How to Measure Bicycle Rims

Rims may seat tires slightly smaller or larger than their width.

Learning how to measure your bicycle rims allows you to purchase replacement rims or upgrade your wheel components at will. There are numerous ways that rim measurements are annotated and three dimensions that are important: the diameter, the inner width and the outer width. The diameter is the size of the rim from the bead seat, where the tire rests inside of the rim; this measurements is abbreviated B.S.D. The inner width of the rim is taken from one end of the tire bead seat to the other at the widest point. The outer width is measured from one side of the exterior wall to the other, so it will be slightly wider than the inner width. When you're purchasing tires, the width does not have to be matched exactly.

Things You'll Need

• Magnifying glass
• Tape measure

Instructions

1. Examine the sidewall of the tire and the interior of the rim for the imprinted measurements. Look for two numbers; the first is the rim diameter and the second is the recommended tire width.

2. Remove the wheel from the frame. Take the tire off the rim.

3. Place the rim on a flat surface.

4. Measure the bare rim from one outer edge to the other using a stiff tape measure. Subtract .3 inches from this measurement to approximate the B.S.D.

5. Stand the rim on edge and measure the circumference of the rim in the bead seat.

6. Convert the imperial measurement (inches) to metric (mm) by multiplying by 25.4.

7. Round the measurement to the nearest millimeter. Provide the measurement in millimeters to your parts provider; this is your metric (UK) rim size. Convert to a B.S.D. (US) size as follows:

   1979mm = 27 inches,

   1756mm = 26 inches and

   1954mm = 29-inch tires.

Tips & Warnings

• To measure a rim without removing the wheel, place the tip of the measuring tape at the axle. And measure the radius using a spoke as a guide for a straight edge. Multiply by 2 and subtract .3 inches.

• If you are not confident in the accuracy of your measurements, take your bike to the local repair shop. A professional will make these measurements for free.

References

• Sheldon Brown: Measuring Bicycle Rim Sizes
• YouTube: Bicycle Parts Wheels And Tire Size
• REI: How To Choose A Bike Tire
• EBicycles: How Do I Measure A Bike Frame?

Resources

• YouTube: How To Remove A Bicycle Wheel