Here’s a list of the most common wheel sizes for each discipline:

MTB

26″, 27.5″ and 29″: 26″ can still be found on older bikes and on some smaller models. 27.5″ offer a balance between the agility of 26″ wheels and the rolling resistance and stability of 29″ wheels. 27.5″ wheels are used on many mountain bikes but the 29″ size is the most popular choice for MTB. They offer greater performance over rough terrain and obstacles, as well as better traction on climbs and stability at high speed.

Road & Gravel

700c: Standard wheel size in road and gravel cycling.

650b: Popular in gravel as they can be used with wider tires and provide greater comfort and traction on uneven terrain.

Things are a bit different for smaller bikes. If you want to look into all the available wheel sizes, from 12″ to 24″