March 14 is Pi Day!  

Why March 14?  In countries that that report their dates in month/day format, March 14 (3/14) corresponds to the first three digits of the number pi (3.14).  Pi Day was instituted in 1988 by physicist Larry Shaw.  Those who observe Pi Day typically celebrate by eating round foods (like pizza, doughnuts, and of course, pie) and taking part in some pi-related activities like those listed below.

What is Pi?

I am embarrassed to admit that despite my many years of math classes (even at the college level), I never really understood the significance of pi until I was well into my thirties.  I could use pi to calculate the area of a circle and the like, but It wasn’t until I began to home-school my own sons that I finally learned what pi actually was.

Pi (often written as π) is the number equal to the ratio of a circle’s circumference to its diameter.

The distance around a circle is its circumference.  The distance across the center of a circle is its diameter.
The distance around a circle is its circumference. The distance across the center of a circle is its diameter.

In other words, if you measure the distance around a circle (its circumference) and the measure across a circle (its diameter), the ratio of the circumference to the diameter will always be pi. This is true for all circles, no matter how large or small.

The ratio of a circle's circumference to its diameter equals pi
The ratio of a circle’s circumference to its diameter equals pi

I remember years ago when my sons were first introduced to the concept of pi, their math curriculum suggested a hands-on activity to demonstrate this fact.  I will share it with you.

Calculating Pi Using the Measurements of a Circle

You will need:

  • A circular object (a cup, a plate, the lid of a jar or coffee can).  Try a few different objects so you can see that the technique works no matter what circle you use.  Make sure your object is a true circle. This won’t work with objects that are ellipses, ovals, etc.
  • Yarn (string can also be used)
  • Scissors
  • A ruler or measuring tape

What to do:

Yarn is used to measure a circular object's circumference and its diameter.
Using yarn to measure an object’s circumference (top) and diameter (bottom)
  • Wrap a piece of yarn around the circumference of your circular object.  Here, we are using the round lid to a jar. Use your scissors to cut the yarn.  The length of the yarn will then equal the circumference of your circular object.  We’ll call this our circumference piece.
  • Using your ruler, measure and note the length of your circumference piece.
  • Take some new yarn and stretch it across the diameter of your circular object.  Cut the yarn to equal the length of the object’s diameter. We will call this a diameter piece.
  • Use your ruler to measure and record the length of your diameter piece.
  • Repeat the process to make 3 more diameter pieces of yarn.
  • Stretch out the circumference piece of yarn.  Beside it, lay your diameter yarn pieces end to end.  

How many diameter pieces did it take to equal the length of your original circumference piece?  I’m gonna guess it took 3 whole pieces and a little bit of the 4th piece. In other words, it took pi diameter pieces to equal the circumference piece. You just demonstrated pi.

You can also use a calculator to divide the length of the circumference piece that you measured by the length of one of your diameter pieces.  

For example, when I used my ruler to measure the length of my pieces, my circumference piece measured 42.7 centimeters while one of my diameter pieces measured 13.6 centimeters. Using my calculator, I divided the length of my circumference piece by the length of my diameter piece.

42.7 centimeters 13.6 centimeters = 3.14* . In other words, pi.

More Facts About Pi

Pi is an irrational number that has no end.  It is an infinitely long, non-repeating number.  In recent years, pi has been calculated to over one trillion digits!  The website piday.org has pi listed to the first million digits. Rajveer Meena holds the Guiness World Record for memorizing the most digits of pi.  In 2015, he recited nearly 70,000 digits of pi.  It took him almost 10 hours to do it.

Check out this video to see what a million digits of the number pi looks like.

The first recorded history of pi traces back to the ancient Babylonians.  Evidence has been found that indicates that the ancient Greeks, Egyptians, and Chinese all calculated pi.  You can read more about the history of pi here.

How to Celebrate Pi Day

The internet abounds with ideas for ways to celebrate Pi Day.  Here are links to a few posts I’ve found that include ways to celebrate the day. This is by no means an exhaustive list. Do a quick search and you can find many more ideas.

Ways to Celebrate Pi Day 2019   

25 Best Ways to Celebrate National Pi Day  

Math Geek Mama shows how to play a fun game called The Race to Pi on her website. As an added bonus, by playing this game you’ll be well on your way to memorizing the first seven digits of pi.  I have played this in a classroom setting and it was lots of fun.

Don’t feel like you have to go out of your way to make Pi Day spectacular. Celebrating Pi Day can be as easy as pi(e).

What’s your favorite way to observe Pi Day? Share your ideas in the comments.

Further Information

In this video from SciShow, we learn three ways “pi can explain practically everything”.

Pi Makes Perfect Sense in These Simple Animations

10 Surprising Facts About Pi

*when rounded to three significant digits

Pi Day is celebrated each year on March 14. What is pi? What is pi used for? How can you demonstrate pi for yourself at home? Learn facts about pi (including who holds the world record for memorizing the most digits of pi) and get ideas for ways to celebrate Pi Day. #piday #pi #math #homeschoo

Leave a Comment