Well, since all I wear is Levi's 501's, 1 size, equals 2 inches, eg. 38 to 36.
BTW (just to brag, more than anything else

) October 09', I was wearing size 40's, that were stretched as far as humanly possible, and were still trying to flip over / down, at the waist line + cutting into me big time ! In other words, I really needed 42"s. 9 months later, I'm wearing 34 comfortably
Oh, and now (below size 34) my 501's are available in half sizes 33"s, 31"s....
But I doubt that will ever matter. As my jeans (after washing) are starting to get tighter on my thighs, which are getting bigger from muscle now, before they are tight on my waist. Geeez, I haven't had that problem for about 27 years !
I'm even thinking that I might have to go to a different style jeans (looser legs) to be able to keep wearing 34"s.
Peace,
Fish