How to measure a bike wheel size and the dropout of a bike

How to Measure a Bike Tire (How to Measure a Bicycle Tire)

In the bicycle field, the wheel size is the size of the bike wheel after you install the tire. So when you get the tire size, you get the bike wheel size.
If you have a bike tire on your wheel, you can generally find required measurement information on the tire wall (side of tire – see picture below).
Tire walls usually display the size as the picture below. 
If you can’t find the sizing on the wall of your tire, here we show you the methods to get your bike wheel size. 

The Standard Method (Easiest and Fastest Ways to Find Bike Tire Size) 

The standard method is to measures the tire width and tire diameter in inches.

How to Measure Bike Wheel Size with the Standard Method

  1. Put your bicycle on its kickstand or against a wall so that it’s stable.
  2. Place the bottom of the tape at the lowest point of the tire (on the ground) and measure in a straight line to the centre of the bike wheel to find your wheel radius (as shown in the image above). Multiply your wheel's radius by two, then it is the wheel's diameter. it is the wheel size.
  3. Measure the tire's width by placing the type across the top of it like the picture below. the number you got is the tire width. Be sure to list the diameter first, followed by the tire width.
  4. Bike wheel diameters are rounded to the nearest half-inch, so if your measurements are just over or under, round up or down to the nearest size.

How To Measure a Bike Wheel (Rim) for Tire Size

If you do not have your bike’s tire to hand, you can determine the tire size by measuring your bike’s rim width in millimetres as well.
Based on the average tire and wheel combinations currently on the market, the following charts illustrate the coinciding tire size and rim length.
Many rims will display their size somewhere, but some rims may be missing this measurement. 
If you cant find measurement on your wheel’s rim, follow the easy steps below that will help you determine the appropriate tire sizes your rim can handle.
  1. With no tire on your bike wheel, position the wheel vertically and hold it steady. 
  2. Take a measuring tape and press it against the inside of the rim, where the tire bead would normally sit.
  3. Measure the internal distance from one side of the rim to the other (as shown in the image above).
  4. Take this width and apply it to the correct table below.

Road Bike Rim Width to Tire Size Chart

Mountain Bike Rim Width to Tire Size Chart
Measure of Wheel
20" -- outer tire: 495mm dia.; rim: 410mm dia.;
24" -- outer tire: 600mm dia.; rim: 517mm dia.
26" -- outer tire: 650mm dia.; rim: 573mm dia.;
700C -- outer tire: 690mm dia.; rim: 635mm dia.
28" -- outer tire: 710mm dia.; rim: 650mm dia.

How to measure the dropout of your bike

The dropout of your bike is important, it can decides if the electric bike kit can fit your bike or not. So before your purchase of an ebike kit, please measure the dropout of your bike first.

Bike dropout is literally called frame spacing, is the inside distance between the two fork ends (drop outs) where the wheel fits in. it is very easy to be measure just by removing the wheel and holding a ruler to measure the place where the wheel came out. please check the following picture.