[DeRock]

Here’s a better tip to measure things without a proper measuring device: spread out your hand on a table and measure the distance between your pinky and thumb. Remember that. Now when you need to measure something just measure it in number of pinky-thumb-stretches. I can quickly get the dimensions to +/- an inch by doing a few quick walks with my hand.

[qznc]

It is hand to remember a few finger/knuckles/elbow/shoulder combinations for common measures. One of your phalanges should be ~1 inch, for example, and one of your finger nails is probably ~1 cm wide.

[bombcar]

There's a reason that the English system of measurement had things like "hand" and "foot" - because when you're not measuring things exactly, close enough and commonly available is fine.

[adzm]

Or be like the mythbusters guy and get a ruler tattooed on your arm!