Bike wheel circumference.

Worked example

A 700x28C tire is 28-622 in ISO terms: width 28 mm on a 622 mm bead seat diameter. Approximate diameter = 622 + 2 × 28 = 678 mm, and circumference = π × 678 = 2,130 mm. The real rolling circumference is about 1% smaller (2,109 mm), and the Sheldon Brown chart gives 2,136 mm for this size. If a computer were set to 2,096 mm while the wheel is really 2,130 mm, it would read 1.62% low: 100 km displayed would be 101.6 km ridden.

How it works

Circumference = π × (bead seat diameter + 2 × tire width). The ISO (ETRTO) size is width then bead seat diameter in millimetres, such as 28-622. Twice the width is added because the tire sits on both sides of the rim. The source notes the actual rolling diameter is about 1% smaller for a road tire, and smaller again at low pressure.

The chart. The table lists circumference in mm for popular sizes; values vary by tire model, so the formula and chart can differ by a few millimetres. Either is accurate to about one or two percent, the source says; a roll-out test (ride one wheel turn with your weight on the bike and measure the distance) is more accurate.

Wrong setting. Distance and speed scale with the set circumference, so the error is (true − set) ÷ set.

Sources and basis (opened 2026-10-08): Sheldon Brown and John Allen, Cyclecomputer/GPS Calibration (ISO/ETRTO method, 1% smaller rolling diameter, tire-size table in mm; the 16 x 1.5 row was left out because it differs from the formula by about 10%, and only popular listed sizes appear).

FAQ

What is the wheel circumference of a 700x28C tire?

About 2,130 mm by the formula, 2,136 mm in the Sheldon Brown chart, and roughly 1% less when actually rolling.

How do I read a tire size like 28-622?

The first number is tire width in millimetres and the second is the bead seat diameter of the rim. For example, 622 mm is the 700C size and 559 mm is the 26-inch size in the chart.

Why is the chart value different from the formula?

The chart is based on rim sizes and tire cross-sections, and different tire models roll differently. The source says either method is generally accurate to within one or two percent.

How do I get the most accurate wheel size?

Do a roll-out test: sit on the bike, roll the front wheel through one turn on pavement and measure the distance, at your usual tire pressure.

What happens if my computer has the wrong size?

Distance and speed are off by the same percentage. At 1.62% in the example, 100 km displayed is 101.6 km actually ridden.