Use your hand as a ruler

Rulers are those impractical tools: you’re never carrying one when you need it. But you know what you do likely have on you? Your hand.

By measuring your hand once and making a mental note of it’s measurement, you can have a trusty measuring device whenever you need it.

Just remember the height of your hand from the wrist to the end of your tallest finger and use your hand to estimate length of an object or space.