Mountain Biking - Tire Width Question

Right now I have the stock tires on my MTB (26" X 2"). In the near future, I'm going to be getting new tires. How do I know what width will fit other than 2" and less? Many of the tires I'm looking at vary from 2.1" to 2.5". I'm sure I could get away with 2.1" but what if I want to go wider? I'm probably going to order online so there's no way I can preview a fit. Thanks in advance.

Well here is the thing

1 - tires don't use a standardized way of measuring. For example Maxxis 2.7 mobster is really around 2.5 while Michelin 2.7 will be closer to 2.8
2 - Different fork companies fit different sizes. (2003 marz won't fit anything bigger than 2.7 while 02 marz fits 3inch tires
3 - Different frames fit different sized tires. My frame for example fits a 26" 2.7 OR 24" 3 inch tire...

so what I am getting at is...without more info I can't help you...I couldn't even hazard a guess. Sometimes it is just trial and error and you walking around with a measuring tape. If it is a newer bike it shoudl EASILY fit 2.1 but may have problems with 2.5. If you havea marzocchi fork no problems but if you have another one...I don't know.

See its confusing...

The post above is good, but here's a simple formula if you want one: your bike will fit 2.2" and smaller, for sure. It's the 2.3's and larger where you're in the range that they'll bind in certain frames or on certain forks when compressed, etc.