The answer depends on two variables – the size of your hands and the format of the hand sanitizer, (whether it is a liquid, gel or foam). The common gel format is used in an amount about the size of a quarter while a foam or liquid can resemble a few tablespoons, but the best indicator is simply coverage: You need enough of the product to be able to cover your hands entirely; front and back, between the fingers, the thumb, and all around the nails. Once you have applied the product, count how long your hands remain wet. In order for the sanitizer to kill the germs that can make you sick, your hands must stay wet with the sanitizer for at least 10 to 15 seconds. If you weren’t able to cover every part of your hands or the product didn’t remain wet for the required length of time, reapply.