About Birthday Math
Birthday Math is a small, cheerful site about the numbers hiding in a birth date: half birthdays, golden birthdays, the weekday you were born and how common your birthday really is.
Most birthday calculators give you a date and stop there. We try to also explain the odd cases, like what happens to a half birthday when the month is too short, and to back every claim about "common" or "rare" birthdays with real birth records instead of guesses.
Where the numbers come from
Birth counts come from two U.S. government sources: the CDC's National Center for Health Statistics for 1994 to 1999 and the Social Security Administration for 2000 to 2014. Together they cover 85.7 million births, one count per calendar day. We use the version compiled by FiveThirtyEight, which is public. When a page ranks or compares days, February 29 is averaged over leap years only, so it can be compared fairly with the other 365 days.
Your own birthday page
Pick a date and you get a page with its own address, like /may-6, or /may-6-1992 with the year. Bookmark it, text it to your mom, drop it in the group chat before someone's birthday.
Corrections
If you spot a mistake, we want to know. Pages show the date they were last updated, and corrections are made as soon as they are confirmed.