Jeans, slacks, shorts, leiderhosen, whatever. The real point is to place the measuring tape through existing (well fitting) belt looped garments to get a good approximate number. It's safer than measuring against the skin, as the tape can slip and slide to give a not-so accurate number.

And NEVER take Levi's or anyone elses numbers to be accurate. A 38 in Levi's can be a 42 in someone else's book. Always measure. You would be AMAZEd how many times we had people order their jeans size only to send it back for altering since it was 3-4 inches too small.

Measure twice... order once.