When you're looking for the right size of bike for you there are a couple of factors that will come into play, the first of which is your inseam, but also the kind of bike and your style of riding. For most of the bikes that we sell at REI, you will see a 'Size Chart' with up to 10 different measurements for each size of bike in that style. To start, you want to look at the 'standover height' of the bike, which directly relates to your inseam, typically expressed in millimeters, so you may need to convert measurements. It is worth noting here that standover height is your inseam with shoes on, not necessarily the inseam most people use when shopping for pants, as an example.
Generally speaking, if you are looking at a road bike or a hybrid bike (not a step-through) then you are looking for around 1" of extra room between your inseam and the standover height of the bike. If you are looking for mountain bikes, you are looking for around 2" of extra room. You can find a lot more detail and useful information in this Expert Advice article, Bike Fitting Basics.
Hopefully this helps get you started, please let us know if you have more questions!
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