Note: this article is about how to measure rims for tires. We have another article which describes how to measure rims for spokes.

What size tire do you need to buy for your bicycle? There are so many different tire sizes and different systems for marking tire sizes that have been used over the years that this is often a serious problem, especially for older bicycles.

We have a major article on this web site explaining the different tire sizing systems. Whenever possible, you should try to match your new tire to the old tire, using the ISO/E.T.R.T.O. size number, if it is marked on the tire or rim.

If the ISO/E.T.R.T.O. size number matches, it means the tire will fit on the rim. Tires are also designated for different widths, but you can interchange tires of different widths as long as you match the ISO/E.T.R.T.O. size number and the widths are not wildly out of line.

But what do you do if you don't have the original tire, or if the markings are not legible? All is not lost! If you have a tape measure, you can measure the rim to determine the ISO/E.T.R.T.O. Bead Seat Diameter. You can measure either the diameter or the circumference.

The rim's diameter will generally be about 10 mm larger than the ISO/E.T.R.T.O. bead-seat diameter, depending on how high the rim flanges stick up above the bottom of the rim channel.

Lay your measuring tape or ruler across the rim from one side to the other. Get the largest measurement, between two points directly opposite one another. Slide one end of the tape back and forth along the rim until the measurement is largest. Measure across a few different diameters and take the average, in case the rim isn't quite round. (If it's way out of round, don't use it. If it's a millimeter or two out of round, the spokes will pull a shallow-section aluminum or steel rim into shape -- not so much for a deep-section carbon-fiber rim..)

Next you get out your pocket calculator or smartphone app, or old-school paper and pencil. As an example, the measured diameter of the rim shown in the photo is 20 7/8 inches, or 20.875 inches. Multiplying by 25.4 gives the diameter in millimeters, 530 mm.

The table below gives common (and uncommon) rim sizes with the corresponding bead seat circumference dimensions. 530 mm is the outside diameter, and the ISO/E.T.R.T.O. diameter is a few millimeters smaller, so this is a 520 mm rim.

If you are working with an empty rim, it is easiest to measure the diameter, but if you have a built-up wheel, the hub will get in the way of the tape measure, making it difficult to get an accurate measurement. For a built-up wheel, it is easier to measure the rim's circumference. It may also be helpful to take a circumference measurement to confirm that the diameter measurement was correct.

Instructions on measuring using the circumference are below the table.

You may measure the circumference of a rim by wrapping a measuring tape all the way around the rim. You derive the diameter from the circumference.

A narrow, metal tape measure -- 1/4 inch or 6 mm wide -- will fit into the well of the rim. (A wide metal tape measure won't fit into the well of the rim and and won't curve smoothly around the rim.)

Don't trust a fabric measuring tape as used in fitting clothing. This kind is usually inaccurate, because the fabric stretches.

Use the metal tape measure as shown in the image below.

Here are the steps to measure using the circumference:

1. The tape has a tab at the end. Hook the tab into the valve hole and wrap the tape all the way around the rim, measuring the total circumference at the bottom of the well.
2. Divide the circumference by pi (3.142) to get the diameter of the well.
   
3. If the tape measure is divided in inches, also multiply by 25.4 to get the diameter of the well in millimeters.
4. Add twice the height from the well to the bead seats (see instructions below).

If you don't have a narrow tape measure, you could wrap a length of thin, flexible electrical wire or bicycle cable inner wire around the rim, mark two places on the wire which line up with one another, lay the piece out flat and measure the distance between the two marks.

Our example rim is a hook-edge rim without clearly-defined bead seats, so we'll measure from the well (but not the bottom of the recessed spoke holes) to the outside of the rim and then subtract twice the typical flange height. Our highly-sophisticated tool for this task is a bicycle spoke. We are also using a small ruler as a bridge across the rim flanges. Holding the spoke with a thumbnail against the ruler gives us a good enough measurement for our purposes.

The measurement can be transferred to the ruler:

Now, calculating, the circumference of the well measured as 64 1/8 inches, (64.125 inches). Multiplying by 25.4 gives 1629 mm; then dividing by pi (3.142) the diameter is 518.5mm. 16mm additional (twice the depth of the well) gives 534 mm, but the bead seat diameter is be about 10 mm smaller, and this is a 520 mm rim.

Measuring rims involves some elementary-school or pocket-calculator math. Sheldon quoted Robert Heinlein at the end of the article:

--Robert A. Heinlein

More about that quote. OK, so Heinlein (actually, a character in one of his books) said that, and Sheldon quoted that, but anyone who cannot accept that other people have different abilities and educational opportunities shows an unfortunate lack of compassion! -- John Allen, whose wife teaches immigrants who never got to attend school in their countries of origin.

But also there's this:

--Albert Einstein

I'm hoping that this article provides an easy guide to some practical math, and helps to dispel math anxiety. Also, a longer discussion of the Einstein quote is online, in case you're interested...