Spoke length calculator
Building a wheel? Spoke length comes from three dimensions: the rim's ERD, the hub geometry and the cross pattern. A typical front road wheel (ERD 602, hub 44/34 mm, 32 spokes, 3 cross) needs spokes of about 294 mm. Enter your own measurements and get the left and right side separately.
ERD (effective rim diameter) is measured to the nipple seats: it is in the rim's specification, or you can measure it with two spokes and nipples. Hub dimensions (flange hole circle diameter and flange distances) come from the hub manufacturer. On a wheel with a cassette the right flange usually sits closer to the centre, which is why the two sides need different spokes.
How is spoke length calculated?
This is the classic trigonometric formula, the same one behind the DT Swiss and Sapim calculators:
l = sqrt( r1^2 + r2^2 + f^2 - 2 x r1 x r2 x cos(a) ) - d/2 r1 - ERD / 2 (rim radius to the nipple seats) r2 - hub flange hole circle diameter / 2 f - flange distance from the hub centre a - spoke angle: 720 degrees x crosses / spoke count d - spoke hole diameter in the flange (assumed 2.4 mm)
It is pure geometry: the spoke is the diagonal of a triangle spanning the nipple seat and the flange hole, and the angle between them follows from the cross pattern. Worked example: ERD 602 mm, a 44 mm flange circle sitting 34 mm from the centre, 32 spokes, 3 cross. The angle is 67.5 degrees and the formula returns 294.0 mm, so you order 294 mm spokes. The result is shown to 0.1 mm so you can see which commercial length is closer.
Accuracy of the ERD matters more than anything else here: a 2 mm error in ERD shifts spoke length by 1 mm on each side. If wheel building is your first venture into home mechanics, a chain replacement is a gentler start, and the chain length calculator sizes that for you.
Frequently asked questions
- What is ERD and how do you measure it?
- ERD, the effective rim diameter, is the diameter of the circle where the spokes end, measured to the nipple seats. Manufacturers publish it, and you can measure it by threading two spokes with nipples fully wound on into opposite holes, measuring between the spoke ends and adding both spoke lengths. A 2 mm error in ERD results in spokes 1 mm too short or too long on each side.
- Why are left and right spokes different lengths?
- On a wheel with a cassette or a disc rotor the hub flanges are not symmetric about the centre: the drive side sits closer to the middle to make room for the cassette. A smaller flange offset means a marginally shorter spoke, usually by 1-2 mm. That is why this calculator computes each side separately.
- How many crosses should you lace a wheel with?
- Three cross is the standard for 32 and 36 spoke wheels, giving good lateral stiffness and drive transfer. Wheels with 24 or 28 spokes are usually laced two cross. Radial lacing (zero cross) appears on front wheels without a disc rotor, but it needs a hub rated for it, since the flange takes higher loads.
- What if the result falls between available spoke lengths?
- Spokes are typically sold in 2 mm steps. For an in-between result, take the nearer length, and when it lands almost exactly halfway, take the shorter one: the nipple has a few millimetres of thread in reserve, whereas a spoke that is too long can protrude past the rim bed and puncture the tape or tube.