This can't be measured directly: if doing a rollout test, divide the result by 6.283. See further notes by John Allen if doing a rollout test.

Radius in millimeters

700 X 44

354

700 X 38

347

700 X 35

345

700 X 32

342

700 X 28

336

700 X 25

335

700 X 23

333

700 X 20

332

27 X 1 3/8

345

27 X 1 1/4

343

27 X 1 1/8

342

27 X 1

340

26 X 2.125

330

26 X 1.9

324

26 X 1.5

312

26 X 1.25

311

26 X 1.0 (559 mm)

305

26 x 1 (650C)

311

Wide 700 C Tubular

338

Narrow 700C Tubular

335

26 X 1 3/8

330

24

305

24 x 1

279

20 X 1.75 (406)

254

20 X 1 1/4 (451)

257

17 x 1 1/4

211

16 x 1 3/8

204

If using a rollout test for greater accuracy, divide the result by 6.2832 (that is, divide by 2 * π). If you are using a calculator, it's easier and more accurate to use the π button!

For the Velotronic Easy, divide by 3.142 (or π) and also do the multiplication for miles. The exact multiplication factor for miles is 2/1.609344.