Most hardtail mountain bikes will use 26inch, 27.5inch or 29inch wheels.

26inch wheels are the most common. 27.5inch are only slightly bigger, but can make a small difference rolling over uneven terrain. These are nimble and manoeuvrable.

27.5 inch wheels (also known as 650b) are gradually taking over from the 26inch wheel size, so expect these to become more prevalent.

29ers (29 inch wheels) are similar to the size of road wheels. These roll quickly, and can smooth out the feel of hard-tail bike, riding more easily over rough terrain.