What Do the Numbers on the Side of My Tyres Mean?

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What Do the Numbers on the Side of My Tyres Mean?

One of the most common questions we get asked is, "What do the numbers on the side of my tyres mean?" The simple answer is, they tell you the size. Which as you can imagine is pretty important. It is also one of those things that seem to stress people out and get them confused

So sit back, strap in, grab a cuppa and we'll explore the fascinating world of tyre sizing. We may be a bit guilty of hyperbole there.

What size of tyres fits my rims?

If you want to buy new tyres for your bike, it can be a good idea to get ones that actually fit your wheel rims. This piece of information might seem obvious, but we see lots of people who manage to get this wrong.

Which can bring you a whole host of issues. 

So to make this easy and to save you from reading anymore, here is an excellent guide. We were going to make our own, but we quite like the Schwalbe one, so we'll use it. It still stands true for other brands of tyres.

bicycle tyre size guide

That chart is excellent, but you still haven't told me what the numbers mean.

Okay, hold your horses, we just wanted to break you into the number slowly. Did you grab that cup of tea yet?

bicycle tyre sizing

Like everything in the bike trade, there are standards, and for those standards, there are more standards, and then there will be things that don't like any of those standards, so they are not a standard more a small standard used in a niche by someone who wants cycling to be more expensive. Is that even possible?

Bloody standards, no one ask us about bottom brackets, or we'll just go and have a cry in the corner.

Tyres were great for this. Back in the days of yore, the time many British people seem to want to go back to, tyre sizing had a lot of standards. Some made sense but most were just whatever somebody decided sounded good on a wet Wednesday afternoon in March. We don't want to go back to these times as buying tyres was a nightmare thanks to these standards.

If you get stressed buying tyres now, you'll have been knackered back in the days of yore.

E.R.T.O. is the standard

Somebody had enough of this, and well they decided to something about it. They created the European Tyre and Rim Technical Organization or E.R.T.O. As it's known to its friends.

Then to make things that little bit snazzier, the name of the standard was changed. The International Organisation for Standardisation also called the ISO to annoy those of us with O.C.D, took over the standardisation of tyres. Which means those numbers on the side of your tyre are now part of ISO 5775.

One of the great things about this standard is that we no longer have six different sizes of 26" tyres. Which is amazing if you want to buy one or just retail them.

I've read this far and I still don't know what the numbers mean

Sit back, we're just about to tell you.

ISO 5775 or E.R.T.O comprises of two numbers. The first number is theoretically how wide the tyre inflates. We say theoretically as this is the bike trade and we don't want to be stuck on actualities. That's for squares or some other comment that I imagine young people make.

The second number is the rim size. The second number is the one that confuses people. The reason it confuses people is that it is not the number we use when we talk about rim size.

Road wheels tend to be described as 700c wheels but if we use their E.R.T.O/ISO number then we'll see that they are 622. Why then do we call it a 700c wheel?

That is because that is what the old French system used to call the wheel size. If you know anything about cycling, you know that cycling has a love affair with France. Not because of the Tour, no because that is where the best cheese and wine comes from. (This might not be a fact)

The French system rather than measuring the rim measures the outer part of the tyre. We told you this would be exciting. We're just about to make it even more exciting.

27.5" is one of the "newer" trends on the block and it is usually referred to as 650b. So now we've added a letter to the numbers. What do the letters mean? Well, technically a tyre with no letter is narrow. According to the French as a tyre gets wider you should go up the alphabet. So a 650a is narrower than a 650b and 650 b is narrower than a 650c.

For your information, the inch system is a little less precise than the E.R.T.O and the French system. Have a little look here.





Size marking 


28 x 1.40 

28 x 1 5⁄8 x 1 3⁄8 

700 x 35C 

Outer diameter 


ca. 28 inch 

ca. 700 mm 

Inner diameter 

622 mm 



Tire width 

ca. 37 mm 

ca. 1 3⁄8 inch 

bzw. 1.40 inch 

ca. 35 mm 

Tire height 


ca. 1 5⁄8 inch 



If you are one of those people who still use tubes, then you'll want to use the E.R.T.O system if you want to buy some for your bike. 

Now you can go and amaze your friends with your new-found knowledge.

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