Spoke length calculator
Calculate left and right separately from your measurements and the model assumptions. Pick a hub and rim or enter the values manually; check ERD and offset before ordering spokes.
New to the terms? Glossary → · Build a wheel from start to finish → · How to measure ERD → · Build wizard →
Left and right are viewed from the rider’s position. On a conventional rear wheel, the drive side is right. On a front disc wheel, the disc side is left and the non-disc side is right.
Higher-tension side = 100%; the other is its share.
Check each hub and rim's provenance and notes: not all data has documented manual validation. Verify ERD and offset with the nipples and washers you will use, cross-check against a second calculator and plan on a test build.
How the maths works
The formula below describes the calculator's conventional J-bend geometric model, with equal spoke counts on both sides. It calculates a straight-line distance in 3D, then subtracts half the flange-hole diameter. W is the axial flange-to-spoke-bed distance after any rim offset correction, D the flange pitch-circle diameter, ERD the effective rim diameter, X the cross count, N the total spoke count and s the flange-hole diameter. Each side uses its own dimensions and cross count. The result depends on the measurements and assumptions; it does not check physical clearance or guarantee suitable thread engagement.
L = √( W² + (D/2)² + (ERD/2)² − 2 · (D/2) · (ERD/2) · cos θ ) − s/2 θ = X · 720° / N
FAQ
- Why do the left and right sides get different lengths?
- The two sides can have different centre-to-flange distances, flange diameters, cross counts or spoke offsets. With other inputs equal, a smaller flange-to-spoke-bed distance gives a shorter spoke; that alone does not tell you which side is shorter on your wheel. A rim-brake front is not necessarily symmetric either. Use the current dimensions for each side and check each result independently; never average the two lengths into one.
- What is ERD and why does it matter so much?
- ERD (Effective Rim Diameter) is the diameter of the circle the spoke tips reach inside the nipples, and it is the single largest source of length errors: 1 mm of ERD moves each side by about 0.5 mm. It depends on your nipples as well as the rim, so measure it yourself. How to measure ERD →
- Should I round the result up or down?
- The default selects the shorter 2 mm stock size; Nearest selects the closest. Neither choice guarantees suitable thread engagement. Compare the unrounded result with the available lengths and the spoke and nipple manufacturer's guidance. Check for insufficient engagement, bottoming out and rim-tape clearance on each side.
- Does it handle asymmetric rims and straight-pull hubs?
- Yes, within the model's assumptions. An asymmetric rim's offset spoke bed (OSB) changes each side's flange-to-spoke-bed distance; check the offset and orientation. For straight-pull, the model replaces the J-bend −s/2 correction with an additive per-side spoke offset: positive means longer. A 1 mm change in that offset changes the result by 1 mm. That 1:1 relationship applies to this offset, not to every measurement. The manufacturer's dimensions, offset convention and lacing pattern must match the model.