I'm currently Pre-T. 100-115 lbs. Size 30-31... In girls I can fit a size 1,3.
when i take T im planning gaining weight and working out. My goal probably is at least 10-15 lbs.
I'm thinking about renewing my wardobe and i was wondering should I continue buying size 30-31 jeans or should i move it up a size? did ur waist changed? and how many size did u have to go up?

Why not 'renew your wardrobe' when your weight changes make it necessary? Changes can be slow to happen or not at all. If  they happen quicker, you can still buy clothes then and know that the size you choose will fit.

Yeah agreed.  buy clothes afterwards if at all possible.  If you must buy clothes, try used clothing stores. I've actually lost 10 lbs since starting T 9 months ago and my waist has gone down two sizes.  But that weight loss could be due to the fact that I have a stomach ulcer

Your waist for jeans-wearing purposes isn't the narrowest part, around your belly button, which can get thicker on T - it's the spot right above your hip bone, which should stay relatively constant or possibly even get smaller (That is, unless you actually are wearing your jeans up at your belly button).

just wait....

I've been on T 8 months now and my clothing still fits as well [if not better] than when i bought it.

I'm at about a 30 waist size [never wore girls pants and have no clue on the sizing there] and my ->-bleeped-<-s still baggy.