MTB tire sizes explained
By Christian Jensen - 15.03.2026
If you’re in the market for new tires, or you’re simply wondering what all the numbers on the sidewall actually mean, this is the place to start. Not long ago I wrote about choosing the right inner tube, and this post is the follow‑up. There are two common sizing systems for mountain bike tires: ETRTO, which is now part of the ISO standards ISO 5775‑1:2014 (tires) and ISO 5775‑2:2015 (rims), and the traditional American inch‑based system.
But enough about standards—what do the numbers really mean? As mentioned, mountain bikes use two systems: the ETRTO format and the inch format. ETRTO lists the tire width first, followed by the rim diameter, separated by a hyphen. So if your rim diameter is 622 mm (also known as 29″) and your tire is 56 mm wide, the tire will be labeled 56‑622. Simple enough. Another 56‑622 tire should be very close in width and identical in rim diameter.
However, there are still differences, because these numbers say nothing about the tire’s profile. That’s why one 56‑622 tire may fit perfectly, while another might rub the frame.
If we convert 56 mm to inches, we get 2.2″. And here’s where things get confusing. Some manufacturers list their tires as 2.2″ wide but use a different millimeter measurement because the standards allow it. I’ve seen both 55 mm and 57 mm widths on tires labeled 2.2″. The next point is the 622 mm rim diameter. That’s actually only 24.5″. In other words, the diameter of a “29‑inch” rim is just 24.5″. The same applies to 27.5″ (which is really 23″) and 26″ (which is 22″). Why? Because the “29 inches” refers to the approximate outer diameter of the tire once mounted. A 2.25″ tire adds roughly 2.25″ from the rim edge to the tread surface. So 24.5″ + 2 × 2.25″ = 29″. The same logic applies to the smaller wheel sizes.
To make things even more interesting, a 700C road rim and a 28″ hybrid rim share the exact same rim diameter. And yes—a 28″ hybrid bike uses the same rim diameter as a 29″ mountain bike. This is exactly why the ETRTO system was introduced: to make tire sizing easier to understand.
So next time you glance at the sidewall and see 64‑622, you’ll know that 64 mm corresponds to 2.5″, and that 622 mm means you’re looking at a tire designed for what we commonly call a 29″ wheel.