Tire size is printed on the sidewall of your tire in a standardized code that tells you the width, diameter, and how much air pressure it holds
Every bicycle tire has its dimensions stamped on the rubber itself, usually on the outer sidewall. You do not need tools or math — you just need to know where to look and what the numbers mean. The code appears as something like "700 x 25C" or "26 x 2.0" or "27.5 x 2.1", and each part tells you something different about whether that tire will fit your bike and how it will perform.
The first number is the wheel diameter (the overall size of the wheel), and the second is the tire width (how fat or skinny the tire is). A third measurement — the bead seat diameter, shown as a letter or number — tells you the internal rim size the tire must fit onto. Once you can read these three pieces of information, you can match a replacement tire to your bike or understand what you already have.
Key Takeaways
- Tire size is printed on the sidewall in a code like "700 x 25C" or "26 x 2.0", where the first number is wheel diameter and the second is tire width.
- The letter or third number (C, B, or a number like 559) indicates the bead seat diameter, which must match your rim for the tire to fit.
- Road bikes typically use 700c wheels with narrow tires (23–28 mm wide), while mountain bikes use 26, 27.5, or 29-inch wheels with much wider tires (2.0 inches or more).
- You can also measure tire width yourself with a ruler or caliper by laying the tire flat and measuring across the widest point, though the printed code is always more reliable.
Where to find the tire size on your bike
Look at the outer sidewall of your tire — the curved rubber surface that faces outward when the wheel is on the bike. The size code is molded or printed into the rubber, usually in white or raised lettering. It may wrap around the tire, so you may need to spin the wheel to see the entire marking if part of it is hidden behind the frame or brake.
If the tire is very worn or the lettering has faded, you may need to clean the sidewall with a damp cloth to read it clearly. If you cannot read the sidewall at all, the tire is old enough that replacement is overdue anyway. Write down the code exactly as it appears — including any letters or symbols — because you will need it to order a replacement.
Understanding the diameter and width numbers
The first number in the code is the wheel diameter, measured in either inches or millimeters depending on the tire standard. Road bikes almost always use 700c (which is 622 mm in the metric system, roughly 27.5 inches). Mountain bikes use 26, 27.5, or 29 inches. Older or smaller bikes may use 20 or 24 inches. This number tells you the overall size of the wheel and cannot be changed without replacing the entire wheel.
The second number is the tire width, measured in millimeters for road tires or inches for mountain and hybrid tires. A road tire marked "700 x 25C" is 25 millimeters wide. A mountain tire marked "26 x 2.0" is 2.0 inches wide (about 51 millimeters). Wider tires grip better and absorb bumps but roll slower and require more pedaling effort. Narrower tires are faster but less forgiving on rough ground. You can usually fit a slightly wider or narrower tire on the same wheel, but there are limits — check your frame clearance and rim width before going significantly wider or narrower.
What the letter or third number means
The letter at the end of a road tire code (like the "C" in "700 x 25C") or the number on a mountain tire (like "559" in "26 x 2.0 559") is the bead seat diameter — the internal measurement of the rim that the tire must fit onto. This is the most important measurement for replacement, because a tire with the wrong bead seat diameter will not fit your rim at all, no matter what the other numbers say.
For road bikes, the letters are A, B, or C, with C being the most common on modern bikes. For mountain and hybrid bikes, the number is usually 559 (for 26-inch wheels), 584 (for 27.5-inch wheels), or 622 (for 29-inch wheels). Always match this measurement exactly when buying a replacement tire. If you are unsure, take a photo of the sidewall code and show it to a bike shop — they can confirm the right replacement in seconds.
Measuring tire width yourself with a ruler or caliper
If the sidewall is too worn to read, you can measure the tire width directly. Lay the bike on its side so the tire is flat against the ground. Use a ruler or digital caliper to measure across the widest part of the tire from one edge to the other. Write down the measurement in millimeters or inches, depending on your tool.
This measurement is less precise than the printed code because tire width varies slightly depending on the rim width and air pressure, but it gives you a ballpark figure for ordering a replacement. If you measure 25 mm, you know you need a 25 mm road tire. If you measure 2.1 inches, you need a 2.1-inch mountain tire. Always round to the nearest standard size — tires come in set widths, not custom measurements.
Common tire sizes and what bikes use them
Road bikes almost always use 700c wheels with tires between 23 and 32 millimeters wide. A racing road bike might use 700 x 23c, while a gravel or endurance road bike might use 700 x 35c or wider. Hybrid and commuter bikes often use 700c as well, but with wider tires (35–42 mm) for comfort and grip on pavement and light trails.
Mountain bikes come in three wheel sizes: 26-inch (older and smaller bikes), 27.5-inch (mid-size, common on modern hardtails), and 29-inch (larger and faster-rolling, popular on cross-country and trail bikes). Mountain tire widths range from 1.9 inches on fast-rolling cross-country tires to 2.5 inches or wider on downhill and fat-bike tires. BMX bikes use 20-inch wheels with narrow tires. Cruisers and beach bikes often use 26-inch wheels with very wide, low-pressure tires for comfort.
Why tire size matters when buying a replacement
Matching the tire size exactly ensures the replacement will fit your rim, clear your frame and brakes, and perform the way the bike was designed. A tire that is too wide may rub the frame or brake pads. A tire that is too narrow may not seat properly on the rim or may feel unstable. A tire with the wrong bead seat diameter will not fit the rim at all — you will not be able to install it.
If you want to change your tire size — for example, going from a narrow road tire to a slightly wider one for comfort — check your frame clearance first. Spin the wheel and look for gaps between the tire and the frame or brake. If there is less than a quarter-inch of clearance, do not go wider. Your bike shop can also tell you the maximum tire width your frame and brakes will accept. Going narrower is usually safe as long as you match the bead seat diameter.
Frequently Asked Questions
Can I put a different width tire on the same wheel?
Yes, within limits. You can usually go one or two sizes up or down in width on the same wheel diameter and bead seat diameter. A 700c rim that fits a 25 mm tire can usually fit a 28 mm or 32 mm tire. Check your frame clearance first — the tire must not rub the brakes or frame. Your bike shop can confirm what widths your frame allows.
What if the sidewall code is partially worn off?
Measure the tire width with a ruler and look for any remaining numbers or letters on the sidewall. The bead seat diameter is usually the last thing to wear away. If you can read even part of the code, write it down. Take a photo and bring it to a bike shop — they can often identify the tire from the remaining markings or the wheel size and frame style.
Is 700c the same as 29 inches?
No. 700c is 622 mm (about 27.5 inches), while 29-inch mountain bike wheels are 622 mm as well — they are the same rim diameter but use different tire widths and bead seat diameters. A 700c road tire will not fit a 29-inch mountain bike rim, and vice versa, because the bead seat diameter is different.
Do I need to match the exact tire width, or can I go close?
Tires come in standard widths, so you will usually find an exact match. If your tire is marked 700 x 25c, buy 700 x 25c. If you cannot find that exact size, the next size up or down (23 or 28 mm) will usually fit, but check your frame clearance first. Always match the wheel diameter and bead seat diameter exactly.
What does PSI mean on the tire sidewall?
PSI is pounds per square inch — the air pressure the tire is designed to hold. This is printed separately from the size code, usually as a range like "80–130 PSI". Use a floor pump with a pressure gauge to inflate to the pressure recommended for your weight and riding style, which is usually somewhere in the middle of that range.