About the Spoke Length Calculator
This spoke length calculator gives the spoke lengths you need to build or rebuild a bicycle wheel. Enter the rim's effective rim diameter (ERD), the hub's flange pitch-circle diameters and centre-to-flange distances, the number of spokes and the lacing pattern (radial, 1-, 2-, 3- or 4-cross), and it returns the left and right spoke lengths in millimetres.
It is for home wheel builders, bike mechanics and anyone ordering spokes for a new rim or hub. Rear wheels and disc-brake front wheels are dished, so the two sides usually need different lengths; enter each side's centre-to-flange distance separately. The table shows how the lengths change with other cross patterns.
The result uses the standard three-dimensional geometry formula, assuming a symmetric drilling and both sides laced with the same pattern. Measure ERD yourself or use the rim maker's figure, and take hub dimensions from the manufacturer's spec sheet — a 1 mm error in ERD changes the spoke length by about 0.5 mm.
With the default inputs, the left spoke length is 292.9 mm. Change any value above to recalculate instantly.
How to use the spoke length calculator
- 1Enter the rim ERD from the rim maker or your own measurement.
- 2Choose the spoke count and lacing pattern.
- 3Enter each flange's pitch-circle diameter from the hub spec sheet.
- 4Enter the left and right centre-to-flange distances.
- 5Read the left and right lengths and round to the nearest available spoke size.
Formula and method
A spoke runs in three dimensions from a hole in the hub flange to the nipple seat in the rim. Projected onto the wheel plane, the hub hole sits on a circle of diameter D and the nipple on a circle of diameter ERD; the angle between them, α, depends on how many spokes each one crosses (X) and on the spokes per side (N/2). The law of cosines gives that in-plane distance, and the centre-to-flange offset W adds the sideways component.
Half the flange hole diameter S is subtracted because the spoke bends at the edge of the hole, not its centre. The formula assumes evenly drilled flanges and rim, the same cross pattern on both sides and no rim offset (for offset or asymmetric rims, adjust W by the offset).
- L
- Spoke length, mm
- ERD
- Effective rim diameter (to the nipple seats), mm
- D
- Flange pitch-circle diameter, mm
- W
- Centre-to-flange distance for that side, mm
- X
- Number of crosses (0 = radial)
- N
- Total spoke count
- S
- Flange spoke-hole diameter, mm
Worked examples
Rear 32-spoke 3-cross on a 600 mm ERD rim
With 16 spokes per side, 3-cross gives α = 67.5°. The non-drive side (35 mm offset) works out to 292.9 mm and the drive side (19 mm offset) to 291.4 mm, so you would order roughly 293 mm and 291 mm spokes.
Symmetric front wheel, 28 spokes 2-cross, 622 ERD
A symmetric rim-brake front hub has identical sides, so both lengths are 299.3 mm. With 14 spokes per side, 2-cross puts α at about 51.4°.
Mountain bike rear, 540 ERD, 58 mm flanges
A 27.5-inch mountain bike rim with a 540 mm ERD on a hub with large 58 mm flanges needs about 260.7 mm on the left and 259.8 mm on the right for 32-spoke 3-cross lacing.
Frequently asked questions
What is ERD and how do I measure it?+
ERD (effective rim diameter) is the diameter at which the spoke ends sit inside the nipples. Measure it by putting two spokes with nipples through opposite holes, measuring end to end, and adding twice the length of spoke that sits inside the nipple — or use the rim maker's published figure.
Why are rear-wheel spokes different lengths on each side?+
The cassette pushes the drive-side flange closer to the centre of the hub, so the rim is dished toward it. The drive side has a smaller centre-to-flange distance and therefore slightly shorter spokes; disc front hubs do the same on the rotor side.
Should I round spoke length up or down?+
Spokes come in 1 or 2 mm steps. Most builders choose the nearest available size, and when the result falls right between two sizes they usually go 1 mm shorter, because a spoke that is too long can bottom out in the nipple before reaching tension.
Which lacing pattern should I use?+
Three-cross is the standard for 32- and 36-spoke wheels because it transmits pedalling and braking torque well. Radial lacing is only suitable for front rim-brake wheels with hubs rated for it, and lower spoke counts often use 2-cross.
How accurate is a spoke length calculator?+
The geometry is exact; accuracy depends on your inputs. Errors in ERD or flange dimensions of a millimetre or two are common, which is why measuring the actual rim and hub is the most reliable way to avoid a wrong order.