Pi Day is on Sunday, March 14, 2027, and it falls on March 14 every year, because the date 3/14 matches pi’s first digits, 3.14. It celebrates pi, the ratio of a circle’s circumference to its diameter. Fans mark it with math games, pie and digit-reciting contests.
When Is Pi Day?
It’s always March 14. The most popular moment is 1:59 p.m., since 3.14159 are pi’s first six digits. UNESCO also marks March 14 as the International Day of Mathematics.
Who Started Pi Day?
Physicist Larry Shaw started it in 1988 at the Exploratorium, a science museum in San Francisco. Staff marched in a circle at 1:59 p.m., matching the digits that follow 3.14.
In March 2009, the U.S. House of Representatives passed a resolution recognizing March 14 as Pi Day. The date is also Albert Einstein’s birthday, and early Exploratorium celebrations sang “Happy Birthday” to him too.
What Is Pi, and Why Celebrate It?
Pi is the number you get when you divide any circle’s circumference by its diameter, about 3.14159. It’s irrational, so its digits never end and never fall into a repeating pattern.
Archimedes made one of the first accurate estimates around 250 B.C. He trapped a circle between two many-sided shapes and narrowed the gap. The day turns that idea into something fun, especially for students.
How many digits have been calculated?
About 300 trillion. In 2025, Linus Media Group and storage maker Kioxia set a Guinness World Record with that many digits. The calculation ran for about seven and a half months, TechRadar reported.
How to Celebrate Pi Day
- Bake a pi pie: cut the π symbol into the top crust of an apple or cherry pie.
- Measure circles: wrap a string around cans, plates and lids, divide by the width and see how close you get to 3.14.
- Hold a digit contest: see who can recite the most digits of pi from memory, with a pie as the prize.
- Celebrate at 1:59: pause for a snack or a toast at 1:59 p.m., just like the first Pi Day marchers did.
- Try NASA’s challenge: JPL publishes a yearly set of space-themed pi puzzles for students.
- Host a pie potluck: ask friends to bring sweet or savory pies, from key lime to chicken pot pie.
- Estimate pi with toothpicks: older students can drop toothpicks on lined paper, a method called Buffon’s needle.
- Do pizza math: measure a pizza’s radius and work out its area with πr² before you eat it.
How Many Digits of Pi Does NASA Use?
Just 15 decimal places, or 3.141592653589793, for its most precise navigation, according to NASA’s Jet Propulsion Laboratory.
That’s plenty. For a circle 40 billion miles across, the error would be about 1.5 centimeters. Measuring the whole known universe to the width of a hydrogen atom would take only 37 decimal places.
Frequently Asked Questions
Do you need to memorize pi?
No. For most school math, 3.14 is enough. Even NASA’s precise navigation stops at 15 decimal places.
Why is Pi Day linked to Einstein?
Albert Einstein was born on March 14, 1879, so many Pi Day events also celebrate his birthday.
What are the basic circle formulas?
A circle’s circumference is π times its diameter, and its area is π times the radius squared (πr²).
Is Pi Day an official holiday?
No. The House passed a resolution recognizing it in 2009, but it isn’t a federal holiday. Schools and offices stay open.