Measuring?

I realize this is an ignorant question, but how do you "measure" your horses. We don't really have anything to measure him by, but a tape measure...lol. But, how many inches = a hand? We're wanting to get Sundance height the next time we're on the farm, but I wasn't too sure about the measurements.

1 hand=4"

Ah....lol. Simple enough! Thanks so much!