Formula to guess Month of Birthday from Singapore NRIC

Latest Update: We have created a JavaScript App to Guess Birthday Month from NRIC


Here is a Math Formula trick to have fun with your friends, to guess their Month of Birthday given their NRIC, within two tries.

(only works for Singapore citizens born after 1970)

The formula is: take the 3rd and 4th digit of the NRIC, put them together, divide by 10, and multiply by 3.

For an example, if a person’s NRIC is S8804xxxx, we take 04, divide by 10 to get 0.4

Then, 0.4 multiplied by 3 gives 1.2

Then, guess that the person is either born in January (round down 1.2 to 1) or February (round up 1.2 to 2). There is a high chance that you are right! Usually, round up for the first six months (Jan to Jun), and round down for the last six months (Jul to Dec).

This formula was developed and tested by me. There are some exceptions to the rule, but generally it works fine especially for people born from 1980 to 2000.

Hope you have fun with maths, and impress your friends!



Author: mathtuition88

Math and Education Blog

8 thoughts on “Formula to guess Month of Birthday from Singapore NRIC”

    1. Hi, thanks for your comment!
      Unfortunately deriving the day of birthday from Singapore NRIC is not possible to my knowledge.
      For deriving month of birthday, you can follow the formula listed on this site, it gives a high percentage of accuracy.


  1. You need to explain how the “theory” works, or how you derived it. eg. How did you determine when to round up/round down? Is there an official link describing how the 3rd and 4th digit is derived to get an explicit month? Are you just implementing “magic” (coincidental) numbers here without any actual basis?

    Anyway, i have another method using some real sg-birth stats and averages. (again, speculation only). See link below to compare:

    Liked by 1 person

    1. Nice method you have there. No doubt yours is more accurate as it uses more information. My method is a “party trick” formula that can be calculated mentally without computer, derived based on empirical evidence in a job tabulating thousands of NRIC and birth dates.


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