How to Measure Bike Wheel Size

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The size of your bike’s wheel is something many casual riders often ignore—if a replacement or upgrade is needed, you can simply take it to the shop and leave it to the professionals.

However, if you prefer to do your upgrades and replacements, knowing how to measure bike wheel size is crucial.

The old-school way of how to measure bike wheel size is to have the bike upright and grab a tape measure. You then measure the wheel’s radius and double it. If you remember your math from school, diameter = 2 x radius!

However, there’s a bit more to it than that. I’ll show you how to measure your bikes’ wheel size properly, whether using the traditional system or following the ISO method.

Bike Wheel Sizing Systems

Taking a tape measure and measuring your wheel might seem like a pretty straightforward thing to do.

However, it’s essential to know how to get the measurements right between the two systems.

Traditional System

This system is based on the outside diameter of the wheel. The measurement of the diameter always comes before the width when representing the numbers.

Note that units are in inches and are always in full increments. Meaning, if you get a number between inches, you’ll have to round it up or down, depending on the number.

Although this system is the traditional way to measure your tires, it isn’t the most accurate. Rounding off the numbers essentially means you’re going to be off by up to nearly half-an-inch in some instances.

As bike tires began evolving, companies created their own sizing structure. This made it even more challenging to find a compatible tire for your bike.

In fact, in the United States alone, at least six different wheel measurements are marked as “26-inch.”

ISO

To eliminate doubt or inconsistencies regarding wheel sizing, ISO 5775 was put in place by the International Organization for Standardization to prevent overlapping information.

Unlike the traditional system, ISO is presented as a code, with the wheel’s nominal width first and nominal diameter second. The two numbers are separated by a hyphen and often followed by the tire’s recommended inflation pressure marked in kilopascals (kPa).

For example, “28-590 inflate to 400 kPa” sizing means the wheel has a nominal width of 28mm and a nominal diameter of 590mm with a recommended inflation pressure of 400 kPa.

The nominal width measurement is the width of the inflated tire without the tread. Additionally, the nominal diameter measurement is the inner diameter of the tire mounted on the rim, and it’s the most important number to get right.

Knowing your wheels’ nominal diameter measurement can help you identify a correct replacement size, even if other details vary or aren’t accessible.

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How to Measure Bike Wheel Size: The Traditional Way

Now that we know about the two sizing systems, it’s time to learn how to measure your bike’s wheel size.

For both methods, you need these wheel measurements:

  • Diameter.
  • Width.

Measuring the wheels doesn’t require any specific tools or sick math skills—all you need is:

  • Tape measure.
  • Pen.
  • Paper.

Step 1: Stand the Bike Upright

Lean your bike against a wall, secure it on the kickstand or get a friend to hold the bike while you measure. The bike should stand securely in place so it won’t tip over you as you measure the wheel.

If you’re alone and need to hold the bike with one hand, use a retractable metal tape measure. A metal tape measure’s stiffness will make it easy for you to measure the wheel using one hand while holding the bike with the other.

Step 2: Measure the Radius

Start by measuring the distance from the ground right beneath the tire to the center of the wheel. This is the radius of the wheel in inches.

Multiply the number by two to get the diameter of the wheel.

Fun fact: Apart from BMX bikes, which are known for having smaller wheels, most adult bikes will have a wheel size between 26 and 29 inches.

Step 3: Measure the Width

Place the tape measure on the flat part of the wheel across the tread to measure the width.

The tire’s width will usually depend on the intended use. For instance, mountain bikes require more traction than street bikes and are usually fitted with deeper treads and knobs.

However, a wider tire is preferred for rough terrain, whereas a narrower tire will provide a smoother and faster ride on tarmac.

Step 4: Put the Numbers Together

Remember, we’re going with the traditional system here. Therefore, you have to put the diameter first and the width second.

How to Measure Bike Wheel Size: The ISO System

Keep in mind that ISO measurements are in millimeters, so ensure you have a metric tape measure or ruler.

If you don’t have one, you can measure in inches and then multiply the number by 25.4 (since there are 25.4 millimeters in an inch).

ISO is the most reliable way to measure your wheel size at home, so follow these steps to get it nailed:

Step 1: Secure the Bike Upright

Before you get started, make sure the bike is standing upright against a wall, in a stand or kickstand.

Step 2: Measure the Radius

Take your metric measuring tape and measure from the center to the inner edge of the tire. This will give you the wheel’s radius.

Multiply that by two to get the diameter.

Keep in mind; most adult bikes have a wheel diameter ranging between 650 to 700 millimeters in the ISO system.

Step 2: Measure the Width

Using the same method as the guide above, measure the width of the tire. Remember, go across the tread, measuring from one side to the other.

The width isn’t as important as the diameter—most bikes are compatible with various widths, as long as the difference isn’t too drastic.

Step 3: Understand the Numbers

With the ISO system, you’re listing the width first and diameter second.

Also, keep this in mind when you’re buying a new wheel, so you don’t accidentally get the numbers the wrong way around.

Additionally, ensure the wheel you’re buying states its ISO measurements. If it’s by the traditional system, the sizes could be off.

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How to Measure the Circumference of a Bike Wheel in Three Easy Ways

The circumference is the outside distance around a wheel. Knowing your bicycle wheel’s circumference is essential if you want to properly calibrate a speedometer, odometer, cyclometer or GPS.

When you change wheel size or width, the circumference will be affected and give you inaccurate data.

Method 1: The Power of Pi

One of the quickest and easiest ways to measure the circumference of your wheels is by using pi.

All you have to do is multiply the diameter—from one outer edge of the tire to the other, as explained in the first guide—with pi:

Formula: Circumference = Diameter x pi

While, technically, pi doesn’t end, you can use it to two decimal places: 3.14

For example, if your wheel’s diameter is 29 inches, the circumference is:

29 x 3.14 = 91.06 inches.

Method 2: From Dot to Dot

This method might be slightly messier than multiplications, but it’s effective.

  1. Simply dot some wet paint onto a spot on the bikes’ tread.
  2. Push the bike forward slowly in a straight line for two rotations, making sure there are two dots on the ground.
  3. Grab your measuring tape and measure from one dot to the other.
  4. The result is your wheel’s circumference.

One slight issue with this method is the wet paint you’ll get on the ground. Make sure you work fast and wipe it off as soon as possible. If the color begins to set, you can use paint thinner or brake fluid, then scratch it off with a rough sponge.

Method 3: A Piece of String

If you don’t feel like doing calculations or cleaning paint off your garage floor, you might prefer this method.

All you need is a string long enough to wrap around the outside of the wheel.

  1. As you wrap the string around the wheel, try to keep it centered—some treads have grooves around the outside that could give you a wrong measurement.
  2. Once the string is wrapped around the wheel, mark it with a pen or cut it off where the two ends meet.
  3. Lay the line on a flat surface and measure it from one end to the other.

Instead of a string, you could also use a sewing tape measure.

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Getting It Right

Knowing how to measure bike wheel size is essential, whether you’re a casual rider or a biking pro.

Before measuring the wheels, determine which system you’re following: the traditional or ISO method.

Some tire manufacturers will mark the wheels with an ISO code or traditional measurements for your convenience. However, I still recommend doing the measurements yourself, especially if you’re buying wheels online.

 

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