Mountain Biking - Roadie to Mountain Morph!

I'm a roadie getting his first mountain bike! When I started on the road I found a few sites that offer sizing calculators. By now, I now and understand the geometry of a road bike. But... Is there any site with a calculator for mountain bikes? For example, I know my inseam is 31.1" So, do I need a 16" or 18" bike??

It depends on the specific bike and how you want it to fit. I'd guess an 18" would be good for the inseam you list, but you'd really have to get on a few and try it out. I have ~30" inseam and like a 17.5-18" frame, depending on the manufacturer. If you're just starting out on a MTB, I'd suggest not oversizing the bike. On a roadie, you don't need to reach the ground unless at a stop light (then you can just find the curb). On a MTB, you sometimes need to get a foot on the ground real quick...you don't want to be cutting it too close on the standover.

http://www.competitivecyclist.com/za/CCM?PAGE=FIT_CALCULATOR_INTRO

With their powers combined, I AM CAPTAIN PLANET! This topic screams "do that!"

It depends on the specific bike and how you want it to fit. I'd guess an 18" would be good for the inseam you list, but you'd really have to get on a few and try it out. I have ~30" inseam and like a 17.5-18" frame, depending on the manufacturer. If you're just starting out on a MTB, I'd suggest not oversizing the bike. On a roadie, you don't need to reach the ground unless at a stop light (then you can just find the curb). On a MTB, you sometimes need to get a foot on the ground real quick...you don't want to be cutting it too close on the standover.

Seems like a good tip. Thanks! I'll keep it in mind when it's time to choose the bike.