You can measure bike wheel diameter with a tape measure stretched across the wheel from one edge to the other, passing through the center hub. For most purposes, though, the number that actually matters is the bead seat diameter of your rim, which is printed on the tire sidewall in a standardized format. Which measurement you need depends on why you’re measuring: buying new tires, setting up a cycling computer, or identifying an unknown wheel.
Check the Tire Sidewall First
Before you grab a tape measure, look at the side of your tire. Most tires made in the last few decades have an ISO number printed on the sidewall in the format “XX-XXX,” where the second number is the bead seat diameter in millimeters. This is the internal diameter of the rim where the tire hooks on, and it’s the most useful number for buying replacement tires or tubes. A tire marked 23-622, for example, is 23mm wide and fits a 622mm rim.
If your tire is worn beyond reading or you have a bare rim, you’ll need to measure manually.
Measuring With a Tape Measure
To get the overall diameter of the wheel (rim plus tire), stand the wheel upright on a flat surface and measure from the ground to the very top of the tire. Double-check by measuring across the full wheel through the center hub. These two numbers should match closely. This gives you the total outer diameter, which is helpful for checking frame clearance or getting a rough size identification, but it won’t tell you which tires fit your rim.
To measure the rim’s internal diameter, which determines tire compatibility, stretch the tape from the inside edge of the rim on one side straight across to the inside edge on the opposite side, passing through the hub. This gives you the approximate bead seat diameter.
For more precision, measure the radius instead. Hold the end of your tape measure at the inside edge of the rim where the spoke nipple sits, and read the distance to the center of the hub axle. Multiply by two. If you get roughly 280mm for the radius, your bead seat diameter is about 560mm, which would point to a 559mm (26-inch mountain bike) wheel.
Common Wheel Sizes and What They’re Called
Bike wheel naming is notoriously confusing. The same rim gets different names depending on the era, country, and marketing. Here are the sizes you’ll encounter most often, identified by their ISO bead seat diameter:
- 622mm: The most common size on the road today. Called 700c on road bikes and hybrids, 29-inch on mountain bikes, and sometimes 28-inch in European markets. These are all the same rim diameter.
- 584mm: Known as 650b in traditional cycling, or 27.5-inch in mountain biking. Originally used on French utility bikes and tandems, now widespread on trail bikes.
- 559mm: The classic 26-inch mountain bike wheel. Fits tires from 1 inch wide all the way up to 5-inch fatbike tires. Less common on new bikes but still very much in use.
The key thing to understand: a “29-inch” mountain bike tire and a “700c” road tire use the exact same 622mm rim. The difference is only tire width. Likewise, a “27.5-inch” mountain bike tire and a “650b” touring tire both fit a 584mm rim. If your measurement lands close to one of these three numbers, you’ve identified your wheel.
Older and Less Common Sizes
If your bike is vintage or imported, you might encounter sizes that don’t match the big three. A common trap is the old 27-inch road wheel, which has a 630mm bead seat diameter and is not interchangeable with 700c (622mm) despite looking nearly identical. Some older British and European bikes used 590mm rims (26 x 1 3/8), which are a different size from the 559mm mountain bike “26-inch.” The numbers on the tire sidewall are the only reliable way to tell these apart, since two wheels can look the same size and be completely incompatible.
When in doubt, bring the wheel to a bike shop. A mismatch of even a few millimeters in bead seat diameter means the tire won’t seat safely on the rim.
Measuring Wheel Circumference for a Bike Computer
Cycling computers and speed sensors need the wheel’s circumference, not its diameter. You can calculate circumference from diameter (multiply by 3.1416), but the most accurate method is a rollout test, because it accounts for tire compression under your actual body weight and inflation pressure.
To do a rollout, sit on your bike next to a wall for balance. Rotate the front wheel so the valve stem is pointing straight down, and mark that spot on the ground. Roll forward in a straight line until the valve returns to the bottom. Mark that spot. The distance between the two marks is your wheel circumference. Using a long carpenter’s tape measure helps you steer straight and measure in one step.
For better accuracy, roll forward multiple revolutions and divide the total distance by the number of turns. Use the front wheel rather than the rear, since the rear wheel can slip slightly under pedaling force or skid during braking, throwing off the measurement. A typical 700c wheel with a 25mm road tire has a circumference around 2,100mm, but yours could differ by 10 to 30mm depending on tire model, pressure, and rider weight.
Tools That Help
A standard tape measure handles most jobs. For the rollout method, a long carpenter’s tape (at least 3 meters) makes things easier. If you’re building wheels or need to measure internal rim width precisely, digital calipers give you accuracy down to fractions of a millimeter. For casual identification of your wheel size, though, a tape measure and a look at the tire sidewall are all you need.