Bicycle tire sizes are marked on the sidewall in two or three numbers
Every bicycle tire has its size printed on the rubber itself, usually on the side wall. The marking tells you the diameter and width of the tire, and sometimes the pressure range. You do not need tools to read it — just look at the sidewall and write down what you see. That number is your tire size, and it is the first thing you need before buying a replacement or checking whether a tire will fit your bike.
The format depends on which standard the tire uses. Most modern bikes use the ISO (metric) system, which looks like 700x28c or 650x50b. Older bikes and some mountain bikes use the inch system, which looks like 26x2.0 or 27.5x2.1. Both tell you the same information — diameter and width — but in different units.
Key Takeaways
- Tire size is printed on the sidewall in either metric (700x28c) or inch (26x2.0) format, and you read it directly from the rubber without tools.
- The first number is the tire diameter, the second is the width, and the letter (if present) indicates the bead seat diameter for metric tires.
- Tire pressure is marked as a range in PSI (pounds per square inch) and varies by tire width and rider weight.
- You measure actual tire pressure with a floor pump or hand pump that has a built-in gauge, not by squeezing the tire by hand.
Reading metric tire sizes (700x28c format)
The metric system is standard on road bikes and hybrid bikes. The first number — usually 700 — is the wheel diameter in millimeters. The second number is the tire width in millimeters. So 700x28c means a 700mm diameter tire that is 28mm wide. The letter at the end (c, b, or a) refers to the bead seat diameter, which is a technical measurement of the rim interface. For practical purposes, you only need to match the letter when replacing a tire on the same wheel.
Common road bike sizes are 700x23c, 700x25c, and 700x28c. Mountain bikes and gravel bikes use the same 700 diameter but wider tires, often 700x35c to 700x50c. The wider the tire, the more grip and comfort it provides, but the slower it rolls. If you are replacing a tire, you can usually go up or down by a few millimeters in width on the same wheel, but check your frame clearance first — wider tires need more space between the frame and wheel.
Reading inch tire sizes (26x2.0 format)
Older bikes and many mountain bikes use the inch system. The first number is the wheel diameter in inches, and the second is the tire width in inches. A 26x2.0 tire is 26 inches in diameter and 2 inches wide. You will also see sizes like 27.5x2.1 and 29x2.4 on modern mountain bikes — these are still measured in inches even though they include decimals.
The inch system is less precise than metric, so tire widths can vary between manufacturers. A 26x2.0 from one brand might measure slightly different from another brand's 26x2.0. If you are replacing a tire, bring the old tire with you or take a photo of the sidewall marking to may support you get the right fit. Mixing inch and metric sizes on the same bike does not work — the wheel diameter must match exactly.
Finding the tire pressure range on the sidewall
Below the size marking, the tire sidewall also shows a pressure range in PSI (pounds per square inch). This range looks like "80-130 PSI" or "50-85 PSI" and tells you the safe operating pressure for that tire. Road tires are usually narrower and run at higher pressure — 80 to 130 PSI. Mountain bike and gravel tires are wider and run at lower pressure — 30 to 65 PSI. The exact pressure you choose within that range depends on your weight, the terrain, and your preference for speed versus comfort.
Do not exceed the maximum pressure listed on the tire. Overinflating can cause a blowout or make the tire wear unevenly. Underinflating below the minimum can cause the tire to pinch or roll off the rim. If you do not see a pressure range on your tire, the rim itself usually has one printed on the inside — check there. Some tires also list a pressure range for tubeless setups, which is different from the range for tube tires.
Measuring tire pressure with a pump gauge
Tire pressure is measured with a floor pump or hand pump that has a built-in pressure gauge. Squeezing the tire by hand tells you almost nothing — even a significantly underinflated tire can feel firm. A floor pump is the most accurate tool and is worth buying if you own a bike. Most floor pumps have a gauge on the barrel that reads pressure as you pump. Hand pumps are portable but less accurate, and some require you to read the gauge while holding the pump head against the valve.
To measure pressure, unscrew the valve cap on the tire, press the pump head firmly onto the valve, and read the gauge. For Presta valves (the thin ones on road bikes), you may need to unscrew the small top nut first. For Schrader valves (the thick ones on mountain bikes), just press the pump head on. If the gauge bounces or reads differently each time, the pump head is not seated properly — try again with more pressure.
Checking tire width with calipers or a ruler
If you want to verify the actual width of your tire — not just what the sidewall says — you can measure it with a ruler or calipers. Place the tire on a flat surface and measure the widest point from edge to edge. Actual tire width is usually close to the marked width, but can vary by a millimeter or two depending on the rim width and how much air is in the tire. This measurement matters mainly if you are trying to fit a tire into a tight frame or checking whether a used tire is the size the seller claimed.
Calipers give a more precise measurement than a ruler, but for most purposes a ruler is fine. Measure the tire when it is inflated to the pressure you normally ride at, since pressure affects the width slightly. If you are buying a replacement tire and want to know whether it will fit your frame, measure your current tire width and add a millimeter or two for safety.
Matching tire size to your wheel and frame
Before buying a new tire, confirm that the size will fit your wheel and frame. The wheel diameter must match exactly — a 700c tire will not fit a 26-inch wheel, and a 27.5-inch tire will not fit a 29-inch wheel. The width has more flexibility. Most wheels can accept a range of widths, usually within 5 to 10 millimeters. Your frame, however, has a maximum tire width it can clear. Road bikes typically have 2 to 3 centimeters of clearance on each side of the tire, so a 28c tire is near the limit. Mountain bikes have more clearance and can fit much wider tires.
If you are unsure whether a tire will fit, check your bike's manual or the frame manufacturer's website. You can also measure the clearance yourself: inflate your current tire to normal pressure, measure the gap between the tire and frame at the tightest point, and subtract a few millimeters for safety. That tells you how much wider you can go. When in doubt, stick with the same width as your current tire or go slightly narrower.
Frequently Asked Questions
What if my tire has two different numbers on the sidewall?
Some tires show both metric and inch sizes. For example, a tire might say "700x28c (27.5x1.10)". Both refer to the same tire — the metric size is the primary marking, and the inch equivalent is listed for reference. Use whichever system your bike uses, and ignore the other.
Can I use a tire size that is not listed on my rim?
Rims are designed for a specific range of tire widths. Using a tire that is too narrow or too wide can cause the tire to slip, wear unevenly, or damage the rim. Check your rim's sidewall for the recommended tire width range, or contact the rim manufacturer. Staying within that range is safer than guessing.
Does tire pressure change with temperature?
Yes. Tire pressure increases in warm weather and decreases in cold weather, roughly 1 PSI for every 10 degrees Fahrenheit. If you ride in winter, check your pressure more often. You may need to add air as the temperature drops to stay within the recommended range.
What is the difference between PSI and bar?
PSI (pounds per square inch) and bar are two ways to measure pressure. One bar equals about 14.5 PSI. Most pumps sold in the United States show PSI, but some European pumps show bar. Check your pump's gauge to see which unit it uses, or convert using the ratio above.
How do I know if my tire is the right pressure?
Use a pump gauge to read the actual pressure, then compare it to the range printed on your tire sidewall. Aim for the middle of the range as a starting point. If you want a faster ride, inflate toward the higher end. If you want more comfort and grip, inflate toward the lower end. Adjust and test until you find what feels right.