Most of us have been in the situation with a flat tyre. The most common reason for a flat tyre is the inner tube but sometimes the tyre needs to be changed too. The easiest way to find a right size tyre is to order a new one with the same specs than the old one.
There are basically 3 different size marking methods for tyres and that can be a mess for person without the knowledge. Tyres use different measures in different countries and you need to know the basics to find the right one.
Tyre size standards
ISO
ISO consists from two different numbers that indicate the tyre dimensions. First number with two digits tells the width of the tyre and the second 3 digit number tells the tyre’s inner diameter (rim diameter).
For example 23-622 is a common road bike tyre size where 23 is the width in millimeters. 622 is the diameter of the rim that the tyre fits in millimeters. 622 is the most common in many bikes because it means that the wheel is 28 inches. The numbers are not exact because there are different kind of tyre structures that can increase the actual wheel size. Basically they just indicate that the tyre will fit to the rim.
Here is a chart of different ISO tyre sizes
622 = 28”
559 = 26”
507 = 24”
406 = 20”
305 = 16”
203 = 12”
It could sound like a great idea to order a wider tyre for smoother riding but you have to think about the structure of the rim and bike frame. Some wider tyres will not fit inside the narrow road bike forks. Even if they fit it can be that the tyre will not stay inside the rim if it is too wide for it.
French way
Other standard that is quite common is the french marking that giver the outer diameter of the tyre with the width. Compared to the earlier example in ISO standard 23-622 it has the same width and diameter but now we are talking about the outer diameter and not the rim diameter. So the markings will be 700 x 23c in French. This method is quite self explanatory except the letter at the end which indicates the inner diameter. Because we are talking about the same tyre size than before the “c” means 622mm.
Inches
Thanks to all of those that do not use the metric system we have to have the markings in inches too. This method consists of three different numbers that indicates the outer diameter, tyre height and tyre width. Normally the dimensions are marked in classical fractional numbers and not decimals.
