# Chances of 2 people sharing a birthday=50%? **URL:** \ **Category:** Factual Questions\ **Created:** [September 29, 2004, 3:35pm UTC](https://boards.straightdope.com/t/chances-of-2-people-sharing-a-birthday-50/266675 "2004-09-29T15:35:38Z")\ **Posts on this page:** 14\ **Page:** 3
**Author:** ![Bryan\_Ekers](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bryan_ekers/32/183_2.png) [@Bryan\_Ekers](https://boards.straightdope.com/u/Bryan_Ekers)\ **Post date:** [October 1, 2004, 4:28am UTC](https://boards.straightdope.com/t/chances-of-2-people-sharing-a-birthday-50/266675/41 "2004-10-01T04:28:14Z")
> [@TJdude825](#): > > What if the Oct. 1 part is important? > > Before **Fuji** told us anything, the probability that he had a birthday of Oct. 1 was 1/365. The probability that his boss did was 1/365. The probability of both of these (1/365)^2 = 1/133,225 = 7.51 \* 10^-6 > > In laymen’s terms: Very small. Yes, but that’s true of any two birthdays. Say Fuji’s birthday was May 7th and the boss’s was August 23rd. What are the odds of these two people having those birthdays? 1 in 133,225. ---
**Author:** ![MartinL](https://avatars.discourse-cdn.com/v4/letter/m/ebca7d/32.png) [@MartinL](https://boards.straightdope.com/u/MartinL)\ **Post date:** [October 1, 2004, 8:03am UTC](https://boards.straightdope.com/t/chances-of-2-people-sharing-a-birthday-50/266675/42 "2004-10-01T08:03:39Z")
> [@Fuji Kitakyusho](#): > > My boss and I (the only two people in our company) discovered after working together for six months that we were both born on October 1. > > What are the odds? I don’t know, but: Happy Birthday! 🙂 ---
**Author:** ![t-keela](https://avatars.discourse-cdn.com/v4/letter/t/a3d4f5/32.png) [@t-keela](https://boards.straightdope.com/u/t-keela)\ **Post date:** [October 1, 2004, 9:15am UTC](https://boards.straightdope.com/t/chances-of-2-people-sharing-a-birthday-50/266675/43 "2004-10-01T09:15:34Z")
So let’s see…Fuji, his boss, Pushkin and the temp, all conceived on New Years… I wish I still had a distribution chart on birthdays. Sounds like there might’ve been some partying going on in the world that night…hmm. but next morning… :smack: I’ve got two nephews and a couple of cousins with birthdays this weekend. My grandfather too, may he RIP. oh yeah, happy birthday y’all 🙂 ---
**Author:** ![TellMeI\_mNotCrazy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tellmei_mnotcrazy/32/3419_2.png) [@TellMeI\_mNotCrazy](https://boards.straightdope.com/u/TellMeI_mNotCrazy)\ **Post date:** [October 1, 2004, 10:43am UTC](https://boards.straightdope.com/t/chances-of-2-people-sharing-a-birthday-50/266675/44 "2004-10-01T10:43:41Z")
Well since the question has pretty much been answered I’ll throw in some neat birthday coincidences that have occured in my life: (all are different years) My sister and I have the same birthday - hers 10 years after mine. My father and ex-mother in law My brother and ex’s grandfather (with whom ex and I lived) My daughter and my older sister Those aren’t big stretches - no fifth cousins fourteen times removed. Makes everyone’s birthday easier to remember though. ---
**Author:** ![muttrox](https://avatars.discourse-cdn.com/v4/letter/m/a8b319/32.png) [@muttrox](https://boards.straightdope.com/u/muttrox)\ **Post date:** [October 1, 2004, 11:53am UTC](https://boards.straightdope.com/t/chances-of-2-people-sharing-a-birthday-50/266675/45 "2004-10-01T11:53:23Z")
> [@t-keela](#): > > First of all. A classroom of kids is not a random sample. Since it is a classroom the odds are nearly 100% that they will be the same age. So the birth year is practically a given. There are other restrictions in a classroom setting which increases the odds of an exact month. Cuttoff months and days for advancement as an example. I don’t recall the exact month but there is usually a specific date that is the cutoff date for the age at which a child can start school. **All of that aside…** the biggest variable which can be applied to even a random sample is the fact that there are times when people simply have sex more often than others. Like Valentine’s day for instance. Practically every new married couple is going to have sex on Valentines Day. Combine all these things and the odds are increased that two kids in a classroom will have the same date of birth. somewhere in mid-late October. Then there’s June newliweds and Christmas etc. but V-tines is the biggie. In my own family (extended as well) there are at least a dozen birthdays that month. [/QUOTE] ---
**Author:** ![muttrox](https://avatars.discourse-cdn.com/v4/letter/m/a8b319/32.png) [@muttrox](https://boards.straightdope.com/u/muttrox)\ **Post date:** [October 1, 2004, 11:57am UTC](https://boards.straightdope.com/t/chances-of-2-people-sharing-a-birthday-50/266675/46 "2004-10-01T11:57:12Z")
Premature eposteation. > [@t-keela](#): > > First of all. A classroom of kids is not a random sample. Since it is a classroom the odds are nearly 100% that they will be the same age. So the birth year is practically a given. Birth year is irrelevant, this phenomona only concerns itself with month and year. > [@](#): > > There are other restrictions in a classroom setting which increases the odds of an exact month. Cuttoff months and days for advancement as an example. I don’t recall the exact month but there is usually a specific date that is the cutoff date for the age at which a child can start school. Again, competely irrelevant as long as each year does in fact encompass all 365 days. > [@](#): > > the biggest variable which can be applied to even a random sample is the fact that there are times when people simply have sex more often than others. Like Valentine’s day for instance. Practically every new married couple is going to have sex on Valentines Day. Combine all these things and the odds are increased that two kids in a classroom will have the same date of birth. > somewhere in mid-late October. Then there’s June newliweds and Christmas etc. but V-tines is the biggie. Please provide a cite that this is a large scale phenomona. Considering the other myths about when people have sex (the great NY blackout ferinstance), this needs to be backed up. ---
**Author:** ![CurtC](https://avatars.discourse-cdn.com/v4/letter/c/ce73a5/32.png) [@CurtC](https://boards.straightdope.com/u/CurtC)\ **Post date:** [October 1, 2004, 3:11pm UTC](https://boards.straightdope.com/t/chances-of-2-people-sharing-a-birthday-50/266675/47 "2004-10-01T15:11:58Z")
> [@Fuji Kitakyusho](#): > > My boss and I (the only two people in our company) discovered after working together for six months that we were both born on October 1. > > What are the odds? You have to be more specific about what exactly you want the odds for. If you’re asking what are the odds that a given two people whare the same birthday, then it’s 1 in 365. The odds that you both share the same birthday, and that this date is October 1, is 1 in 133225. The odds that you will eventually have a boss sometime in your life with the same birthday as you is, what, 1 in 15, depending on how many bosses you’ll have. The chance that you will, at some point in your life, be paired up with another person who shares the same birthday is very high. When I got out of college and got a real job, I was surprised to find out that there were three others in the building (out of maybe 150 people) who shared my birthday (four of us total), and my secretary and I were born on the same exact day. ---
**Author:** ![Crandolph](https://avatars.discourse-cdn.com/v4/letter/c/7cd45c/32.png) [@Crandolph](https://boards.straightdope.com/u/Crandolph)\ **Post date:** [October 6, 2004, 8:40am UTC](https://boards.straightdope.com/t/chances-of-2-people-sharing-a-birthday-50/266675/48 "2004-10-06T08:40:24Z")
> [@moggyAU](#): > > in fact i know many people who share the same birthday, some born in the same hospital on the same day and same year … and how are the quintuplets these days? 😃 ---
**Author:** ![guito13](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@guito13](https://boards.straightdope.com/u/guito13)\ **Post date:** [October 6, 2004, 2:47pm UTC](https://boards.straightdope.com/t/chances-of-2-people-sharing-a-birthday-50/266675/49 "2004-10-06T14:47:00Z")
I share the same birthday with my sister 3 years apart…and my daughters have the same birthday…but…they are twins 🙂 ---
**Author:** ![t-keela](https://avatars.discourse-cdn.com/v4/letter/t/a3d4f5/32.png) [@t-keela](https://boards.straightdope.com/u/t-keela)\ **Post date:** [October 6, 2004, 9:14pm UTC](https://boards.straightdope.com/t/chances-of-2-people-sharing-a-birthday-50/266675/50 "2004-10-06T21:14:58Z")
> [@muttrox](#): > > Please provide a cite that this is a large scale phenomona. A cite that couples in the US get romantic on Valentine’s day? A cite that the world celebrates on New Year’s Eve? You’re kidding right? These things need to be proven to you. ---
**Author:** ![TellMeI\_mNotCrazy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tellmei_mnotcrazy/32/3419_2.png) [@TellMeI\_mNotCrazy](https://boards.straightdope.com/u/TellMeI_mNotCrazy)\ **Post date:** [October 6, 2004, 9:19pm UTC](https://boards.straightdope.com/t/chances-of-2-people-sharing-a-birthday-50/266675/51 "2004-10-06T21:19:55Z")
> [@t-keela](#): > > A cite that couples in the US get romantic on Valentine’s day? > A cite that the world celebrates on New Year’s Eve? > > You’re kidding right? These things need to be proven to you. I don’t think it can safely be assumed that there’s a much higher incidence of sex on Valentine’s day, I was married 10 years, and never happened to have sex on Valentine’s Day that I can recall. Regardless, say every couple DID have sex on Valentine’s Day; the percentage of those couples for who Valentines was a fertile period, and further, the percentage of fertile couples who actually conceive, I have a hard time believing that there’s any significant increase in Valentine’s babies over any other day of the year. ---
**Author:** ![TellMeI\_mNotCrazy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tellmei_mnotcrazy/32/3419_2.png) [@TellMeI\_mNotCrazy](https://boards.straightdope.com/u/TellMeI_mNotCrazy)\ **Post date:** [October 6, 2004, 9:21pm UTC](https://boards.straightdope.com/t/chances-of-2-people-sharing-a-birthday-50/266675/52 "2004-10-06T21:21:29Z")
That should have said …the percentage of those couples for who Valentines was a fertile period, and further, the percentage of fertile couples who actually conceive is the same as any other day, so I have a hard time believing that there’s any significant increase in Valentine’s babies over any other day of the year. ---
**Author:** ![teleute12](https://avatars.discourse-cdn.com/v4/letter/t/3d9bf3/32.png) [@teleute12](https://boards.straightdope.com/u/teleute12)\ **Post date:** [October 7, 2004, 12:32am UTC](https://boards.straightdope.com/t/chances-of-2-people-sharing-a-birthday-50/266675/53 "2004-10-07T00:32:31Z")
I used to know a gal on a different message board who had the same birthday as me, down to the year. It was a community of about… well, there are 304 people registered to the board, but a better grouping would be the 60 people on my chat list. What are the odds that, in a group of 60 people, two people would have the exact same birthday? ---
**Author:** ![Wendell\_Wagner](https://avatars.discourse-cdn.com/v4/letter/w/8491ac/32.png) [@Wendell\_Wagner](https://boards.straightdope.com/u/Wendell_Wagner)\ **Post date:** [October 7, 2004, 3:47am UTC](https://boards.straightdope.com/t/chances-of-2-people-sharing-a-birthday-50/266675/54 "2004-10-07T03:47:59Z")
No, a cite that there is a significant tendency for more people to be born on some days than on other days. I’ve never read of such a tendency, and you’d think that it would be well known if it were true. Your theory sounds clever, but I know of no evidence that it’s true. I don’t even know of any evidence that people have more sex on some days than others, let alone that more people are born on some days than others. [Previous page](https://boards.straightdope.com/t/chances-of-2-people-sharing-a-birthday-50/266675.md?page=2)