If you get a tape measure and put it round you roughly where the largest area of your behind is, then wiggle it up and down slightly to see what the biggest number is. That is what I always suggest using. Now, some of us expand somewhat when we sit down. Some more than others !
it,s always a good idea to keep hold of the tape measure and then sit down. Allow the tape measure to expand if needed. That is the finished measurement you will need in your trousers. Measuring when standing is all well and good but you have to sit down too!