Why do we have to keep getting haircuts? Can’t we make our hair stop growing? Well, we can’t: our hair and nails grow every minute of our lives — not fast enough to see them moving, but fast enough that if we let them go, we’ll end up like shaggy cavemen with claws. So our fan Jay A. asked us, which grows faster, our hair or our nails? Hair grows about 1/2 inch each month, but it’s a little different for everyone. If you measure your hair today and wait a couple of months for a haircut, you can find out your own hair speed! Meanwhile, fingernails grow about 1/8 of an inch each month. The longer the finger bone, the faster that nail grows, so your pinky fingernail grows the most slowly. Finally, your toenails grow only 1/4 as fast as your fingernails — good thing, because they’re harder to reach!
Wee ones: If you’ve clipped 7 of your fingernails so far, what number nail comes next?
Little kids: If you always want your hair to be between 9 and 14 inches long, is 17-inch hair okay? Bonus: If you just got a haircut in February and you want to grow out your hair for 5 months, when will you get your next haircut?
Big kids: If your hair grows 1/2 inch each month, how much should it grow in 1 year? (Reminder if needed: A year has 12 months.) Bonus: If your hair is 10 inches long and you think it would look awesome if it were 24 inches long, how many years and months do you need to let it grow?
The sky’s the limit: If your hair grows 1/2 inch each month, your fingernails grow just 1/8 of an inch, and your toenails grow 1/4 as fast as your fingernails, how many times as fast as your toenails does your hair grow?
Wee ones: Number 8.
Little kids: No, 17 inches is too long (longer than 14). Bonus: In July.
Big kids: 6 inches. Bonus: It needs to grow 14 inches, which would take 28 months, or 2 years 4 months!
The sky’s the limit: 16 times as fast, because your toenails grow just 1/4th of 1/8th of an inch, or 1/32 of an inch each month. Another way to see it is to flip all the fractions: hair takes just 2 months to grow an inch, fingernails take 8 months, and toenails take 4 times as long as fingernails, or 32 months — compared to just 2 months for hair.
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.