How To Measure Bike Tire Size: New Step-by-Step Guide

Bike tires are one of the most crucial parts of a bike because they directly affect the bike’s performance. As bike tires are in constant contact with the trails, they often wear out quickly and thus, need to be replaced often. You need tires at the correct size that will fit the bike rim.

Suitable bike sizes will ensure the bike’s optimum performance and the rider’s safety. It is essential for you, the passionate bike riders, to be familiar with bike tire measurements. In this article, you will find everything you need to know about how to measure bike tire size so that you can provide your companion with the best care.

Check out this video for an informative bike tire size-101:

Watch video: Bike Tires 101 : The basics of bike tire sizing

Read more:

International Organization for Standardization (ISO) for A Bicycle Tire

International Organization for Standardization (ISO) for A Bicycle Tire
International Organization for Standardization (ISO) for A Bicycle Tire

There are many ways to express the tire dimensions. In fact, too many ways. In a traditional sizing system, tire size can be measured in inches (26″) or millimeters (650). The inch-based size is expressed in decimal (26 1 x 1.75) or in fraction (26 x 1 3/4). But unlike in math, 1.75 and 1 3/4 stand for different bike tire sizes.

What’s more, each country has its own standard for bike tire sizes. These numbers present the same bicycle tire size:

  • English size 28 x 1-5/8 x 1-3/4
  • French size 700B x 40C
  • German size 28 x 1 ½ x 1,75

The old sizing systems are complicated and inconsistent. Such a messy system creates a lot of confusion and difficulties when it comes to choosing the right bike tire size. If you are new to biking, these unclear technical numbers can scare you off.

Thus, the world of biking was in need of a more transparent and standardized system for measuring bike tires. Here came the introduction of the ISO (International Organization for Standardization) by the ETRTO (European Tire and Rim Technical Organization).

Under the ISO standard system, the bike tire sizes are expressed through two numbers in millimeters: tire width – bead set diameter

Tire width

The Tire width
The Tire width

Tire width is the width of a bike tire after being inflated in millimeters. One bicycle wheel size can take more than one tire width. It means that you can choose how wide or narrow your bike tires depend on your bike and your riding style.

The tire width plays a role in determining the bike’s weight, rolling resistance, and overall riding comfort. Compared to a narrower tire, a wider tire is heavier. Still, it is better at absorbing shocks, preventing flatting and increasing comfort.

A bike tire with narrower width will reduce the total weight of a bike so you can accelerate quickly. If you are the type of rider who pays more attention to speed, you will want narrow bike tires.

25mm is the most common tire width of reading bikes. Wider bike tires above 30mm are more suitable for touring, cyclocross, and mountain bikes.

But this tire width is also determined by the (inner) rim width. A 13mm-wide rim can support a 16mm-28mm tire, but anything above that will be too much of a burden.

If you are interested in the connection between tire width and rim, check out this video:

Watch video: Understanding Tire Sizes | Tech Tuesday #145

Bead seat diameter (BSD)

Bead seat diameter (BSD)
Bead seat diameter (BSD)

The diameter of the rim’s bead seat is the more critical number that you need to pay attention to. The number is a definitive guide for you to choose a tire that will fit into the bike’s rim.

The BSD measures the tire diameter at the tire bead – the edge of the tire that connects it to the rim. The BSD is specific for each bike tire.

Unlike the flexibility of tire width, the bead seat diameter of a bike is specific. Why? – If the tire is too small for the rim, both the tire and the rim will be damaged from shocks and give you a bumpy ride. If the tire is too big for the rim, it will get flat suddenly and put the rider at risk of losing control.

How does ISO diameter work? – It’s straightforward. All bike tire producers nowadays carve the ISO sizes on the sidewalls of the bike tires. For example: on your bike tire’s sidewall, you find this number 47-599. And

This means that the tire width is 47mm, and the rim’s bead seat diameter is 599. If your bike wheel diameter is also 599 (exactly!), you can use this tire.

But if you don’t like this tire because the 47mm-wide tire is too heavy, you can choose a tire with ISO size 40-599. Or, if you wish for more stability and comfort, opt for a wider tire with ISO size 60-599.

How Do I Know My Bike Tire Size?

How Do I Know My Bike Tire Size
How Do I Know My Bike Tire Size

The simplest way to find out your bike tire size is to ask the bike seller when you purchase. But what if the information slips your mind after a long time? Check your bike’s sidewall.

All bike tire producers either paint or carve the bike tire measurements on the tires’ sidewalls. You will find the measurements expressed in ISO and, sometimes, followed by numbers in traditional measurement systems.

For example:

  •  Mountain bike tires: 37-597 26 x 1 3/8: In the ISO system, the tire is 37mm wide with a bead seat diameter of 597mm. Or in a traditional way, the tire diameter is 26 inches, and the width of the tire wheel is 1 3/8 inches.
  • Road bike tires: 37-622 700 x 35C: In the ISO system, the tire is 37mm wide with a bead seat diameter of 622mm. Or in a traditional way, the outer tire diameter is 700mm, and the tired width is circa 35mm.

Luckily, most of the bike tire manufacturers now will provide bike tire sizes in ISO standard bike tire measurement systems. The measured numbers of tire width and diameters are presented in millimeters and are kept consistent across all types of bikes. You will be able to find the most suitable tires for your bike in a blink.

This is a great video that help you to understanding tire sizes better:

Watch video: Understanding Tire Sizes | Tech Tuesday #145

How Do I Measure My Tire Sizes?

How Do I Measure My Tire Sizes
How Do I Measure My Tire Sizes

You change your bike tires after using them for a long time, and they are already worn out. If you are lucky, you can still see the tire size numbers on the sidewall. But in many cases, the numbers have faded and almost disappeared. What should you do now?

Option 1: You can have your bike tires changed at the bike shop. This is the fastest and simplest way, but it hurts your pocket.

Option 2: You measure your tire size, get a new pair and replace them yourself. This way requires more time and effort. But you can save quite a bit while enjoying taking care of your own companion.

If you like DIY, here are some ways you can use to measure your bike tire size. Following these steps, you will find out the tire diameter and tire width – the two essential components of tire size.

Method 1: Traditional standard (with outer diameter)

Method 1 Traditional standard (with outer diameter)
Method 1 Traditional standard (with outer diameter)

Equipment: a retractable metal ruler (in inches), a pen, and a piece of paper (or anything to take note)

Step 1:

Stand the bike up with the kickstand or lean the bike against the wall. Secure the bike position so it won’t suddenly crash on you while you do the measuring.

Step 2: Measure the tire’s outer diameter:

Measure from the contact point of the tire and the ground to the center point of the bike wheel and to the tire’s outer edge in the opposite direction.

There is another way to measure the outer diameter. Measure the distance between the contact point of the tire and the ground to the center part of the wheel. Multiply the result by two.

Step 3: Measure the tire width

Measure the tire widths
Measure the tire widths

Place the ruler in a straight line across the tread from one side to the other side to measure the tire’s flat part. That’s the tire width.

Step 4:

Write down the measurements in this order: outer diameter x width in inches (ex: 26 x 1.5). When you look for a tire, make sure to keep the order and the unit of length in mind.

Method 2: ISO (with inner diameter)

Method 2 ISO (with inner diameter)
Method 2: ISO (with inner diameter)

Equipment: a retractable metal ruler (in millimeters), a pen, and a piece of paper (or anything to take note)

Step 1: 

Stand the bike up with the kickstand or lean the bike against the wall.

Step 2: Measure the tire’s inner diameter:

Measure from the beam – the inner edge of the tire to the center point of the bike wheel. From here, you can extend the tape and form a straight line across the center point to the opposite beam. Or you can take the result of the first measurement and multiply by two. Either way, you will get the tire’s inner diameter.

Step 3: Measure the tire width.

Measure the tire width
Measure the tire width

Place the ruler in a straight line across the tread from one side to the other side to measure the flat part of the tire. That’s the tire width.

Step 4:

Write down the measurements in this order: width – inner diameter in millimeter (ex: 50-559). When you look for a tire, make sure to keep the order and the unit of length in mind.

ISO is a standard sizing system. If you measure your bike tire in ISO, you can find a suitable tire quickly and easily. But what if you only have a tape in inches? You can convert your measurements to millimeters by multiplying them with 25.4 (1inch = 25.4 mm).

Method 3: Circumference of bike wheel

Method 3 Circumference of bike wheel
Method 3 Circumference of bike wheel

Method 1 and 2 are more than enough for you to find out your tire size so that you can replace your old worn-out tire with a new suitable one. But if you intend to install a bicycle speedometer, you will need to measure the wheel’s circumference.

Equipment: a metal ruler (in inches or millimeters), a pen, and a piece of paper (or anything to take note)

Step 1: Measure the tire’s outer diameter. Like in method 1, you measure the distance between the contact point of the tire and the ground up to the central part of the wheel. Multiply by two.

Step 2: Multiply the result to 3.24 (pi number) to get the measurement of the wheel’s circumference.

However, suppose you already measure the tire size either in traditional sizing systems or ISO. In that case, you can look up the corresponding circumference of the wheel on online charts or in wheel size online calculator.

How Do I Measure Bike Tires for My Kids?

How Do I Measure Bike Tires for My Kids
How Do I Measure Bike Tires for My Kids

Kids’ bike tires are not much of a difference compared to adult bike tires. They are just smaller in size. You can follow either method 1 or 2 in part “How do I measure my bike tire sizes?” to measure the tire size of your kid’s bike.

In general, bike tires for children will have the ISO size of around 203mm-547mm. Depending on the height of the kid and the type of bike, parents can choose different bike sizes for their kids.

How To Measure Bike Tire for Tube?

How To Measure Bike Tire for Tube
How To Measure Bike Tire for Tube?

Unless you ride with tubeless tires, an inner tube is a must for your bike tire. Inner tubes increase and stabilize the air pressure inside the tire. This helps the tire to keep its structural shape and strength.

With appropriately-inflated inner tubes, you can ride while loading more stuff on your bike. Moreover, you can enjoy the comfort and safe ride. But only when the tube and the tire are compatible in size. If not, the tube can be overstretched or pinched, increasing the risk that your wheel will get punctures.

You can either find the bike size on the tire’s sidewall or measure it yourself following the methods I show you above. Then you only need to choose an inner tube with a compatible size.

On the inner tubes, you will find information about the tire size that are compatible with the tubes. For example, here is what you might find on an inner tube: 700 x 18-23c.

The number shows that this inner tube is suitable for a 700mm tire with a width between 18mm-23mm. This number is presented in a similar manner on the bike’s sidewall. So, you can understand it easily.

This is a great guide for choosing suitable inner tube for your bike tire:

Watch video: What Size Bike Inner Tube Do I Need? || REI

FAQs

Is it easy to read bicycle tire sizes?

Yes, it is easy to read tire sizes. They are printed directly on tires’ sidewalls. Most of them are expressed in ISO standard system in this form: inner diameter – tire width (in millimeters)

Are the tire sizes of different bike types similar?

No, tire sizes of different bikes are different. Thanks to their diversity, they can provide optimum performances in different conditions.

Is rim measurement necessary for bike tire measurement?

No. To measure bike tires, you need only two components: tire width and tire (inner/outer) diameter.