How do they decide the right size MTB for you ?

I was told a MTB needs to be 2 sizes smaller then your road bike. That is if you're gonna use it for off-road. If you're only gonna ride on the road then get the same size as your road bike.

What is your inseam measurement? If using a MTB for off road purposes you need at least 2" clearance over the top tube.

Quoted post said:

Originally posted by Lenny
I'm in the process of buying my first MTb and I am dubious about the arbitary method of the bike shops deciding the right size frame.

Generally, all they do is look you up and down and say 19". or 20" or 21" .. ( usually depending on what's sitting on the showroom floor ) I'm 6'1"" tall and don't want to buy the wrong size.

Should I find a bike shop that has something called The Original Fit Kit - which at least has a scientific basis for measurement...

I suggest you read the fine article on fit at Peter White Cycles Website, URL:
http://www.peterwhitecycles.com/fitting.htm
David Ornee, Western Springs, IL

There are a few good sites to help figure out what size you should buy. I like the fitting system at Wrench Science

One thing I've heard over and over when considering MTB size is that top tube length makes a bigger difference than frame height. You can adjust this to some extent with stem length, but changing stem length much changes the handling characteristics.

I'm between 6' and 6'1" and I'm riding a Trek 21" hardtail. It's a little too big for me, but I bought a stem that's a little shorter and I can get by for now just fine. It was too good a deal to pass up at the time.

http://www.coloradocyclist.com/bikefit/index.cfm

Thanks guys for all the helpful replies and links to fitting websites....

I ended up buying a 21" Giant XTC 2 Disc

You can calculate it by inseam and height. At 6'1" it is almost certain you will require something in the vecinity of 19"-20" with a top tube length of approx 24".