29ers and 26 inchers also not what you think

Here are the actual rim diameters of the three main mountain bike wheel sizes: 26in, 27.5in and 29in. It’s all a bit confusing.

>>> Best mountain bike wheels: a buyer’s guide

Actual rim diameters of mountain bike wheel sizes

  • 26in = 22in
  • 27.5in = 23in
  • 29in = 24.5in

We just happened to have a wheel of each main wheel size lying around in the same place for once so we thought we’d grab our tape measure and see what the rim diameters actually are. The numbers above are the results.


27.5in rims are actually 23in diameter (where the tyre bead sits)


29in rims are actually 24.5in diameter (where the tyre bead sits)


26in rims are actually 23in diameter (where the tyre bead sits)

(Apologies for the slight variation in mm measuring accuracy. I needed a third hand but didn’t have one! The different inner depth of the rim walls is also an issue with these pics.)

Why are wheel sizes wrong?

Wheel sizes. Nothing has come closer to causing the mountain bike world to self-destruct more than folk arguing about them.

But did you know that your wheel size of choice isn’t what’s written on the tyre sidewalls?

This is not a new problem. Dating back to so-called 26in wheel mountain bikes, the name of wheel sizes have been misleading.

The reason why mountain bike wheels ended up being called 26in is that it was a rough approximation of the diameter of the outside of the tyre ie. a 22in diameter rim with a 2in tyre on it. Remember you need to factor in the tyre twice so 2 x 2in = 4in. Thus a 22in rim + 4in of tyre = 26in.

This rather odd way of describing mountain bike wheel sizes continued when Gary Fisher came out with the “twenty niner”. Again, the wheel size number stated (“29 inches”) referred to an approximation of the diameter of the outside of the tyre, this time fitted with 2.25in tyres (as that was the average tyre width at the time). So 24.5in rim + 4.5in of tyre = 29in.

Then the 27.5in (AKA 650b) wheel size came along. This is arguably the most cynically mislabelled wheel of the lot. By calling it 27.5in the bike industry gave the impression that it was halfway between a 26in wheel and 29in wheel. It isn’t. “27.5in” rims are only 1in wider in rim diameter than 26in. They’re 1.5in smaller in diameter than 29in rims. So not a halfway house really.


See that “Rim Size: 584 x 23c”? That’s this rim’s ETRTO.


Truth be told, we didn’t really need to break out the tape measure. We could have just looked at something called the ETRTO measurements.

ETRTO stands for European Tyre and Rim Technical Organisation. You may have noticed this acronym on your rims. ETRTO is the best measurement of wheel size. The ETRTO number is the inner diameter of a tyre. Kind of like a measurement of tyre bead diameter.

  • 26in = 559mm ETRTO
  • 27.5in = 584mm ETRTO
  • 29in = 622mm ETRTO

So there you go. That’s how big your wheels actually are.

29ers are only 1.5 inches bigger than 27.5 bikes. 29ers are 2.5 inches bigger than 26in bikes. 27.5 bikes are only 1 inch bigger than 26in bikes.

Does this mean we can start the wheel size debate all over again? Oh sh..