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 Website 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 generally interchange tires of different widths as long as you match the ISO/E.T.R.T.O. size number.

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

  The rim's diameter will generally be 6-8 mm larger than the Bead Seat Diameter, depending on how high the rim flanges stick up above the bottom of the rim channel.

  If you are working with an empty rim, this is the easiest way to measure, but if you have a built up wheel, the axle will get in the way of the tape measure, making it difficult to get an accurate measurement.

• Circumference

  For a built-up wheel, it is easier to measure the rim's circumference by wrapping your tape measure all the way around the rim.

  Depending on the design of the rim and the width of your tape measure, you may or may not be able to measuer the actual bead seat part of the rim, but you should be able to get pretty close, and you can fudge a bit to allow for the way the tape fits into your particular rim.

  Below is a table of common (and uncommon) rim sizes with the corresponding bead seat circumference dimensions:

Anyone who cannot cope with mathematics is not fully human. At best he is a tolerable subhuman who has learned to wear shoes, bathe, and not make messes in the house.

--Robert A. Heinlein