How To Measure The Wheels of My Bicycle?

In this post, you will learn how to obtain rim measurements with tools that you probably already have in your home workshop. This time we will limit ourselves to the seat-to-seat diameter and the internal width, useful for determining the deck measurements.

Another reason it would be useful to know the dimensions of a hoop is to calculate the measurement of the radii or rays, but that will not be covered at this time.

Before going into detail, it is worth clarifying that for the purposes of this article we refer to the hub, spokes (or spokes), and rim assembly as the rim, without the tube or cover.

Common wheels on current bicycles

In the best of cases, when replacing a tire, we can take the measurements of the old tire as a basis. However, knowing and knowing how to determine the measurements of a wheel becomes important when we do not have the measurements of the tire at hand, whether this information is illegible or we have the pure wheel.

Not having measures cover the first thing you will need is to get the diameter of the bead seat or bead seat diameter (BSD) . This measurement must correspond to the diameter in millimeters of the roof. If not, it will not be supported. In the following tables you will find the diameters in the ETRTO system of today’s common bicycle wheels:

English systemETRTOInner width (mm)Commonly used in:
29 “62219 – 40Mountain bikes, adventure
27.5 “58422 – 35Mountain bikes
26 “55920 – 40Mountain bikes, crusiers, and other models
24 “54021 – 26BMX, youth MTB bikes
twenty”40619 – 25Folding bicycles
16 “34919English folding bikes
English systemETRTOInner width (mm)Commonly used in:
700C62213 – 25Road, touring, hybrid bikes
650B58418 – 25Gravel bikes, French utility bikes, tandems, touring and cargo bikes.
650C57113Triathlon, time trial, high performance road bikes for smaller riders.

How to determine bicycle rim diameter (BSD)

BSD calculation based on the overall diameter

When we have the pure rim, without the reinforced rim, the simplest thing is to use a tape measure to determine its diameter. Remember that the data we need is the seat to seat diameter (BSD), which is more or less 10 mm less than the total diameter of a ring.

On the left side, we can see the diagram in which the calculation of the BSD is represented by subtracting 10 mm from the total diameter of the ring. The same diagram appears on the right side, showing a cross-section of the ring.


  1. Measure the total diameter of the ring (T), from its outer ends, in millimeters . To find the diameter, it is suggested to move one of the ends of the tape a little and locate the largest measurement.
  2. Repeat the diameter measurement at different points on the rim and calculate an average value . The hoop may not have a perfect circumference, so having multiple measurements will give you a more accurate value.
  3. Get the seat to seat diameter (BSD). For this, subtract 10 mm from the average value that you have obtained as the total diameter.
  4. Locate the measurement in the ETRTO system that is closest to the value obtained.

Calculation of BSD based on circumference

When you have a reinforced wheel, the hub complicates the diameter measurement, it is much easier to measure the circumference. This can be done with a thin tape measure or, failing that, with a cable or similar, so that it does not stretch and achieve a more accurate measurement.

Procedure :

  1. Measure the circumference of the base of the hoop in millimeters. Place the end of the tape on one of the edges of the valve hole and go around the entire ring to measure the circumference of its base.
  2. Find the diameter of the base of the hoop (B). Divide the value of the circumference obtained in the previous step by π (3.1416).
  3. Measure the height from the base of the hoop to the edge of its flanks (H). You can use a radio (radius) or some similar object.
    1. Place the end of the spoke perpendicular to the base of the hoop.
    2. Using a ruler, mark the point where the edge of the flanks intersects the radius.
    3. Measure the height with the ruler.
  4. Determine the total diameter of the ring (T). To do this, multiply the height from the base to the flank, which you just calculated, by two and add the value obtained to the diameter of the base of the ring.
  5. Get the seat to seat diameter (BSD). Subtract 10 mm from the value obtained for the total diameter of the ring.
  6. Locate the measurement in the ETRTO system that is closest to the value obtained .

How to determine the inner width of the rim

By selecting the width of the covers you have greater flexibility. However, knowing the internal width of the rim will be a good starting point in choosing the appropriate tire width.

This measurement has no greater science, it is only a matter of measuring the distance between the internal faces of the flanks of the ring.

Leave a Comment