Day of Birth Calculator

How the weekday of a date is found

Weekdays repeat every seven days without exception, so the weekday of any date can be found by counting days from a date whose weekday is known. This calculator converts your date of birth into a day number — the count of days since Thursday 1 January 1970 — using an exact formula for the Gregorian calendar (the widely used days_from_civil algorithm), then takes the remainder after dividing by seven. No lookup tables or external services are involved, so it works the same for 1850, 2000 or 2400.

The same day number gives the day of the year (1 January is day 1; 31 December is day 365, or 366 in a leap year) and the ISO week number.

ISO week numbers

ISO 8601 weeks start on Monday, and week 1 is the week that contains 4 January (equivalently, the first week with a Thursday). As a result the first days of January can belong to the last week of the previous year, and the last days of December to week 1 of the next. 31 December 2024 is in ISO week 1 of 2025, and 3 January 2021 is in week 53 of 2020 — the calculator shows the week-year whenever it differs from the calendar year.

Examples

Old dates and the calendar change

The calculator applies Gregorian rules to every date, including those before a country adopted the Gregorian calendar. Most of Catholic Europe switched in 1582; Great Britain and its colonies, including those in North America, switched in September 1752, when 2 September was followed by 14 September. A birth recorded in the older Julian calendar (“Old Style”) therefore shows a different weekday here unless it is first converted to the Gregorian date.

Your birthday’s weekday in future years

A common year is 52 weeks plus one day, so a birthday moves forward one weekday each year, and two when a 29 February falls in between. The table in the results lists the weekday of your next ten birthdays. For a countdown to the next one, see the Birthday Countdown Calculator; for your full age and days lived, the Date of Birth Calculator.

How to use the day of birth calculator

1. Enter the date of birth — day, month and year.
2. Press Calculate. The result shows the weekday you were born on, the day of the year, the ISO week number, and the weekday of your next ten birthdays.
3. Check older dates carefully: for anyone born before their country adopted the Gregorian calendar, convert the date first (see "Old dates" above).

The same calculator works for any date, not only birthdays: an anniversary, a historical event, or the weekday a document says something happened on.

Checking it by hand: the Doomsday rule

You can find the weekday of a date in your head with John Conway's Doomsday rule. Each year has a "doomsday" — a weekday shared by easy-to-remember dates: 4/4, 6/6, 8/8, 10/10 and 12/12, the last day of February, plus 9/5, 5/9, 7/11 and 11/7.

Example: 15 August 1947.

1. The century anchor for the 1900s is Wednesday.
2. Take the last two digits, 47. Divide by 12: 3 remainder 11. Divide the remainder by 4: 2 (ignore the fraction).
3. Add them: 3 + 11 + 2 = 16. Take 16 modulo 7 = 2. Two days after Wednesday is Friday — that is 1947's doomsday.
4. 8 August is a doomsday date, so it was a Friday; 15 August is exactly one week later, also a Friday.

The calculator agrees: 15 August 1947 was a Friday. The rule is a fun way to check a result, but the calculator's day-number method is the one that is guaranteed exact for every Gregorian date.

More dates people look up

Common mistakes

• Swapping day and month. 05/06 is 5 June in India and the UK but 6 May in the US. Type the month by name if you are unsure.
• Using a converted or corrected birth date. If your certificate was re-issued with a different date, use the date the calculator should check, and keep the original for official purposes.
• Mixing calendars. Dates in the Hijri, Shamsi or Indian national calendars need converting first; the Hijri date converter and Shamsi date converter handle two of them.
• Reading meaning into the weekday. Rhymes like "Monday's child is fair of face" are folklore and fun — not a description of anyone.

Summary

The weekday of any date is found by counting days from a date whose weekday is known and taking the remainder after dividing by seven. The calculator does this exactly for every Gregorian date and adds the day of the year, the ISO week and the weekday of your next ten birthdays. For dates before the calendar change, convert from the Julian calendar first, and use the Doomsday rule if you enjoy checking the answer in your head.