Rarity of February 29 Birthday?

Having, or knowing someone with, a February 29th birthday is rare, right?  Actually, no.  February 29th comes around every four years during leap years.  This year, 2016 is a leap year, as was every 4 years prior to 2016.

Let's look at the rarity of a February 29th birthday.  Since February 29th only occurs every 4 years, one can reasonably estimate that the odds of being born on February 29th are four times lower than being born on any other day of the year.  This assumes that birthdays are spread equally across the year, which I will in this estimate.  During a non-leap year, an individual has a 1/365 (0.27%) chance of being born on a specific day.  Multiply this by 1/4 to get a 1/1461 (0.068%) chance of being born on February 29th.  Yes, it is lower, but not ridiculously lower to conclude February 29th birthdays are very rare.

How many people have February 29th birthdays?  The Earth's population currently sits at 7.3 billion, multiply this by the chance of being born on February 29th and you get approximately 5 million people with February 29th birthdays.  That's a lot!  In the U.S. alone, with a population of 319 million, we can expect around 220,000 people with February 29th birthdays.

What are the odds you know someone with a February 29th birthday?  Let's consider the average number of friends each facebook user has (338).  Multiply this by the odds of having a February 29th birthday and you find that a quarter of one person was born on February 29th.  Obviously one can't have a quarter of a person, but it shows that it isn't that unexpected to know someone with a February 29th birthday.  Add in the people you know who you aren't Facebook friends or people you know who don't have a Facebook account, and it's quite likely you do know someone with a February 29th birthday.

So is a February 29th birthday something to marvel over due to its rarity?  Nope.  Not at all.