Spoke length calculator
Enter the rim's effective rim diameter (ERD), the hub's flange measurements and the lacing pattern. The calculator gives the spoke length for the left and right side, which differ on almost every rear wheel and disc-brake front wheel.
Runs in your browser: nothing you enter is sent anywhere.
How to use
- Enter the ERD from the rim’s specifications.
- Enter the spoke count and the lacing pattern (number of crosses).
- Enter the diameter and the centre-to-flange offset of each flange from the hub’s specifications.
- Keep the spoke hole diameter at 2.6 mm unless your hub says otherwise.
- Read the left and right lengths and round them to the spoke sizes you can buy.
L = √(R² + r² + W² − 2·R·r·cos θ) − d ÷ 2
where R = ERD ÷ 2, r = flange diameter ÷ 2, W = flange offset, θ = 720° × crosses ÷ spokes and d is the spoke hole diameter. The first three terms describe a straight line from the hole in the flange to the nipple seat in the rim; half the hole is taken off because the spoke bends through the near side of the hole.
Example
ERD 600 mm, 32 spokes 3-cross, both flanges 58 mm, offsets 35 mm (left) and 20 mm (right), holes 2.6 mm:
- θ = 720 × 3 ÷ 32 = 67.5°, cos θ = 0.38268; R = 300, r = 29;
- left: 300² + 29² + 35² − 2 × 300 × 29 × 0.38268 = 85,407.3, √ = 292.25 − 1.3 = 290.9 mm;
- right: 300² + 29² + 20² − 6,658.7 = 84,582.3, √ = 290.83 − 1.3 = 289.5 mm.
Tips
- Measure twice: an ERD that is off by 2 mm changes the spoke length by about 1 mm.
- Straight-pull hubs use a different geometry; use the hub maker’s own calculator for them.
- Order a spare or two of each length — a short spoke is the most common building mistake.
Frequently asked questions
What is ERD and how do I measure it?
Effective rim diameter is the diameter at which the spoke ends sit inside the rim, measured across the rim through the nipple seats. Rim makers publish it; to measure, put two spokes with nipples through opposite holes, measure from end to end and add the distance to the nipple seat on each side.
What are flange diameter and offset?
Flange diameter is the diameter of the circle through the centres of the spoke holes on one hub flange. Offset is the distance from the centre of the hub (half way between the locknuts) to the middle of that flange. On a rear hub the drive-side offset is smaller, which is why the spokes are shorter there.
How do I round the result?
Spokes are sold in 1 or 2 mm steps. Most builders round to the nearest available length, and round down slightly when the result is close to the next size or the rim is thin, so the spoke does not poke through the nipple. Check your rim and nipple maker's advice.
What does the number of crosses mean?
It is how many other spokes each spoke crosses on its way from the hub to the rim on the same side. 3-cross is common for 32 and 36 spoke wheels, 2-cross for 24 and 28, and 0 (radial) for some front wheels. The pattern needs crosses × 4 to be smaller than the spoke count.