Chinese gender predictor
Works out your Chinese lunar age and the lunar month you conceived in, using a real lunisolar calendar rather than your age plus one. It gives a boy or girl answer for fun, and states the only sourced accuracy figure there is: fifty per cent, which is chance, measured across 2,840,755 births.
Chinese gender predictor
No sign-up · PrivateYour lunar age depends on where your birthday sits relative to Chinese New Year, which is why this is not simply your age plus one.
Roughly the day you ovulated. If you are not sure, use one of the other two methods — they get there from dates you are more likely to know.
The first day of bleeding, not the last.
Ovulation is assumed 14 days before the next period, so a 32-day cycle moves the conception date four days later than a 28-day one.
Conception is taken as 266 days before the due date — 280 days minus the two weeks between a notional last period and ovulation.
The conception date works out as on or before your own date of birth. Check both dates — it is usually the year.
Nothing you type leaves your device. The whole calculation runs in your browser.
How this is calculated
FormulaHow the two lunar numbers are worked out
The Chinese chart asks for two things: the mother’s lunar age at conception and the lunar month of conception. Almost every calculator online estimates both, badly. This one computes them.
Getting to a conception date
You can enter the conception date directly, or the tool derives it. From a last period it is the first day of bleeding plus 14 days, adjusted by your cycle length: conception = LMP + 14 + (cycle − 28), so a 32-day cycle moves it four days later. From a due date it is due date − 266 days, which is the 280 days of Naegele’s rule minus the fourteen days between a notional last period and ovulation.
Lunar age is not your age plus one
Nominal age is counted in Chinese years: lunar age = (Chinese year now − Chinese year of birth) + 1. Chinese years begin at Chinese New Year, which falls between 21 January and 21 February depending on the moon, not on 1 January and not on your birthday. Somebody born on 15 January 1994 was born in Chinese year 1993, because Chinese New Year 1994 did not arrive until 10 February; somebody born that April was born in Chinese year 1994. Their lunar ages differ by one for the rest of their lives.
The lunar month, including leap months
The calendar is computed from first principles in your browser, with no lookup table to go stale. New moons come from the phase series in Meeus’ Astronomical Algorithms, chapter 49. The sun’s apparent longitude comes from Meeus’ low-precision solar theory, which fixes the December solstice and the major solar terms. Month 11 is the lunar month containing the day of the December solstice. If thirteen new moons fall inside a solstice-to-solstice year, the leap month is the first one after month 11 that contains no major solar term, and it takes the number of the month before it. Everything is evaluated in Beijing civil time, UTC+8, because that is what the Chinese calendar is defined in.
December 2014 shows why the day, not the instant, is what counts. The winter solstice fell at 23:03 UT on 21 December and the new moon at 01:36 UT on 22 December, so the solstice came first. In Beijing both land on 22 December, which puts the solstice on the first day of the new month and makes Chinese New Year 2015 fall on 19 February. Comparing raw instants instead gives 20 January, a month out. This calculator returns 19 February and matches the published Chinese New Year date for every year tested between 1970 and 2030.
The prediction itself
There is no verifiable original chart to reproduce, and published copies of the grid differ from one another, so this tool does not claim to transcribe one. It applies a rule of its own, stated in full: add the lunar age to the lunar month; an odd total gives boy and an even total gives girl. Statistically that is the same coin as any grid you will find elsewhere, because every published grid performs at chance. One documented limitation: the difference between terrestrial and universal time is not applied, which is at most about a minute across 1900 to 2100 and can only matter when a new moon falls within a minute of Beijing midnight.
The accuracy figure, stated plainly
Fifty per cent. That is chance, and it is the only accuracy figure on this page, because it is the only one anybody can point to a study for.
In 2010, Eduardo Villamor and colleagues tested the Chinese lunar calendar method against 2,840,755 singleton births registered in Sweden between 1973 and 2006. They estimated each mother’s lunar age and lunar month of conception and applied a published version of the algorithm. Kappa, the statistic used for agreement beyond chance, came out at 0.0002, with a 95% confidence interval of −0.0009 to 0.0014 — an interval that straddles zero. In their words, the method “is no better at predicting the sex of a baby than tossing a coin”, and they “advise against painting the nursery based on this method’s result”.
It did not work better for anyone. The same study checked whether accuracy varied by year of conception, maternal age, education, body mass index or parity, and it did not. A second analysis on 1,000 births using a website-customised version of the algorithm returned a kappa of −0.02, which is fractionally worse than guessing.
If another site tells you seventy per cent, or eighty-five, or ninety-three, ask it for the study. There is not one.
The other method people arrive here for
The Shettles method proposes that timing intercourse close to ovulation favours a boy while two to four days before favours a girl. Shettles himself claimed a 75% success rate for couples trying for girls and 80% for boys.
In 1995, Allen Wilcox, Clarice Weinberg and Donna Baird published a prospective study in the New England Journal of Medicine: 221 women, 625 ovulatory cycles dated by daily urinary hormone measurements, 192 pregnancies and 129 live births. Their finding was that cycles producing male and female babies had similar patterns of intercourse in relation to ovulation, and their conclusion was that “for practical purposes, the timing of sexual intercourse in relation to ovulation has no influence on the sex of the baby”.
The mechanism Shettles proposed did not survive either. His theory rested on X-bearing and Y-bearing sperm differing in shape and speed; later work on sperm morphology found no shape difference between them.
Where the chart is supposed to come from
Nobody can say. The origin stories — a Qing dynasty scroll, a royal tomb near Beijing, seven hundred years of continuous use — are repeated on every site that publishes the grid and verified by none of them. The grids themselves are not identical to one another either, which is precisely why the Swedish study had to run a second analysis using a website-customised algorithm.
So this tool does not pretend to reproduce a historical chart. It applies a rule stated openly on the page: add the lunar age to the lunar month, odd means boy and even means girl. That is honest about what it is, and statistically it is the same coin as every grid it could have copied.
The part that is real
The lunar arithmetic is worth having even if you ignore the prediction completely, and it is where nearly every other calculator goes wrong.
Lunar age is not your age plus one. It is counted in Chinese years, and a Chinese year begins at Chinese New Year, somewhere between 21 January and 21 February depending on the moon. Two people born in the same Western year can therefore have different lunar ages, permanently. Someone born on 15 January 1994 was born in the year of the Rooster, because Chinese New Year 1994 did not arrive until 10 February; someone born that April was born in the year of the Dog.
The lunar month is not the Gregorian month either. Lunar months begin at a new moon, so they drift against the calendar, and roughly every third year an extra leap month is inserted to keep them in step with the seasons — which shifts the number of every month after it. Charts have no row for a leap month at all, which is one more place where two websites will hand you different answers about the same pregnancy.
How the calendar is computed
From first principles, inside your browser, with no lookup table that can go stale. New moons come from the standard lunar phase series; the sun’s apparent longitude fixes the December solstice and the twenty-four solar terms. Month 11 is the month containing the solstice, and where a year has thirteen months the leap month is the first one after month 11 with no major solar term in it. Everything is evaluated in Beijing civil time, because that is how the Chinese calendar is defined, and using instants rather than Beijing days puts Chinese New Year 2015 a month early.
What actually tells you
The mid-pregnancy scan. In the UK the NHS offers a screening scan between 18 and 21 weeks; it exists to check your baby’s development rather than to answer this question, and some units will not tell you if the view is not clear. Blood tests that analyse cell-free fetal DNA can indicate sex earlier, though they are offered for screening reasons rather than curiosity, and availability differs by country.
Nothing about the shape of your bump, what you crave, how you carry, the heart rate, or a wedding ring on a string changes what is already there. Enjoy the chart for what it is: a fifty-fifty guess with good stories attached, and a genuinely interesting piece of calendar mathematics underneath it.
Sources
- Accuracy of the Chinese lunar calendar method to predict a baby's sex: a population-based study — Paediatric and Perinatal Epidemiology (Villamor E, Dekker L, Svensson T, Cnattingius S, 2010), accessed
- Accuracy of the Chinese lunar calendar method to predict a baby's sex (abstract with kappa values) — Wiley Online Library, accessed
- Timing of sexual intercourse in relation to ovulation. Effects on the probability of conception, survival of the pregnancy, and sex of the baby — New England Journal of Medicine (Wilcox AJ, Weinberg CR, Baird DD, 1995;333:1517-21), accessed
- The Shettles Method of Sex Selection — Embryo Project Encyclopedia, Arizona State University, accessed
- Accuracy of the Chinese lunar calendar method to predict a baby's sex (repository record) — University of Michigan Deep Blue, accessed
- 20-week screening scan — NHS, accessed