How to Calculate the Day I Was Born

By Samuel Markings; Updated April 24, 2017
The known, fixed number of days in a month helps in calculating your birth weekday.

Unless your parents told you, it is unlikely that you know the day of the week you were born. However, you can harness simple mathematical algorithms to use calendar constants -- such as 7-day weeks and 12-month years -- to calculate the day you were born.

Write down your date of birth. For the sake of this example, let's use the date March 10, 1966.

According to mathematicians Burkard Polster and Marty Ross in the Plus Magazine article "On What Day of the Week Were You Born?" the table below should be used to add the relevant table entry to the day-of-month born:

Jan: 6 Feb: 2 Mar: 2 Apr: 5 May: 0 Jun: 3 Jul: 5 Aug: 1 Sep: 4 Oct: 6 Nov: 2 Dec: 4

Following the example, the day of the month is 10 and the month is March so :

10 + 2 = 12

Use the following table to find the number that should be added to the result of section 2:

1900: 1 1910: 6 1920: 5 1930: 3 1940: 2 1950: 0 1960: 6 1970: 4 1980: 3 1990: 1 2000: 0 2010: 5

Following the example, since the date we are using is in the 1960s, we use the number associated with this in the table above:

12 + 6 = 18

Add the final number of the year to the result obtained. Following the example, the year was 1966, so the final number of the year is 6:

18 + 6 = 24

Next we need to take into account the number of leap years. For this step the following tables will be required:

Even numbers:

0: 0 1: 0 2: 0 3: 0 4: 1 5: 1 6: 1* 7: 1 8: 2 9: 2

Odd numbers

0: 0 1: 0 2: 1 3: 1 4: 1 5: 1 6: 2 7: 2 8: 2 9: 2

Write down the decade number of the date. If the decade number is even, use the the table for even numbers, and find the table entry that corresponds to the final number within the year. If the decade is odd, the table for odd numbers and find the table entry that corresponds to the final number within the year.

Following the example, the decade is the 60s, and therefore 6 is the decade number, which is even. We will therefore use the even table above. The final year number of 1966 is 6; we will therefore use the number that corresponds to 6. The entry has been marked with a star. Add the number found within the table to the result of section 3:

24 + 1 = 25

Finally divide the answer obtained in step 4 by 7 and take note of the remainder. Following the example:

25 / 7 = 3 remainder 4

If the date written was January or February of a leap year (these are the years in even decades that end in 0, 4 or 8) then subtract 1 from the answer. The value of the final remainder corresponds to the day of the week. Day 4 is Thursday, and therefore this is the day that corresponds to March 10, 1966.

About the Author

Samuel Markings has been writing for scientific publications for more than 10 years, and has published articles in journals such as "Nature." He is an expert in solid-state physics, and during the day is a researcher at a Russell Group U.K. university.