A bicycle's tire uses a measurement of overall diameter and width. The best way to determine the wheel size is to examine a properly fitting tire and look for the size printed on its sidewall. Physically measuring the tire should be a last resort, since this measurement could differ from the actual wheel size, especially if the current tire is not the original one.
Turn the bicycle over and balance it on the seat and handlebars.
Examine the side of the tire and look for the printed size. The size will use the format "27 x 1 1/2" or "700 x 20," for example.
Read the first number, which will be the overall diameter measured from one outside edge of the tire to the other across its widest point. This number might be in inches or millimetres. Inch measurements will usually be in the teens or 20s, such as 16, 22, or 28. Millimetres will be in the hundreds, such as 600, 650 or 700. You can convert millimetres into inches by dividing the millimetres by 25.4.
Read the second number, which will be the wheel rim width. This can also be in inches or millimetres. Small fractional numbers, such as 1 1/2 or 1 3/8, will be inches. Larger numbers, such as 20 or 38, will be millimetres.
Measure the diameter of the entire wheel, including rim and inflated tire. This should be measured through the centre from the outer edges of the tire. This measurement, used only if the size printed on the tire is no longer legible, can be used to determine wheel size.
Measure the diameter of the rim in a similar manner if you only have a rim. However, the diameter of the rim is not measured from the outside edge, since this varies in height. Instead, measure across the rim diameter from the level of the bead seat on one side to the level of the bead seat on the other. The bead seat is where the tyre's bead -- it's stiff inside edge -- would sit on the inside of the rim. This diameter measurement is different than that of the wheel, but it will give you a measurement to offer a sales person, who can use it to reference the appropriate wheel size.
Measure the width of the wheel between the inner edges of the rim. You also can measure between the outer edges of the rim for a more complete picture of the wheel's size. These measurements are best measured with the tire off, and again, only if you cannot read the size on the tire.