You want to be sure your kitchen scale is calibrated right, or your recipes aren't going to turn out very well. Baking expert Alice Medrich recommends using standard US coins to check.
A standard penny weighs 2.5 grams, while a nickel weighs 5 grams and a quarter weighs 5.67 grams. You can find out the weight of all coins at the United States Mint official page. It's easy to remember to check a penny or a nickel because the weights are always standard, much like you can use a dollar bill as a ruler.
Merdich adds that you shouldn't use misshapen coins as their weight will be different. When in doubt, add more coins before deciding your scale is off. If it's inaccurate, recalibrate your scale. But if you're weighing a single penny, there's something you need to know, as Merdich discussed on Food52:
Only scales meant to be accurate to .5 grams (or less) will register the correct 2.5 grams for a single penny. If your scale is meant to be accurate to only 1 gram (like mine), the weight of a single penny will be rounded up to 3 grams. Stay calm and add a second penny: If the two pennies weigh 5 grams, then all's right with the world.
While five grams aren't going to matter in checking your weight, it's still an easy way to check your bathroom scale and track your weight accurately. Just use more coins than a single penny.