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How to Measure Bike Tire Size: Traditional & ISO System

It’s that time again. You need to change your bike tires. But what size are your tires? Worry not. We’re here to show you all about bike sizing systems as well as teach you how to measure your bike tire size. Measuring bicycle tires is not difficult, and it will be even easier once you read our guide on the issue. So read on, then get your bike, a tape measure, and get to work.

Tire Sizing Systems

In this article, we separated tire sizing systems into two parts. The first part consists of the traditional systems, while the second includes the ISO method. You can read below to find out more about them.

Traditional Tire Sizing Systems

There are many tire sizing systems from the past since, at one point, every country that manufactured bicycles was using its own system of tire sizes. Every size system was different, so the same tires would have different sizes in different countries. As a result, the same numbers would often be used for different-sized tires.

The traditional sizing systems measure the tire’s outside diameter and the tire width. The diameter would be measured in inches or millimeters, while the width of the tire would be presented by a second number or code. For example, 700C  is a nominal outside tire diameter that shows that the tire width is 700mm, and the letter “C” refers to the tire width, one a scale from A to D, A being very narrow, and D being wide. This system that included letter codes as well was called the French system.

Furthermore, the width would sometimes be expressed in decimal and sometimes as a fraction, which caused many mismatches. Although mathematically equal, different size numbers and designations often referred to different size tires and were not interchangeable.

The ISO Tire Sizing System

In need of a standardized system, the International Organization for Standardization (ISO) developed a universal metric-based tire and rim sizing system. This system is more accurate and reliable than the other measuring methods. It was originally developed by the European Tyre and Rim Technical Organisation (ETRTO). The system uses two numbers, a two-digit number and a three-digit number, separated by a hyphen. The first number indicates the width in millimeters. The number shows the inflated width for tires, while for rims, the number shows the inner width between the rim flanges. The second number is also expressed in millimeters (mm) and shows the rim’s bead seat diameter. This is the number to look out for when wanting to change tires. If this number matches, the tire will fit into the rim.

For example, a 700 x 20C tire in the standard tire system would be a 20-622.

The tire should fit the rim properly and not be too narrow or wide (for the rim). When the tire is too narrow for the rim, there are high chances that the tire will get damaged from the road. Meanwhile, if the tire is too wide for the rim, there are chances that the brake shoes will cause sidewall wear, risking losing control. The tire width should be somewhere between 1.45/2.0 x the inner rim width. If you measure the total width from bead to bead width (with beads pulled apart), approximately, it should be about 2.5 x the ISO width.

To help you better understand the ISO tire sizing system, below we present two examples, one of an ISO tire size and one of an ISO rim size.

  1. 57-584

Start by looking for the bigger number. In our case (57-584), the bigger number is 584. This number indicates the bead seat diameter (often called a BSD) is 584 mm when measuring bead to bead. The other number that we have left shows that the inflated tire width is 57 mm.

  1.  622×15

Start from the smaller number; 15. In this case, we understand that the inside diameter of the flange, from flange to flange, is 15 mm. The second number, the bigger one, shows the diameter of the rim’s bead seat, which in our case is 622mm.

How to Measure Bike Tire Size

how-to-measure-bike-tire-size

You can measure your bike tire size in four easy steps. All you will need is the bike wheels, a tape measure, and the text below.

Start by securing the wheel

To get the right and accurate measurements of your bike tire, you need to make sure that the wheels will not move while you do your work. First, stand the bike upright. You can do that by leaning it against a wall or using the kickstand. You can even get someone to help for faster results.

Use your tape measure to determine the diameter

Next, use a tape measure to determine the length from the center of the wheel to the tire’s inner edge. To get the diameter measurement, proceed multiplying that length by two. That’s traditional sizing. However, if you are interested in determining the ISO diameter, measure in millimeters from the center of the wheel to the tire’s inner edge and double it.

Proceed by measuring the tire width

This means measuring the flat surface across the tire’s tread from one side of the tire to the other. Remember that traditional width measurements are in inches, while the ISO method uses millimeters. Measure accordingly. Also, keep in mind that the width will vary depending on the use of the tire. For rougher terrains, wider-width tires are used, while narrow-width tires are used for smoother and faster rides.

Combine those measurements

Since you measured the diameter and width of the bike tire, the last step is combining the measurements to find your tire size. Keep in mind that the order of your measurements will determine the size system. As mentioned earlier, the traditional sizing places the diameter first and the width second. The ISO sizes put the width first and the diameter second.

It might seem challenging having to deal with bike tire measurements, especially when considering the many sizing systems for them. Still, by following our guide on how to measure bike tire size, you can quickly come to a clear conclusion.

Whether it be to determine the size of your tires based on the traditional system or the one set by the International Organization for Standardization, in four easy steps, you will have your tire size numbers.

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