Reader Terrance Glover writes in:

Here's a tip for you (without having to print our a ruler).

Use a dollar bill to measure. Every piece of US paper money in your wallet/pocket/sock is 6" long. Folded in half, and you have a 3" measuring stick, and so on. I can't tell you the number of times I've used this trick at HomeDepot to get a rough idea of the size of something.