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Howard Sutherland
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.
I 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 510 mm larger than the beadseat 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.
Next you get out your pocket calculator or smartphone app, or if you would like to show that you attended elementary school before such things existed, paper and pencil. 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 builtup wheel, the hub will get in the way of the tape measure, making it difficult to get an accurate measurement. For a builtup 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.
Traditional Designation  Applications/Notes  ISO Bead Seat Diameter  Bead Seat Circumference (Rim measurement) 


28 x 1 1/2, 700 B  English, Dutch, Chinese, Indian Rodbrake roadsters (Also marked F10, F25, 700 B) 
635 mm  1995 mm  78.5 inches 
27 x anything  Older road bikes, went out of fashion in the early 1980s  630 mm  1979 mm  77.9 inches 
700 C, 28 x 1 5/8, 29 inch (28 x 1 1/2 F.13 Canada) 
Most newer adult bikes for road use use this size, including most road bikes and hybrids. 29 inch are fat tires, same rim diameter. Also 700C tubular.  622 mm  1954 mm  76.9 inches 
26 x 1.25, x 1.375  Very Rare U.S. size, 1940s and older. Not available.  599 mm  1881 mm  74.1 inches 
26 x 1 1/4 EA.1, 26 x 1 3/8 (S6), 650  Schwinn "lightweights", older English "club" bikes  597 mm  1875 mm  73.8 inches 
26 x 1 3/8 (E.A.3), 650 A  Most 3speeds except Schwinn, departmentstore or juvenile 10 speeds  590 mm  1853 mm  73.0 inches 
700 D, 26" tubular  Oddball size formerly used on some GT models. Not available. 590 mm or 584 mm tires may work. Also, older Mavic 26" tubulars  587 mm  1844 mm  72.6 inches 
650B, 26 x 1 1/2, 26" tubular  French utility bikes, tandems, and loadedtouring bikes; some older Raleigh and Schwinn mountain bikes.This size is currently undergoing something of a renaissance. Some 26" tubulars.  584 mm  1834 mm  72.2 inches 
26 x 1, 650 C, 26" tubular  Triathlon, time trial, high performance road bikes for smaller riders. Some 26" tubulars.  571 mm  1793 mm  70.6 inches 
26 x 1 3/4  Schwinn cruisers  
26 x 1.00 x 2.125  Most mountain bikes, cruisers  559 mm  1756 mm  69.1 inches 
24 x 1 1/4, 24 x 1 3/8 (S5)  Rare British or Schwinn juvenile  547 mm  1718 mm  67.7 inches 
600A  French juvenile, very close to the nearest British size.  541 mm  1699 mm  66.9 inches 
24 x 1 1/8, 24 x 1 3/8 (E.5)  British juvenile, most wheelchairs. French 600A is 541 mm, close enough. Saavdra 25" tubular  540 mm  1696 mm  66.8 inches 
24 x 1, 24 x 1 1/8, 24" tubulars  High performance wheels for smaller riders; Terry front, most 24" tubulars.  520 mm  1633 mm  64.3 inches 
24 x 1.5 x 2.125  Juvenile mountain bikes, BMX cruisers  507 mm  1593 mm  62.7 inches 
22 x 1 3/8  Wheelchair  501 mm  1573 mm  62.0 inches 
550 A, 22 x 1 3/8  European juvenile, folding bicycles  490 mm  1539 mm  60.6 inches 
550C, 22 x 1 1/4", 22"tubular  European juvenile and racing bicycles (rare); 22" tubular  470 mm  1477 mm  58.1 inches 
22 x 1.75; x 2.125  Rare juvenile size...Schwinn  457 mm  1436 mm  56.5 inches 
20 x 1 1/8; x 1 1/4; x 1 3/8  Juvenile lightweights, BMX for light riders, some recumbents, Bike Friday Pocket Rocket  451 mm  1417 mm  55.8 inches 
500 A  European juvenile, folding  440 mm  1382 mm  54.4 inches 
20 x 1 3/4, 20"tubular  Rare Schwinn juvenile, specialty racing bicycles, older Easy Racers recumbents  419 mm  1316 mm  51.8 inches 
20 x 1.5 x 2.125  Most BMX, juvenile, folders, trailers, some recumbents  406 mm  1275 mm  50.2 inches 
18 x 1, 18 x 1 3/8  Wheelchair  400 mm  1257 mm  49.5 inches 
450 A  European juvenile  390 mm  1225 mm  48.2 inches 
17 x 1 1/4, 18" tubular  Alex Moulton AM series, 18" tubular for specialty racing bicycles.  369 mm  1159 mm  45.6 inches 
18 x 1.5 x 2.125  Birdy folding bikes  355 mm  1115 mm  43.9 inches 
16 x 1 3/8  Older Moulton, Brompton & other folders, recumbent front, Greenspeed trikes, juvenile  349 mm  1096 mm  43.2 inches 
400 A  European juvenile  340 mm  1068 mm  42.1 inches 
16 x 1 3/8  Very rare mystery tire  337 mm  1059 mm  41.7 inches 
16 x 1 3/8  Very rare Polish juvenile  335 mm  1052 mm  41.4 inches 
16 x 1 3/4  Rare Schwinn juvenile. Probably the same rim diameter as 16" tubulars.  317 mm  996 mm  39.2 inches 
16 x 1.75 x 2.125  Juvenile, folders, trailers, some recumbents  305 mm  958 mm  37.7 inches 
12 1/2 X anything.  Juvenile, scooters, trailers.  203 mm  638 mm  25.1 inches 
10 x 2  Wheelchair casters  152 mm  478 mm  18.8 inches 
8 x 1 1/4  Wheelchair casters  137 mm  431 mm  16.9 inches 
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:
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 hookedge rim without clearlydefined 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 highlysophisticated 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 about 10 mm smaller, and this is a 520 mm rim.
This article has featured common, inexpensive and improvised tools. More sophisticated tools such as a caliper with a depth gauge can make the work go faster.
Sutherland's sells a handy measuring tape which automatically calculates the diameter of a rim from the circumference. It is intended to measure spoking diameter but may also be used to measure the bead seat diameter.
Howard Sutherland demonstrates the Rim Diameter System in the video below:
Measuring rims involves some elementaryschool or pocketcalculator math. Sheldon quoted Robert Heinlein at the end of the article:
Robert A. Heinlein
More about that quote is online. OK, so Heinlein (actually, a character in one of his books) said that, and Sheldon quoted that, but anyone accepting that other people have different abilities and educational opportunities shows compassion!  John Allen, whose wife teaches immigrants whose educational opportunities were very limited 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...