If there are 23 people in the same room, there is a 50/50 chance that two people have the same birthday. Sounds a bit surprising, but it's mathematically true!
The birthday paradox - also known as the birthday problem - states that in a random group of 23 people, there is about a 50% chance that two people have the same birthday. In a room of 75 there's even a 99.9% chance of two people matching.
The chance of: two people sharing a birthday would be 1 - (364/365), or 0.3%, or 1 in 370. three people sharing a birthday would be 1 - ((364/365)(363/365)), or 0.8%, or 1 in 122.
Since the world population is estimated at over seven and a half billion,1 you should, in theory, share your birthday with over 20 million people (~20,438,356).
The Least Common Birthdays
December 25 (Christmas Day) is the least common birthday, while January 1 (New Year's Day) is the second least common. December 24 (Christmas Eve) also makes the list as the 3rd least common birthday while July 4 (Independence Day) is the 4th least common birthday.
The only events that have a probability of 50% are theoretical events, such the probability for a number selected at random to be odd or even. No real event have such probability. Tossing a fair coin is such a theoretical event because there is no “fair coin” in reality.
Your April baby's birthday is pretty rare.
Your little one is unlikely to need to share the limelight on their birthday because April birthdays are not super common. Only two April birthdates made their way into the top 200 most popular birthdays (April 4th at 144th and April 11th at 189th), while 11 landed below 300.
According to real birth data compiled from 20 years of American births, mid-September is the most birthday-packed time of the year, with September 9th being the most popular day to be born in America, followed closely by September 19th.
23 people.
In a room of just 23 people there's a 50-50 chance of at least two people having the same birthday. In a room of 75 there's a 99.9% chance of at least two people matching. Put down the calculator and pitchfork, I don't speak heresy. The birthday paradox is strange, counter-intuitive, and completely true.
The probability that a person does not have the same birthday as another person is 364 divided by 365 because there are 364 days that are not a person's birthday. This means that any two people have a 364/365, or 99.726027 percent, chance of not matching birthdays.
While the 9th of September is generally regarded as the most common birthday around the world, this actually varies a little bit from country to country – it depends on the hemisphere that the country is in (because this depends on the seasons), the holidays that the country celebrates and its most prevalent religion ...
10 Least Popular Birthdays
While holidays are a popular time for baby making, babies are less likely to be born on popular holidays. Christmas, New Years, Christmas Eve, July 4th, Halloween, and some suspiciously Thanksgiving days all make the top 10 least common birthdays.
We are all players in the Birthday Paradox
If there are 23 people in the same room, there is a 50/50 chance that two people have the same birthday. Sounds a bit surprising, but it's mathematically true! In a room with a certain number of randomly chosen people, a pair of them will probably be born on the same day.
What are the rarest birthdays? Surprisingly, none is in February. They cluster around holidays. December 25, January 1, December 24, July 4, January 2, December 26, November 27, November 23, November 25 and October 31st.
January birthdays are super special.
Despite the star power January has, chances are your little January babe won't share a birthday with too many classmates. Only one January birthday lands among the top 200 most popular birthdays…and that's January 24th all the way down in spot 199!
There are some interesting outliers: January 1st and July 1st are extra common, because people with an unknown birth data are commonly assigned these birthdays.
May babies have relatively rare birthdays.
Only one May birthdate cracks the top 100 popular birthdays (May 23rd lands in 89th place). Meanwhile six May birthdays fall in the bottom 50 (the 4th, 11th, 13th, 26th, 27th, and 31st).
If the horse runs 100 races and wins 50, the probability of winning is 50/100 = 0.50 or 50%, and the odds of winning are 50/50 = 1 (even odds). If the horse runs 100 races and wins 80, the probability of winning is 80/100 = 0.80 or 80%, and the odds of winning are 80/20 = 4 to 1.
February 3rd is the only day where no one in history has ever been born. Despite much scientific study, there is no explanation for this phenomena. Historically it has been referred to as "the empty day" or "nobody's birthday". The years range from late 1700s to the 2010s.
What is a golden birthday? Your golden birthday is the year you turn the same age as your birthday – for example, turning 25 on the 25th, or 31 on the 31st. If you've ever wondered where the tradition comes from, you're not alone.