mean
A correct calculation means the chart was read the way the tradition describes. It does not mean the prediction is likely to be right. The largest test of the chart, on 2,840,755 singleton births in Sweden, concluded it is "no better at predicting the sex of a baby than tossing a coin" (Villamor et al., 2010). Our tool is for cultural interest and fun.
1. Where the lunar dates come from
Every Chinese New Year date and every lunar-month boundary in the calculator comes from the Hong Kong Observatory's Gregorian-Lunar Calendar Conversion Tables, the official astronomical source for Hong Kong. We do not compute moon phases ourselves.
- Chinese New Year dates: 1960–2031, used to work out lunar age for mothers born in those years.
- Lunar-month dates: every day from Jan 29, 2025 to Feb 2, 2030 (1,828 days), the range of conception dates the calculator accepts.
- Leap months: 2025 has a leap 6th month (Jul 25, 2025 – Aug 22, 2025) and 2028 a leap 5th month (Jun 23, 2028 – Jul 21, 2028). The chart has only 12 columns, so we read a leap month as the month it repeats. Some other tools read it as the following month; that is one known reason calculators disagree.
Dates are handled as calendar days in your own time zone, so a date never slips to the day before because of where you are. We test this in US Pacific and China time on every build.
2. The lunar-age rule
Traditional Chinese age (虚岁, xūsuì) starts at 1 at birth and goes up by one at every Chinese New Year, not on your birthday. We use one rule for every date:
Lunar age = lunar year of the date − lunar year of the birth + 1, where a date before that year's Chinese New Year belongs to the previous lunar year.
The popular shortcut, "add one or two to your age", is right most of the time but fails for birthdays in January and February, close to Chinese New Year. The calculator never uses the shortcut. If you want just this step, our lunar age calculator shows it for any date, including a conception date.
3. What each input mode assumes
| You enter | Date the chart uses | Why |
|---|---|---|
| Exact conception date | That date | Nothing to estimate. |
| First day of last period | That date + 14 days | Ovulation is usually about 14 days before the next period (ACOG), which is day 14 of a 28-day cycle. With a longer or shorter cycle the real date shifts, and near a month boundary that can change the column. |
| IVF date | The date as entered | The chart has no traditional rule for IVF. Most parents enter the transfer date; some prefer the fertilization date. Our IVF guide explains both. |
Every result in our English and Spanish calculators now shows these steps under "How this result was worked out": the date used, the lunar age with the years that produced it, the lunar month with its first and last day, and the chart cell.
4. The chart
The boy/girl grid covers lunar ages 18 to 45 and lunar months 1 to 12. We reproduce the traditional chart as published by YourChineseAstrology.com, and the same grid drives the calculator, the full chart page, the year pages and the printable PDFs. Other websites and printed charts sometimes differ in a few cells. We do not claim ours is the original: no historical document confirms the chart's origin, and the popular Qing-dynasty story is a legend.
5. How we check it
Every time the site is built, an automated check compares the calculator with a copy of the Observatory tables (73 years, 903 lunar-month start dates), entered from the source and never generated by the calculator. It checks every Chinese New Year from 1960 to 2031, the lunar month of every supported day, and lunar ages on the day before and the day of Chinese New Year, in two time zones. If anything disagrees, the site cannot be published.
Worked examples in our guides are computed by the same code as the calculator rather than typed by hand, so the text and the tool cannot disagree. The table below is generated the same way.
6. Test cases you can check by hand
Illustrative inputs, not real people. Each row covers a boundary where calculators often go wrong.
| Case | Mother born | Date used | Lunar age | Lunar month | Chart cell |
|---|---|---|---|---|---|
| Ordinary date Mid-year, nowhere near a boundary. | Jun 15, 1994 | Jul 5, 2026 | 33 | 5 | Girl |
| Day before Chinese New Year Still the 2025 lunar year, lunar month 12. | Jun 15, 1994 | Feb 16, 2026 | 32 | 12 | Boy |
| Chinese New Year day Lunar age goes up by one; lunar month 1 starts. | Jun 15, 1994 | Feb 17, 2026 | 33 | 1 | Girl |
| Last day of a lunar month Lunar month 1 of 2026 ends here. | Jun 15, 1994 | Mar 18, 2026 | 33 | 1 | Girl |
| First day of the next month One day later, a different column. | Jun 15, 1994 | Mar 19, 2026 | 33 | 2 | Boy |
| Born before Chinese New Year Chinese New Year 1994 was Feb 10, so her lunar birth year is 1993. | Jan 20, 1994 | Mar 10, 2026 | 34 | 1 | Boy |
| Leap month (2025) Inside the leap 6th month; read as month 6. | Mar 1, 1995 | Aug 1, 2025 | 31 | 6 | Girl |
| Early 2027, before New Year January 2027 still belongs to lunar 2026. | Sep 3, 1996 | Jan 20, 2027 | 31 | 12 | Boy |
| Last-period input Last period June 21 + 14 days = July 5. | Jan 20, 1992 | Jul 5, 2026 | 36 | 5 | Boy |
7. One result, worked through by hand
Take the "born before Chinese New Year" row from the table: an illustrative mother born on Jan 20, 1994, with a conception date of Mar 10, 2026. This is exactly what the calculator does with those two dates.
- Her lunar birth year. Chinese New Year 1994 fell on Feb 10, 1994. She was born before it, so she belongs to the lunar year 1993.
- The lunar year of the conception date. Chinese New Year 2026 fell on Feb 17, 2026, before the conception date, so the date is in lunar year 2026.
- Lunar age. 2026 − 1993 + 1 = 34. Her everyday age that day is 32, so the gap is 2, more than the "add one" shortcut would give.
- Lunar month. Lunar month 1 of 2026 runs from Feb 17, 2026 to Mar 18, 2026, and the conception date falls inside it.
- Chart cell. Row 34, column 1 reads Boy.
A tool that used her everyday age, or that counted March as month 3, would read a different cell. Our guide to why calculators give different results walks through each of those mistakes.
8. Why another calculator may give a different answer
If a different website gives you the opposite result for the same dates, one of these is almost always the reason. Each one moves the lookup to a different row or column.
- It used your everyday age, or the "add one" shortcut, instead of the full lunar-age rule.
- It treated the columns as January–December instead of lunar months.
- It used your last-period date as if it were the conception date, which is about two weeks earlier.
- It read a leap month as the following month instead of the month it repeats.
- It uses a different version of the chart, with some cells changed.
The first four are input differences that the steps above let you check. The last one cannot be settled, because no original document exists to compare against.
9. How the printable charts are made
The free printable PDFs for 2026 and 2027 are generated from the same chart and the same Observatory lunar-month dates as the calculator, not drawn by hand. Page 1 is the full chart; page 2 lists that year's lunar-month dates and the lunar-age rule. The build records a fingerprint of the data each PDF was made from, and if the chart or the dates ever change, the site cannot be published until the PDFs are made again.
10. Your privacy
The calculation runs in your browser. The dates you type are not sent to our server, not stored and not put in the page address. Our analytics records only events such as a calculation being started, finished or rejected, which input mode was used, a share button or PDF download being clicked, and never a date.
11. Known limitations
- Conception dates are accepted from Jan 29, 2025 to Feb 2, 2030; we add years as the Observatory tables are needed.
- Lunar ages outside 18–45 are not on the chart, so the calculator stops there.
- A last-period date is an estimate; if your dates are uncertain, try the days either side.
- The chart itself is folklore. No calculation can make it predict a baby's sex.
12. Corrections log
- October 30, 2026. Worked examples on 14 pages had the wrong lunar month or lunar age (for example, June 10, 2026 described as lunar month 5; it is month 4). All are now computed from the calculator. Chart columns were relabelled from January–December to lunar months 1–12. IVF wording was made consistent across the site. The automated Observatory check was added.
- October 22, 2026. The lunar-age shortcut was corrected for birthdays that fall before Chinese New Year of the birth year.
- October 15, 2026. The calculator's own lunar-age rule and its 2025 lunar-month dates were corrected, so some results shown before that date could differ from today's. Every source on the site was re-checked and claims without a verifiable source were removed.
Found something that looks wrong? Tell us on our contact page with the dates you entered, and we will check it against the tables above.