Bicycle Spoke Length Calculator

ERD is the effective rim diameter, the diameter across the points where the spoke ends seat inside the nipples. It is the single most important rim number for spoke length, and manufacturers or a measured value both work.

Cross count is how many other spokes each spoke crosses on its way from hub to rim. The lacing angle equals crosses times 720 divided by spoke count. Road builds often use 3 cross, and 0 cross is a straight radial lacing.

It uses the law of cosines on the rim radius, flange radius, and center to flange offset, with the lacing angle, then subtracts half the spoke hole diameter. The formula is sqrt(r squared plus R squared plus f squared minus 2 r R cos angle) minus hole over 2.

Rear hubs and many front hubs are dished, so the left and right flanges sit at different distances from the center plane and often have different diameters. Enter each flange's own numbers and run the tool twice for one wheel.

Most hub flanges use a 2.6 mm spoke hole, the default here. If your hub spec lists a different flange hole size, enter it, since the tool subtracts half of this value from the raw geometric length.

Yes. The tool requires a positive even number such as 24, 28, 32, or 36, because spokes leave the hub in balanced left and right groups. An odd or fractional count is rejected with a message.

Spokes come in 2 mm steps, so round to the nearest available length. Many builders round down slightly for 3 cross since a hair short still threads fully, but always sanity check against a second calculator before ordering.

No. The calculation runs entirely in your browser using plain arithmetic, so your rim and hub measurements never leave your device and nothing is stored.

Yes, as long as your ERD reflects the true rim you are building. Asymmetric rims change the effective dish, which shows up in the center to flange numbers you measure, not in the formula itself.