Estimate Distances with Your Arm and This Rule of Thumb - measurements - Lifehacker
http://lifehacker.com/5351728/estimate-distances-with-your-arm-and-this-rule-of-thumb

Quick anatomy lesson: the distance between your eyes is one-tenth the length of your arm. This is important, because it helps you estimate the distance between yourself and any object of reasonably known size to a rather accurate degree. Let's begin this with a confession: I can't estimate distances worth a darn. 100 ft, 300 ft, 20 m, I can't really do it. Thankfully with this trick, we can get pretty accurate distance estimates with only the tiniest bit of math, a general idea of the size of the object you're estimating distance from, and an arm. Basically, all you need to do to estimate a distance using this classic method is hold your arm out and stare at your thumb with an eye closed. Silly, yes, but it makes sense, because you'll switch which eye you look through and sort out how far the object appears to have "jumped" to the side: