What is ERD (Effective Rim Diameter)?

ERD is the diameter of the circle where the spoke nipple ends sit in the rim. It is measured from the top of one nipple seat to the opposite side. Most rims have ERD stamped or available from the manufacturer.

How do I measure hub flange diameter?

Measure the diameter of the circle through the centre of the spoke holes on the flange (PCD - Pitch Circle Diameter). This is usually 38-65 mm for road hubs.

What cross pattern should I use?

3-cross is standard for most wheels. 2-cross works for low-spoke-count wheels (24 spokes). Radial (0-cross) is used for front non-disc wheels only. Higher cross = stronger but heavier.

How accurate does the spoke length need to be?

Within 1mm is ideal. Too short and you cannot get enough nipple engagement. Too long and the spoke may protrude through the nipple. Order to the nearest 1mm or 2mm available.

Why are drive-side and non-drive-side different?

Rear hub flanges are asymmetric — the drive side is closer to centre. This means different spoke lengths for each side. The calculator gives one side; repeat with different flange offset for the other side.

BikeCalcs

Spoke Length Calculator — Precise Spoke Lengths for Wheel Building

Calculate exact spoke lengths from hub and rim dimensions for wheel building.

ERD (Effective Rim Diameter)

Hub Flange PCD

Flange Offset (from centre)

Spoke Hole Diameter

Total Spoke Count

Cross Pattern

How We Calculate This

Spoke Length Formula

R = ERD / 2 (effective rim radius)
r = Hub flange PCD / 2 (flange radius)
d = Flange offset from hub centre
theta = 4 x pi x crosses / (spokes per side)
L = sqrt(R² + r² + d² - 2Rr cos(theta)) - spoke hole radius
Result rounded to nearest 0.5 mm for standard spoke ordering

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Always verify results and consult a professional bike fitter where appropriate.