Two Peas a Day Keeps the Dentist Away

Here's your nightly math! Just 5 quick minutes of number fun for kids and parents at home. Read a cool fun fact, followed by math riddles at different levels so everyone can jump in. Your kids will love you for it.

Two Peas a Day Keeps the Dentist Away

March 15, 2018

How many tooth brushings should you get out of a tube of toothpaste? And if you’re getting more than that, are you using too little toothpaste? What is the right amount of toothpaste, anyway? These questions came up when our fan Maggie H. asked, how many squeezes can you get from one tube? We can figure it out easily using math. Dentists say you should use a blob the size of a pea. Peas are about 8 millimeters wide, which if you do some grown up math (half the width times itself times itself, times 4/3 of pi), that comes to about 0.01 fluid ounces. That gives us about 100 squeezes per ounce. Then it depends on the size of the tube. Now let’s see how that shakes out with your toothbrushing routine!

Wee ones: A pea can fit on the tip of your finger. Find 3 things in your room that can fit on your fingertip.

Little kids: Give your tube of toothpaste 1 big squirt onto a clean counter (so you can still use it!). How many peas of toothpaste do you think you see?  Bonus: If you brush twice on Sunday, once on Monday, twice on Tuesday, once on Wednesday…how many times will you brush Friday to keep the pattern?

Big kids: If you get 300 squirts from one tube, 400 from another, and the 3rd tube’s number is halfway between, how many squeezes did you get in TOTAL from the 3 tubes?  Bonus: If you can get 400 tooth brushings out of your tube starting January 1, and you brush only once a day, what date will be your last day on that tube? (Assume no leap years.)










Wee ones: Items might include a teeny Lego piece, a fluff of lint, or a crumb.

Little kids: Different guesses for everyone…if you have peas handy, line them up to check your answer!  Bonus: You will brush once.

Big kids: 1,050 squirts, since you get 350 from the 3rd tube.  Bonus: On February 4 the following year. You use 365 the first year, leaving you 35 brushings for the next year. You use 31 of those in January, leaving 4 for February.

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About the Author

Laura Overdeck

Laura Overdeck

Laura Bilodeau Overdeck is founder and president of Bedtime Math Foundation. Her goal is to make math as playful for kids as it was for her when she was a child. Her mom had Laura baking before she could walk, and her dad had her using power tools at a very unsafe age, measuring lengths, widths and angles in the process. Armed with this early love of numbers, Laura went on to get a BA in astrophysics from Princeton University, and an MBA from the Wharton School of Business; she continues to star-gaze today. Laura’s other interests include her three lively children, chocolate, extreme vehicles, and Lego Mindstorms.

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