Today I picked up this 1996 GT Outpost Trail, for $10.
(I am not knowledgeable on MTB’s, but thought it best to buy it first and research it later.)

It has a combination High Ten & Chromolly frame, Shimano Altus Group, and Weinmann double wall alloy rims. It is in fine condition.

My question is - What size is it?
How do I measure frame size when the seat tube sticks up an extra 2" like that?

I personally don't count the upper part of the seat tube. A mountain bike is typically a little longer center to center than it is tall, center to center. That looks to be about an 18" bike, no?

Measure it like a road bike: Seat tube from the center of the BB to the center of the TT. Unlike most road bikes, most of the MTBs are measured in inches.

Thank you Luker and E.

That would make it an 18" bike then - not a 20".

For $10 you got a great little bike! I have one just like that but in black, it looks to be 18". If you are into retro mountain biking, you can easily fit an older Rock Shox Mag10 or Mag21 with the 1" threaded steerer.

geek

Thanks Geek, I have to agree people who own these are lucky - it is a great little bike.

As for me though, I plan to sell it in the Spring, after I clean it and overhaul the bearings/cables. (It's too small for me, plus, I already have a larger Marin Pine Mountain equipped just as you describe -along with an untold number of road bikes ).

-If I was to keep it though, I would have entertained making it into an urban commuter. It has an excellent frame for that, and the braze-ons for racks, fenders, etc.

fantastic bike. THis was my first real MTB. Exact colors and everything. Still have it and still ride it here and there. It rides great. Much better than my new specialized. I never liked that thing.

Wow - that is an impressive endorsement for a humble GT Outpost Trail!
(It's amazing more people are not wise to it - but that's why you can still have a good chromolly MTB for such a bargain price.)