Tire size is printed on the sidewall of every bicycle tire in one of three standard formats

Every bicycle tire has its size stamped on the rubber itself, usually on the outer sidewall where you can see it without removing the wheel. The three formats you will encounter are ISO (or ETRTO), which uses two numbers separated by a dash like 37-622; inch-based sizing, which looks like 26 × 1.95; and French sizing, which uses a single number like 700C. Most modern bikes use ISO or inch-based, but all three describe the same measurement: the tire's width and the diameter of the wheel it fits.

You do not need special tools to find your tire size. Walk to your bike, spin the wheel so the sidewall faces you, and look for the printed numbers. Write them down exactly as they appear — including any letters — because that exact code is what you need when buying a replacement tire.

Key Takeaways

  • Tire size is always printed on the sidewall of the tire itself, and you can read it without removing the wheel or using any tools.
  • ISO format (37-622) lists width first in millimeters, then wheel diameter; inch format (26 × 1.95) lists diameter first, then width in inches.
  • The two numbers or measurements describe different things — width tells you how fat the tire is, and diameter tells you what size wheel it fits.
  • When buying a replacement tire, use the exact size printed on your current tire, because even small differences in width or diameter will not fit your wheel.

Understanding ISO (ETRTO) format: the two-number system

ISO format is the international standard and appears as two numbers with a dash between them, like 37-622 or 28-700. The first number is the tire width in millimeters. In the example 37-622, the tire is 37 millimeters wide. The second number is the wheel diameter in millimeters — in this case, 622 millimeters, which is a standard road bike wheel.

This format is the most precise because it removes all guesswork. A 37-622 tire will fit only wheels that are 622 millimeters in diameter, and it will be exactly 37 millimeters wide when mounted and inflated to the correct pressure. If you see 28-700 on a road bike or 50-559 on a mountain bike, those numbers follow the same pattern: width first, diameter second.

The advantage of ISO sizing is that you can compare tires across brands with complete accuracy. A 37-622 from one manufacturer will have the same width and fit the same wheel as a 37-622 from another, which is not always true with the older inch-based system.

Understanding inch-based sizing: diameter × width

Inch-based sizing reverses the order and uses inches instead of millimeters. You will see it written as 26 × 1.95, 27.5 × 2.1, or 29 × 2.2. The first number is the wheel diameter in inches, and the second number is the width in inches. A 26 × 1.95 tire fits a 26-inch wheel and is 1.95 inches wide.

This format is common on mountain bikes and older road bikes. The width number can be deceptive because it is not always precise — a tire marked 1.95 inches might actually be anywhere from 1.9 to 2.0 inches wide depending on the manufacturer and how much air pressure you put in it. This is why ISO sizing has become the standard for new bikes.

One source of confusion: a 26-inch wheel is not actually 26 inches in diameter. The 26-inch measurement refers to the bead seat diameter (the part of the wheel rim where the tire sits), which is about 559 millimeters. This is why you will sometimes see both numbers on the same tire — for example, 26 × 1.95 (559) — with the ISO equivalent in parentheses.

Understanding French (700C) sizing: the single-number system

French sizing uses a single number followed by the letter C, like 700C or 650C. This number refers to the approximate outer diameter of the tire when mounted and inflated, measured in millimeters. A 700C tire is roughly 700 millimeters in diameter, which is the standard for road bikes and many gravel bikes.

The width is not included in the 700C designation itself, so you will see it written separately: 700 × 25C means a 700C tire that is 25 millimeters wide. The letter C is a historical remnant from French tire manufacturing and does not affect how you use the size — it is straightforward part of the label.

700C is equivalent to a 622-millimeter ISO wheel diameter, so a 700C tire will fit the same wheel as a tire marked 622. The 700C designation is still common on road bikes and is often printed alongside the ISO size on the tire sidewall.

How to measure tire width if the sidewall is worn or unclear

If the printed size is faded or missing, you can measure the width yourself with a ruler or caliper. Remove the wheel from the bike, lay the tire flat on a table, and measure straight across the widest part of the tire with the wheel off the ground. Write down the measurement in millimeters or inches, depending on which system your ruler uses.

For the wheel diameter, you have two options. You can measure from the center of the axle (the bolt in the middle of the wheel) straight across to the opposite side and double that number. Or you can look up the wheel size by the number of spokes and the rim width — most bike shops can identify a wheel by sight if you bring it in or describe it.

Measuring is slower than reading the sidewall, but it works if the original marking is gone. Keep in mind that a tire's actual width can vary slightly depending on the rim width and air pressure, so a measurement is less precise than the printed size.

Why tire size matters when buying a replacement

Tire size is not a suggestion — it is a hard limit. A tire that is too wide will not fit inside the frame or brake system. A tire that is too narrow will not seat properly on the rim and can slip or lose air. A tire with the wrong diameter will not fit the wheel at all.

When you go to buy a replacement tire, bring the exact size from your current tire or write it down before you go to the shop. Do not guess or round up. If your tire says 37-622, do not buy a 40-622 thinking it will be close enough — it will not fit. If it says 26 × 1.95, do not assume a 26 × 2.0 is the same — it may be too wide for your frame.

Most bike shops will cross-reference your size with their inventory and show you what is in stock. If you are ordering online, use the exact size as your search term. Many online retailers have a size filter that will show only tires that fit your wheel diameter, which eliminates most mistakes.

Common tire sizes and what bikes they fit

Road bikes typically use 700C wheels with tire widths between 23 and 32 millimeters, often marked as 700 × 25C or 28-700. Mountain bikes use 26-inch (559 millimeters), 27.5-inch (584 millimeters), or 29-inch (622 millimeters) wheels with widths from 1.9 to 2.5 inches or 50 to 65 millimeters. Gravel and hybrid bikes often use 700C wheels with wider tires, typically 35 to 45 millimeters wide.

Smaller bikes for children use 20-inch or 24-inch wheels, marked as 20 × 1.75 or 24 × 1.95 in inch format. BMX bikes use 20-inch wheels with very narrow tires. Cruiser bikes and beach bikes often use 26-inch wheels with very wide tires, sometimes 2.5 inches or wider.

If you are not sure what size your bike uses, the sidewall of your current tire is the most reliable source. If that is not readable, a photo of your wheel sent to a bike shop or posted in a cycling forum will get you an answer in minutes.

Frequently Asked Questions

Can I put a wider tire on my bike if it has the same wheel diameter?

Not always. Even if the wheel diameter is correct, a wider tire might not fit inside your frame or between your brake pads. Check your bike's manual or measure the clearance between the tire and frame before buying. Many bikes have a maximum tire width listed in the specs.

What is the difference between 26-inch and 559-millimeter tires?

They are the same wheel size, just measured in different systems. 26-inch refers to the bead seat diameter (where the tire sits on the rim), while 559 millimeters is the ISO equivalent. A tire marked 26 × 1.95 (559) uses both numbers to show the same wheel fits both sizing systems.

If my tire says 700 × 25C, does that mean it is 25 inches wide?

No. The 25 is in millimeters, not inches. A 700 × 25C tire is 25 millimeters wide, which is about one inch. The C refers to the French sizing system and does not affect the width measurement.

Do I need to match the exact tire size, or can I go one size up or down?

You should match the exact size printed on your current tire. Going up or down in width can cause the tire to rub the frame or brakes, or to lose contact with the rim. The wheel diameter must be exact — there is no flexibility there at all.

Where else can I find my tire size if the sidewall is completely worn away?

Check your bike's manual or the manufacturer's website — most list the stock tire size in the specifications. You can also take a photo of your wheel and ask at a local bike shop, or measure the wheel diameter and width yourself with a ruler or caliper.