Where to Find the Tire Size on Your Bike

The tire size is printed on the sidewall of your tire in large letters and numbers. Look at the outer edge of the rubber where it meets the rim — you will see a code like "700x25c" or "26x2.0" or "27.5x1.95". This marking is on both sides of the tire, so you can read it from either side of the bike.

If your tires are very dirty, wipe the sidewall with a dry cloth first. The numbers should be straightforward to read once the surface is clean. If the printing has worn away completely, you can also check the tire box or receipt from when the tires were installed, or ask the bike shop where you bought the bike.

Key Takeaways

  • Tire size appears as two or three numbers separated by an "x" or a slash, printed on the tire sidewall.
  • The first number is the wheel diameter in millimeters or inches, depending on the tire type.
  • The second number is the tire width, also in millimeters or inches.
  • A third letter or number (like "c" or "b") indicates the bead seat diameter and helps you match tires to your specific rim.

Understanding the First Number: Wheel Diameter

The first number tells you the diameter of the wheel in either millimeters or inches. Common sizes are 700c (road bikes), 26 inches (older mountain bikes), 27.5 inches (modern mountain bikes), and 29 inches (larger mountain bikes). Road bikes almost always use the metric 700c standard, while mountain bikes use inch measurements.

This number must match your rim. You cannot put a 26-inch tire on a 700c rim, and you cannot put a 29-inch tire on a 27.5-inch rim. If you are replacing a tire, the first number should be identical to what was on the old tire. If you are unsure what rim you have, the rim itself is usually marked with its size.

Understanding the Second Number: Tire Width

The second number is the width of the tire, measured in millimeters (for road and gravel bikes) or inches (for mountain bikes). A road tire marked "700x25c" is 25 millimeters wide. A mountain bike tire marked "26x2.0" is 2.0 inches wide, which equals about 51 millimeters.

Wider tires grip better on loose ground and absorb bumps more comfortably, but they are slower on pavement and require more effort to pedal. Narrower tires roll faster on smooth surfaces but offer less grip on dirt or gravel. Your rim has a maximum and minimum width it can safely support, so check your rim specifications before choosing a much wider or narrower tire than what came on your bike.

The Third Code: Bead Seat Diameter and Tire Type

Many tire sizes include a letter or number after the width. On road bikes, you will see "c" (clincher), "t" (tubular), or "s" (semi-tubular). On mountain bikes, you might see "b" (bead) or no letter at all. This code describes how the tire attaches to the rim and is important for matching the tire to your specific rim.

The most common is the clincher (marked "c"), which uses a bead that hooks under a lip on the rim and works with a separate inner tube. Tubular tires are sewn around an inner tube and glued directly to the rim — these are rare on modern bikes. For most riders, you only need to match the first two numbers and make sure the third code matches what your rim accepts.

Reading Metric Versus Inch Measurements

Road and gravel bikes use metric sizing: 700x25c means 700 millimeters wheel diameter and 25 millimeters width. Mountain bikes use inches: 26x2.0 means 26-inch wheel diameter and 2.0-inch width. A few older bikes use a mixed system, like 27x1.25, where 27 is inches but 1.25 is also inches.

The system you have depends on the type of bike and when it was made. Road bikes are almost always metric. Mountain bikes are almost always inches. If you see a "700" or a decimal point like "1.95", you are looking at metric. If you see a whole number like "26" or "29" without a decimal, you are looking at inches. When you buy a replacement tire, match the exact format of your original tire.

Common Tire Sizes and What They Mean

700x23c — Road bike tire, narrow and fast, common on racing bikes. The wheel is 700 millimeters in diameter and the tire is 23 millimeters wide.

700x28c — Road bike tire, slightly wider than 23c, more comfortable for long rides and rougher pavement.

700x35c — Gravel or commuter tire, wide enough for light off-road use while still rolling reasonably fast on pavement.

26x2.0 — Mountain bike tire, older standard, 26-inch wheel with a 2-inch-wide tire. Less common on new bikes.

27.5x2.1 — Modern mountain bike tire, 27.5-inch wheel with a 2.1-inch-wide tire. A middle ground between 26 and 29.

29x2.3 — Larger mountain bike tire, rolls over obstacles more easily but heavier and slower to accelerate.

What to Check Before Buying a Replacement Tire

Write down all three parts of the size code from your current tire before you go to the shop or order online. For example, "700x25c" or "27.5x2.1". This ensures you get a tire that fits your rim and matches your bike's design.

Also check your rim width if you are changing tire width significantly. Most rims are marked on the inside with a recommended tire width range, like "18-25mm". Staying within this range keeps the tire seated safely on the rim. If you want a much wider or narrower tire, ask the bike shop whether your rim can handle it.

Frequently Asked Questions

Can I put a different width tire on my bike if the diameter matches?

Yes, as long as the new width is within your rim's safe range. Most rims have a minimum and maximum width printed inside. Going slightly wider (from 25c to 28c) or slightly narrower is usually fine, but jumping from 25c to 35c might not work safely on a road bike rim.

What does the "c" in 700x25c mean?

The "c" stands for clincher, which is the most common tire type. It means the tire has a bead that hooks under a lip on the rim and uses a separate inner tube. Most modern bikes use clincher tires.

Are 26-inch and 27.5-inch tires interchangeable?

No. The first number must match your rim exactly. A 26-inch tire will not fit a 27.5-inch rim, and vice versa. Check your rim size before ordering a replacement.

How do I know if my bike uses metric or inch measurements?

Road bikes almost always use metric (700c). Mountain bikes almost always use inches (26, 27.5, or 29). Look at your current tire sidewall — if you see "700" or a decimal point like "1.95", it is metric. If you see a whole number like "26" or "29", it is inches.

What if the tire size on my rim does not match the tire size on my tire?

The tire size should always match the rim size. If they do not match, the tire was installed incorrectly or does not belong on that bike. Do not ride it — take the bike to a shop to have the correct tire installed.