Birthday Calculator

What the birthday calculator can do

Use the radio buttons to choose one of five questions. Each one shows only the fields it needs.

• Age from DoB. Your exact age in years, months and days on a chosen date (today if you leave it empty), plus totals in months, weeks and days.
• Next birthday. The date of the coming birthday, the days remaining, the weekday it lands on and the age you will turn.
• What year was I born. The birth year, worked out from your age at the start of the current year.
• What day was I born. The weekday of any date, so you can find out which day you were born on.
• Date of birth. An estimated birth date from an age that is known on a particular date.

Exact age from a date of birth

The age is counted in complete years, then complete months, then the remaining days. For someone born on 15 May 1990, the age on 19 October 2026 is 36 years (the 2026 birthday has passed), 5 months (to 15 October) and 4 days, written as 36 years, 5 months and 4 days. In single units that is 437 months, 1,900 whole weeks or 13,306 days.

If you need the age as of a past or future date, such as for an application deadline, type that date into the Calculate age on box. The age calculator offers the same calculation with a running-age figure added.

Finding your next birthday

The next birthday is the first anniversary of the birth date that is later than today. A birthday that falls on today's date is treated as already celebrated, so the following one is a full year away.

Example. Born on 15 May 1990, measured on 19 October 2026: the next birthday is 15 May 2027, 208 days away. It falls on a Saturday, and the person will turn 37.

People born on 29 February (sometimes called leaplings) only have a real birthday in leap years. In other years the calculator moves the date to 1 March, so a 29 February 2000 birth shows its next birthday as 1 March 2027. Many leaplings choose 28 February instead, and some countries set a legal rule for which day counts, so check the local convention if it matters.

Working out the year you were born

If you know how old you were on the first day of the current year, the birth year is the previous year minus that age. This avoids the usual off-by-one problem, because you cannot yet have celebrated a birthday of the new year on 1 January.

year of birth = (current year − 1) − age on 1 January

Example. Someone who was 30 on 1 January 2026 was born in 2025 − 30 = 1995. The one exception is a birth on 1 January 1996, which also makes a person exactly 30 on 1 January 2026, so the answer holds for everyone except people born on that single day.

Which day of the week were you born on?

Weekdays repeat in a fixed cycle of seven, so the weekday of any date follows from the number of days between that date and a reference date whose weekday is known.

Example. Between 15 May 1990 and Monday 19 October 2026 there are 13,306 days. Dividing by seven gives 1,900 whole weeks and a remainder of 6, so the birth date is six weekdays before a Monday: counting back gives Sunday, Saturday, Friday, Thursday, Wednesday and finally Tuesday. The calculator confirms that 15 May 1990 was a Tuesday.

The same tool works for dates in the future, which is useful when you want to know whether an upcoming date falls on a weekend. For a plain weekday lookup you can also use the weekday calculator.

Estimating a date of birth from an age

Pick a date on which the age is certain, enter that age in years, months and days (fill in only the parts you know), and the calculator subtracts it from the date. The result is exact only when all three parts are given.

Example. A person was 30 years, 2 months and 5 days old on 19 October 2026. Subtracting 30 years gives 19 October 1996, two months back gives 19 August 1996, and five days back gives 14 August 1996. If only the 30 years are known, the estimate is 19 October 1996, the same day and month, 30 years earlier.

The date with a known age can also be a date of death, which is a common way to estimate the birth date of a relative from a record such as a gravestone. When subtracting months from a date at the end of a month, the result rolls forward: one month before 31 March 2026 is 3 March, because there is no 31 February.

A date estimated from an incomplete age can be off by up to a year, a month or a day, depending on the least precise unit you entered. Confirm with official records where it matters.