how do i measure my saddle seat size?

[luckyslove2]

ok, i am getting a new saddle any day now, but i was wondering how i should measure my rump to get the correct saddle size for me, i already know the size for my horse. (i need to know wheather i need a 14",15",16",17", or an 18") do i measure from hip to hip or what? i have never had a saddle of my own before, i use the community one at our stable. so i wouldn't be able to measure from pommel to cantle or whatever...
please leave nice answers!

thanks to all answers in advance!