So here's the deal, I'm 16 and I'm hitting 78-79 consistently on the mound and 81-82 from the outfield. I'm a sophomore 6' 1" and about 140 lbs. Last year I was sitting 70-73 max but after actually picking up some weights I'm where I'm at today. I'm still growing like my dad and brother did until their senior year I just want to know if I really could put on weight and muscle like my brother who is 6' 3" 215 lbs how hard would I be throwing. I'm not a huge velocity guy more of a sinker ball pitcher actually but it's always good to get an idea on your potential you feel?
So many factors contribute to velocity progression that it is actually impossible to be able to predict how hard you'll throw with the added size/strength. Will adding quality weight help gain velocity? Yes most likely, considering you already have a pretty good fastball for someone weighing 140 pounds. At 6'1 140 you should probably aim for gaining about 1-1.5 pounds every week. I highly recommend checking out LankyLefty's (think it's titled Till I collapse) blog on this site considering he was in a similar spot early in his high school career and now hes around 215 pounds and pitching in professional baseball. I'd also recommend posting a video of your mechanics from a side view and front view.
If you post your mechanics people may be able to give a rough estimate, but without seeing them you can't say; for all we know, you just step and flick the ball and have 20+ to add from mechanics, or they're flawless.