How Many People Were Born The Same Day I Was?

// December 20th, 2011 // Information, Personal

I recently became 28, and started thinking to myself, what is the probability of meeting a random person that was born on my birth date.

Normally we would say that meeting someone that shares your same birthday has the probability of 1/365, which means that you have a 0.274% chance of meeting someone that shares your birthday.

However, in real life, peoples’ birthdays aren’t distributed equally. For example, I found a dataset online with 481040 birthdates for an insurance company. This dataset shows that Aug. 18 had the highest number of births, while Dec. 26 had the least number of births.

The chances of me meeting someone that shared my same birthdate became 0.266% which meant that my birthday had a lower number of births than the average. That also means that for every thousand people I meet, there are 2.6 that share my birth date.

Then I started thinking, what are the chances of me meeting someone that was born the same year I was born. Now we could just assume that the average lifespan of a human is 68 according to the World Health Organization, and say that my chances of meeting someone are 1/68. However, in real life that’s not the case. The number of births increase exponentially. According to this wikipedia entry:
“The population of the United States of America is exponentially increasing at an average rate of one and a half percent a year (1.5%). This means that the doubling time of the American population (depending on the yearly growth in population) is approximately 50 years.”
Plus the numbers of deaths per year are not the way they were a century ago. We have fewer wars now, and the number of deaths has also decreased due to advancements in medicine.

Back to the question, what are the chances of me meeting someone that was born the same year I was born? Recent stats show that there are 7 Billion humans on this earth. The year I was born had about 78 million births. According to the UN data, it was estimated that 105 die between birth and the age of 5 for every 1000 births during the year I was born. It is also estimated that there are about 50 deaths per 1000 people between the ages of 5 and 29 for various reasons for natural causes to wars (I’m not 29, but its close enough). So, we can say that around 850 per 1000 people born in the same year I was born are still alive today. That means 66.3 million of the 7 billion people on this earth were born the same year I was born.

So, the chances of me meeting a person born the same year I was born are 0.947%.

Finally, what are the chances of me meeting someone that was actually born on the same day I was born? If we take the probability of being born on the same day I was born, and apply that to the 66.3 million that were born the same year I was born. Then we would come to the result of having about 176280 people in the world that were born on the same day I was born and are still living today.

The chances of me meeting someone born on the same day I was born are 0.0025%.That means that there are only 25 people in every million, that were born on my exact birth date! So, given the population in Montreal of 1.6 million, there are only 40 people!

5 Responses to “How Many People Were Born The Same Day I Was?”

  1. that was interesting. a close friend of mine was born on Dec 16. (her birth year is either 1985 or 1984 i don’t remember). 1 thing though is that you are taking global stats for your calc (ie. birth rates & death rates, and child mortality rates) however the birth/death pattern may not necessarily be the same in every regions. is some regions (climate zones) people may perfer to (or have more oppertunity to) concive a child durig a particular season due to availability of resources. for example, lets say in a certain rural village in china with a population of 500,000 where the population is 100% dependent on agriculture. their lives are set to match the seasonal nature of the crops that they harvest. in these types of communities u will observe that a large number of births take place at a particulat time of the year (ie. just before harvest). in another part of the world, let say beki-beki-stan-stan (a country somewhere n africa) where the people’s lives styles are completely different and thus would not demonstrate this pattern. the point to this little narrarative is that if you were born in that chinese farming village then the probability of you meeting someone with your birth day or month is very high. however if you migrated to the african nation, the probability would shift.

  2. Feras says:

    Hey Mirza,

    You are right, things differ in different parts of the world. I’m not saying the stats are 100% accurate, but they should be close. BTW, loved the Hermain Cain reference (beki-beki-stan-stan).

  3. :) says:

    First of all i have to admit that i didn’t understand the details but i get the main idea LOL .It was really interesting. I never met anybody with the same day i was born :(
    But i know tow of my friends were born on December 16.
    Happy Birthday To you Feras 4*4 / 12 / 1984 .

  4. Feras says:

    Hi :) ,

    Glad you found it interesting. I don’t think I know anyone born on your birthday (If you are who I think you are). But I do know someone that was born the same week.


  5. Abdulrahman Abahsain says:

    Hey Feras,
    It is interesting to know that if you gather 23 people you will have a 50% chance that 2 has the same birthday but thisis totally different. Check a famous stat problem called “Birthday Paradox” ….

    What will you be thinking about when you are 68? “How many people will die the same day as you?” :p