It should be mentioned (since it was kind of implied earlier) that a 700c tire IS NOT the same as a 27 inch.

27 inch is a 630mm diameter
700c is a 622mm diameter.

(this is bead fit diameter, btw)

On road bikes with tight fits (like a 700x20) that’s a huge freakin’ difference.

a 26 inch tire is the same as a 650c, however.

in case you don’t believe me:

ISO Bead Seat Diameter Traditional Designations
635 mm 28 x 1 1/2
630 mm 27 x anything
622 mm 700 C
599 mm 26 x 1.25, x 1.375
597 mm 26 x 1 1/4, 26 x 1 3/8 (S-6)
590 mm 26 x 1 3/8 (E.A.3), 650 A
587 mm 700 D
584 mm 650B, 26 x 1 1/2
571 mm 26 x 1, 26 x 1 1/2, 26 x 1 3/4, 650 C
559 mm 26 x 1.00- x 2.125
547 mm 24 x 1 1/4, 24 x 1 3/8 (S-5)
540 mm 24 x 1 1/8, 24 x 1 3/8 (E.5), 600 A
520 mm 24 x 1, 24 x 1 1/8
507 mm 24 x 1.5- x 2.125
490 mm 550 A
457 mm 22 x 1.75; x 2.125
451 mm 20 x 1 1/8; x 1 1/4; x 1 3/8
440 mm 500 A
419 mm 20 x 1 3/4
406 mm 20 x 1.5- x 2.125
390 mm 450 A
369 mm 17 x 1 1/4
355 mm 18 x 1.5- x 2.125
349 mm 16 x 1 3/8
340 mm 400 A
337 mm 16 x 1 3/8
317 mm 16 x 1 3/4
305 mm 16 x 1.75- x 2.125
203 mm 12 1/2 X anything.
152 mm 10 x 2
137 mm 8 x 1 1/4

For a reference table of tire (tyre) sizes, try this link. It’s the UK National Cyclists` Organizations website that has the most complete chart I’ve seen anywhere; it’s even got more the Sheldon Browns! With a discussion of tire compatibility, how tires are measured, recommending the use of ISO ratings, as well as minimum pressure recommendations (for road use). For instance, a 26×1½ is a whole inch bigger than 26×1.5.

By the way, did you know that a Coker tire (which is not on the chart, by the way ) is not really a 36 inch tire? I measured mine and the 36 inch measurment seems to be the overall diameter, not the rim as with most other tires.

Pretty much all tire sizes are based on the outer diameter of the tire, more than the rim size. The rims are always smaller than their nominal “size.” For example, 24" rim is somewhere around 22" diameter. Further confusion for people trying to figure out what size things are.

So there’s the size marked on the tire, which is in most cases close enough. Except for the exceptions. Kind of like the English language.