I spend most of my time on a road bike, but have several buddies who are pretty heavy into Mountain Biking & keep giving me a hard time about getting out & joining them. I see most of miles still being on my road bike, but am trying to find a used bike that I can go out with them on. Something probably off Craigslist or ebay; somewhere I can get a full suspension bike with disc brakes for less than 1200 bucks.

Anyway, as I started my research on mountain bikes, I quickly discovered they are not sized the same way as road bikes. My roadie is 54cm & I have noticed they are in different increments (17.5? 19.5? etc). I know that a Large Cannondale is too big (my old mt. bike is a Cannondale-large & it is comfortable for around town, but just a little too big for riding hard in the mountains.). Also I realize the most popular answer is going to be go to LBS & get measured, but I am trying to save some money & definitely don't want a new bike. I am just looking for an idea of my size so I don't drive 2 hours to look at a bike that is way off the map as far as sizing.

Any suggestions? Thanks in advance.

As you have already discovered, road frames are sized in centimeters and mountain frames are sized in inches -- go figure. If you are comfortable on a 54cm road frame, my guess is that you should be looking at a 17-18" MTB. Much larger than that and you will not have the requisite clearance above the top tube to prevent possible injury to your manhood...

Agreed.

I ride a 54cm Trek road frame and a 17.5" Trek MTB frame.

Manufacturers differentiate in sizing though.

This might be of help to you, thanks to good ol' Sheldon Brown.

https://www.sheldonbrown.com/frame-sizing.html

Old School / ProFail

Thanks for the info. Exactly what I was looking for. I realize they will all be just a tad different, but with current gas prices I didn't want to drive 90 miles to try something that was going way too big or small. At least now I have a range.

And I do want to take care of the manhood!

on most models you can fit on 2 sizes it's just what your comfortable on ..mountain bikes are odd... the numbers sometimes don't represent the actual size of the frame .. that 17.5 inch trek fuel actually has a 16.5 inch seat tube.

With modern ultra-compact frames standover becomes less of an issue, and what you want to pay attention to is TT length. Of course, that can also be effectively adjusted with stems and different setback seatposts. I love this about modern mountain bikes, it's way easier to shop used.

Generally you want the smallest size you can fit on. According to other people, at least. I feel more secure on larger bikes.