National Pi Day is recognized today in the U.S. and around the world. Here's what to know about the quirky holiday, and how to celebrate.

What is pi? Pi can calculate the circumference of a circle by measuring the diameter — the distance straight across the circle’s middle — and multiplying that by the 3.14-plus number.

What is Pi Day, and why do we celebrate it every March 14? Here's what to know as math lovers around the world mark Pi Day.

What is Pi Day, and why do we celebrate it every March 14? Here's what to know as math lovers around the world mark Pi Day.

Scientists have calculated pi to 105 trillion digits, although most of us are more familiar with the approximation 3.14. But how do we know that pi is an irrational number?

Pi Day is celebrated on March 14 to honor the famed irrational number. Celebrate the day with these interesting facts about pi and Pi Day.

The calendar says it's time to dig into a popular dessert and celebrate math. So, what is the significance of pi?