MTB Spoke Length Calculator

Pick your hub and rim from the workshop database, set spoke count, cross pattern and any rim offset, and the calculator returns four lengths — left and right, front and rear. Under the hood it uses the standard wheelbuilding formula: each spoke is the three-dimensional hypotenuse from the hub flange hole to the rim’s nipple seat, worked out from the flange diameter (PCD), the centre-to-flange distance, the rim’s effective diameter (ERD) and your lacing pattern, minus half the flange hole. The recommended column then rounds that figure and applies your nipple and washer adjustments, so you get a length you can actually buy.

ERD — effective rim diameter — is the diameter measured at the point in the rim where the end of the spoke sits: across the nipple seats, not the rim’s inner or outer edge. It’s the most sensitive number in the whole calculation. An ERD that’s out by 2 mm moves every spoke length by about 1 mm — the difference between threads buried safely in the nipple and threads poking at your rim tape.

There’s a quiet assumption built into every ERD figure: it was measured with some nipple in mind, and it rarely says which one. Different nipple designs seat the spoke at different depths, so the same rim behaves as if it has a different ERD depending on the nipples you lace with — a shift that can run to several millimetres. SpokeCalc’s nipple seat diameter explainer covers this in detail, and the classic wheelbuilding references land in the same place: an ERD figure on its own is incomplete without knowing the nipple it assumes.

Because most wheels are dished. Disc rotors and cassettes push the hub flanges off-centre, so one flange sits closer to the rim’s centreline — the spokes on that side run steeper and shorter. Offset-drilled rims claw some of this back, which is what the rim offset field models: offset shifts the nipple bed sideways, evening up the lengths and the spoke tensions with them.

Our workshop default rounds down. A touch short still engages plenty of thread in the nipple; a touch long can bottom out against the nipple head or push through into the rim bed. The calculator also applies your nipple type’s length adjustment and adds a millimetre when you run nipple washers. The actual column always shows the raw figure, so you can see exactly what the rounding did.

Yes — custom wheel and bike builds are core workshop work. Bring us a hub and rim (or let us spec them) and we’ll sort the right spokes as part of the build, then lace, tension-balance and stress-relieve the wheel properly. If your hub or rim isn’t listed in the calculator yet, book the workshop and we’ll spec the build for you.