The size of the bike tire is one of the commonly ignored aspects among casual riders.

In most cases, riders usually go with whatever bike tire that will come along or leave the task to the local bike shop when it’s time to replace or upgrade a tire.

However, if you’re planning on purchasing new tires online, things can get a tad challenging. This is because it’s an obvious requirement to know your bike tire size.

It doesn’t help that previously; every country had a unique sizing system. And, the same tires would be referred using different numbers.

While every tire now has the size marked outside, these numbers easily wear off over time.

And if you don’t have the size recorded, you may have a difficult time picking the ideal tire size.

Fortunately, there are ways to measure the bike tire size if it’s not available to you.

 And in this post, we shall walk you through the common methods of measuring bike tire size.

But before then, it would be prudent that we have a rudimentary understanding of the bike tire size classification systems.

Traditional Sizing Systems

The traditional sizing system is a standard system based on the measurement of the outside diameter.

When representing the measurements, the diameter comes first and the width second. 

Most noteworthy, their units are in inches and they need to be full-inch increments.

This means that you round off to the nearest inch depending on whether the figure is over or under. 

However, the evolution of tires and rims negated the use of this sizing system due to compatibility issues.

By then, each company had its sizing structure, and this made it particularly hard to purchase a tire.

The ISO

To eliminate the inconsistencies in the sizing, the International Organization for Standardization (ISO), for developing and publishing international standards came in.

They created a standardized sizing system to regulate and guide the tire measurement system.

It came up with a system that unifies the sizes with two numbers separated by a dash.

The first a two-digit shows the width and the other a three-digit, the diameter. 

The three-digit is the most valuable figure. It represents the bead diameter where the tire fits the rim.

It helps to identify tires that can fit your bike even when the other details vary. 

It comes in handy if you have little or no information of your tires.

It’s important to note that bike size measurements that use the ISO system are in millimeters. 

Although bicycle tires are available in vastly different sizes, a majority have their diameter falling between 12-26. For the width, the size is between 1.75-2.215 inches.

Step by Step Procedure of Measuring Tire Size

When measuring the size of your tire, there are two main measurements of interest.

This is the tire diameter and width. Another important one is the circumference. 

The process of obtaining the measurements is pretty straightforward.

You also don’t need any specialized tools. A tape and writing materials are sufficient. 

Step 1 : Place your bike in an upright position

Set and place your bike in an upright position or lean it on a wall.

When your bike is upright and firmly held, you’ll easily measure the size of the bike without much interference.

So, ensure that it’s firm to prevent movement and tipping. Get some help if possible, for a quicker process.

If you are doing it alone, we recommend that you use a tape to hold it in place.

A retractable metal tape works best as opposed to a plastic one -it’s sturdier. This will give you assured hands free operation.

Step 2 : Determine the diameter of the tire

In the second step, we determine the diameter of the tire.

First, place the tape against the center of the wheel.

From here, extend the tape in a straight line towards the edge of the bike tire.

This is only the radius, multiply the number by 2 to get the diameter.

When measuring to get measurements that correspond to the ISO standards, it’s slightly different.

Place the tape from the center of the wheel to the tires’ inner edge as opposed to the outer. Then double it to obtain the diameter. 

Step 3 : Measure bike tire width

The next measurement is the width, which is pretty easy.

You simply need to take your tape and place it on the ground across the tier’s tread going in a straight direction.

The width measurements when using the ISO is presented in millimeters and in inches when using the traditional method. 

The size of the width will vary greatly depending on the tire’s use.

For instance, bikes intended for rougher terrain have a wider width, while those with a narrow width promise a smoother, faster ride.

Combining the Measurements

Once you have the bike tire diameter and width, the final part is combining the measurement.

How you represent the data will vary depending on what system you prefer.

Remember, the traditional and ISO have different representations.

As mentioned earlier, the traditional system of bike sizing  places the diameter first, and the width second. The ISO is the vice versa. 

And, there you have it- your measured bike tire size.

To avoid having to take these measurements regularly, note them down for future reference. 

Calculating the Circumference of a Bicycle Wheel

Your bike circumference is handy, especially if you are looking to calibrate your bike’s speedometer, GPS or odometer.

Changing your bike tire size will provide inaccurate data.

So, regardless of whether you’ve purchased a new cyclometer or simply need to calibrate the existing one because you’ve switched tires, you’ll first need to determine the wheel’s circumference.

There’re two ways to determine the circumference of a bike tire, and this is dependent on whether you know the dimensions of the diameter.

And since we’ve already shown you how to calculate the diameter, you can easily know the circumference of your bike tire by multiplying the diameter by pi.

C= πD

Pi (π) is equal to 3.14, and the circumference of a 29-inch tire is reached by multiplying 29 by 3.14.

Another formula for calculating the circumference is 

                                    C=2r or πr2

Where r is the radius, the figure you get before doubling the distance. 

How to Measure the Bike Tire Circumference

You only need to physically measure the circumference of your bike tire if you’re not able to obtain the diameter. 

There are two main methods to go about this;

Using a String

To measure the circumference, evenly wrap a string around the outside edges of the tire.

Mark or cut the string on the spot where the string reaches the straight point.

The distance from one end to the marked point is the circumference of your bike tire. Measure this using a tape.

Using Wet Paint

Using a brush or stick, mark a section of your bike tire and the ground with wet paint.

Ensure that the quantity applied can make a mark when the tire comes in contact with the ground.

Carefully push the bike while following a straight line, until the bike makes the second dot on the ground.

This indicates that the bike has made a full rotation.

Using a tape, measure the length between the first and second dot. This length represents the circumference of your bike’s tire.

Note that not much paint is needed in the process.

Therefore, there’s no need to incur costs to purchase paint. You can use alternatives such as ink.

The aim is to make a visible mark to aid in identification. 

Getting the Bike Diameter from the Circumference

From determining the circumference using any of the above methods, you can easily calculate the diameter.

Use the same formulas shown previously but work backwards by using the figures you have. The formula transform as follows;

D=C

Now that you have your bike tire size what next? You’ll use the measurements you’ve obtained when replacing your tires.

However, you might find the combined measurement you have does not match what is available locally.

So, what do you do? Analyze the bike size tire charts and identify what is close to the figures.

Also, to be able to switch between the bike measurements systems, use online bike tire calculators. You’re bound to find brands that use different systems. 

Conclusion

The process of determining the tire size is painless and quick.

The measurements you can obtain using this process are the diameter, width and circumference of the bike tire. 

There are 3 main steps to follow when measuring the width and diameter. You only need a tape and writing materials to note down your findings. 

If you follow the above steps, you can now accurately measure your bike tire size whenever the need arises.

There are differences in determining the bike tire diameter depending on the system you choose.

The difference comes in with positioning the tape between the center and the edge. Another contrast is in the units.  

Once done with the measurements, combine your findings to conform to either the standard or ISO classification. 

For the circumference, simply make the calculations if you have the diameter. If not, make use of a string or wet paint to get the circumference.