If you pay attention to the goings-on within the bicycle industry, particularly the MTB side of things, you may have spotted some chat about a new wheel size. But before I dive into that, it might be worth just recapping where we are with bicycle wheel sizes as of right now. I’m not going to cover every single wheel size since the beginning of bicycle history, just the common sizes from the past 20-odd years, sizes you’re likely to have come across if you consider yourself a bike rider.

Let’s start with road bikes, at the beginning of my bike riding life, most road bikes had 27” wheels. This was close to the 700c size that most road bikes currently use. 27” was a popular size in the UK and the US but become pretty obsolete as the 700c designation that was common in Europe started to take over. The diameter of the 27” was actually a little bigger than 700c. There’s also a 650c size (not to be confused with 650b - see below) which is pretty rare but makes an appearance on some time-trial and tri bikes as well as race bikes for juveniles and smaller riders.

When mountain bikes got really popular in the 80s and 90s they all had 26” wheels, in the 2000s several manufacturers introduced the 29” wheel that most MTBs currently use. Interestingly the 29” wheel size is exactly the same size as the road 700c size. Fast forward a few years and we have an in-between size, 27.5” (also known as 650b). Not only is this popular in the mountain bike world, but has become pretty common on gravel bikes.

You might be a little confused at this point with the way we’re describing tyre sizes, you’ll have noticed that we’ve been mixing units and jumping around quite a bit. For tyres that are listed in inch sizes (eg 27”, 29”) these numbers refer to a very rough estimate of tyre diameter. Same with 700 or 650 except in this case it’s the tyre dimater in mm. The ‘c’ or ‘b’ suffix designates the width of the rim for that size of tyre. It’s not a number you can rely on but it is a convenient way of grouping tyres together.

Thankfully there are some standards that very accurately define tyre and wheel sizes and all current manufacturers adhere to these standards. The ISO wheel standard for bicycles describes a dimension that relates to the diameter of the rim where the bead of the tyre sits. Sometimes known as BSD (Bead Seat Diameter). An example of this would be ISO622 which is what all 700c and 29” rims/tyres fit into. Expanding on the ISO standards is the ERTRO standard. This takes the ISO dimension and adds a tyre width value. Like ISO, this is described in mm. An example of an ERTRO size would be 32-622 which is a 700c wheel with a 32mm tyre. I’ve added a chart at the bottom of this page that lists some common wheel sizes and their current standard designations. Check the sidewall of your tyre and you’ll see the ERTRO size.