How to Determine Wheel Size on Bike

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As a biking enthusiast, you may be wondering how to determine wheel size on bike. Whether you want to purchase a replacement or spare tire for your bicycle, you must first determine the exact size of your bike wheel. 

Many accessories also require this information for proper fit, knowing your wheel size is an integral part of bike ownership and maintenance. Measuring your bike wheel can be accomplished by two methods. Luckily, both are very simple.

Measuring the Wheel Using the ISO Method

The International Organization for Standardization method, or ISO method, will allow you to determine wheel size on a bike in metric units, specifically millimeters (mm). When measuring your tires at home, ISO sizing is considered the most reliable and best option. To determine wheel size on a bike using this method, only a few simple steps are required:

1. Gather a Measuring Tape with Metric Units

If you don’t have one, you can convert from inches to millimeters by multiplying the measurement in inches by 25.4.

2. Measure From the Center of the Wheel to the Inner Edge of the Tire

To determine the diameter, double the measurement. You may also see a measurement called bead seat diameter (BSD), which is the diameter of the wheel measured from the bottom of the channel on the rim in which the tire bead fits. This can be a difficult dimension to measure with a tire installed, but it is generally 10-12 mm less than the wheel diameter.

3. Measure the Width of the Wheel Across the Flat Part of the Tread on the Tire

Keep in mind that tires of slightly different widths can be used on the same wheel, so this measurement may not be exact. If you have a bike wheel rim without a tire installed, you can also measure the inner rim width, which is the distance between the two inner edges of the channel on the rim where the tire will sit.

4. First List the Width and Then the Diameter, Both in Millimeters

For example, a 39 mm x 700 mm-wheeled bike would have a width of 39 mm and a diameter of 700 mm. Keep in mind that when looking at dimensions expressed using the ISO, a BSD may also be provided. The BSD is marked as either a value after the width, separated by a hyphen, or as a value before the width, separated by a cross (x).

Measuring the Wheel Using the Standard Method

The standard method of how to determine wheel size on a bike expresses the measurements in inches (in). This process is very straightforward and, like the ISO method, requires only a few steps: 

1. Using a tape measure, in inches, measure the distance from the ground beneath the tire to its center point. Multiply this measurement by two to get the diameter. 

2. Measure the width from one side of the tire to the other, across the flat part of the tread. Again, this can vary based on the type of tire on your wheel.

3. List the diameter and then the width in inches. When purchasing a tire for your wheel or other bike accessories, remember that standard sizes place the diameter first, followed by the width. For example, if the size is 26 in x 1.75 in, it means that the wheel has a diameter of 26 in and a width of 1.75 in.

Calculating the Circumference of a Bicycle Wheel

If you plan to use an odometer, speedometer, or GPS on your bike, you need the wheel circumference for accurate calibration. You can measure the circumference in multiple ways depending on the supplies you have available.

One of the simplest ways to measure the circumference is with a string. Evenly wrap it around the outside edge of the tire and mark or cut the string at the point where it reaches back to its starting point. Then measure the length of the string to determine the circumference of the wheel including the tire.

Another method to measure the circumference of your wheel is using a dot of wet paint. Put a dot of wet paint on a spot on the tire tread. For at least two rotations, carefully push the bicycle in a straight line and the paint should mark the ground twice. To determine the wheel circumference, including the tire, measure the distance from the first spot of paint on the ground to the second.

You can also calculate the circumference of the wheel by multiplying the diameter by Pi (3.14). This method is unique as it also allows for calculating the circumference of the wheel rim without a tire. To do this, simply measure the diameter of the rim (from the point where it meets the tire) and multiply it by Pi. This formula will yield the circumference of the wheel without taking into account the size of the tire.

Finally, if you have followed one of the above methods on how to determine wheel size on a bike and already know the wheel width and diameter, you may be able to look up the circumference on online charts. 

How to Determine Wheel Size on Bike – Tips and Tricks

While determining the wheel size on a bike is very simple, a few things can help make the process even easier.  

Using a kickstand or leaning the bike against a wall can prevent the bike from falling over as you work. If you need to measure the bike alone, a retractable metal tape measure can be sturdier and more convenient than a flexible tape measure.

If you take the tire into account in your measurements, make sure to inflate it to the recommended pounds per square inch (PSI). The optimal tire pressure should be printed on the sidewall of the tire. Finally, you might be able to save yourself some effort simply by checking your tire sidewalls to see if the measurements you need are listed.

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