The Chinese Lunar Calendar, Explained

Rule one: months follow the moon

A Chinese lunar month begins on the day of a new moon and ends the day before the next one. Because a lunation is about 29.53 days, months alternate between 29 and 30 days in no fixed pattern; the calendar simply follows the sky.

This is why lunar month 1 is not January. Lunar month 1 begins on Chinese New Year, which moves between 21 January and 20 February. By the middle of the year, a lunar month can sit three to seven weeks away from the Western month of the same number.

The gender chart’s twelve columns are these lunar months. Reading your Western month straight off the chart is the single most common way to get a wrong answer.

Rule two: everyone ages on New Year

Traditional East Asian age reckoning, sometimes called sui, works differently from the Western count in two ways:

• A newborn is one year old on the day of birth, not zero.
• Everyone gains a year together on Chinese New Year, not on their own birthday.

So a woman born in June 1995 was 30 in Gregorian terms in early 2026, but her lunar age was 31 until 17 February 2026, and 32 from that day on, months before her actual birthday. Two conception dates a single day apart, either side of New Year, land on different rows of the chart.

Rule three: some years have thirteen months

Twelve lunar months add up to roughly 354 days, about eleven short of the solar year. Left alone, the calendar would drift a full season in under a decade. To prevent that, an extra intercalary month is inserted roughly every third year, seven times in every nineteen.

A leap month takes the number of the month before it. The year 2025, for instance, carried a leap sixth month. The gender chart has no thirteenth column, so a conception inside a leap month is read against its base month number.

Does the moon actually affect a baby’s sex?

No. Biological sex is settled at fertilisation by whether the sperm carries an X or a Y chromosome, and nothing about the lunar month or the mother’s age changes that. The lunar calendar is the indexing system the chart uses, not a mechanism.

What the lunar calendar genuinely is: a careful, centuries-old piece of astronomy that kept an agricultural society in step with both the moon and the seasons. That is worth understanding on its own terms, just not as an obstetric tool.