For men: measure around the fullest part of your chest, with the tape passing just under the armpits and across the shoulder blades.
For women: measure around the fullest part of your bust, while wearing a non-padded bra. This is your bust measurement, used for tops and dresses.
Keep the tape horizontal and parallel to the floor. Breathe normally — don't expand your chest artificially.
Common chest sizes (men): S = 36-38", M = 38-40", L = 40-42", XL = 42-44".
If you're between sizes for jackets, size up. You can't tailor a jacket that's too tight in the chest, but a slightly loose one can be taken in.