Spoke Length Calculator

ERD is the diameter at which the spoke nipple seats inside the rim — it's the critical rim measurement for spoke length calculations. The most accurate way to measure it is to thread two spokes with nipples into opposite holes of the rim, then measure the distance between the spoke ends and add twice the spoke length from the nipple seat to the spoke tip. Your rim manufacturer may also publish the ERD in their specifications.

On most rear wheels, the cassette or freewheel forces the hub flanges to sit off-center — the drive side is closer to the rim centerline. This asymmetry means the drive-side spokes pull at a steeper angle than the non-drive-side, resulting in different required spoke lengths. Front wheels with symmetrical hubs or disc-specific rear hubs may have very similar lengths on both sides.

The cross number indicates how many times each spoke crosses over or under other spokes between the hub and the rim. Radial (0-cross) lacing is the lightest but offers no twist resistance. 3-cross is the most common pattern, balancing strength, durability, and spoke angle. 4-cross is used for heavy-duty applications like tandem bikes or cargo wheels.

Measure from the center of one spoke hole to the center of the directly opposite spoke hole on the same flange. This gives you the Pitch Circle Diameter (PCD), also called flange diameter. Use a digital caliper for accuracy. If the holes are not directly opposite (odd hole counts), measure center-to-center of two adjacent holes and use trigonometry to find the PCD.

Center-to-flange distance is the measurement from the hub's axle centerline to the midplane of each flange. To measure it, find the total flange-to-flange distance (inside face to inside face), then measure each flange's thickness, and calculate accordingly. The sum of both center-to-flange distances should equal the hub's total flange spacing.

Generally, rounding to the nearest whole millimeter is acceptable. A spoke that is 0.5 mm too long may poke through the nipple and puncture the inner tube, so when in doubt, round down rather than up — especially if you're close to the midpoint. Always check that the finished spoke doesn't protrude beyond the nipple slot inside the rim.

Rim offset is the lateral displacement of the spoke hole circle from the rim's centerline. Most standard rims have zero offset, but some asymmetric rims (like those designed for rear MTB wheels) use an offset spoke bed to improve drive-side spoke tension balance. If your rim is asymmetric, use the manufacturer's specified offset value to get accurate spoke lengths.

Usually not. Front hubs are typically symmetrical with both flanges equidistant from center, while rear hubs are offset to accommodate the drivetrain. ERDs may also differ between a matched front and rear rim set. Always calculate spoke lengths separately for each wheel using that wheel's specific hub and rim measurements.