Spoke Length Calculator
Enter your wheel dimensions in millimeters. This calculator returns left and right spoke lengths for standard J-bend wheel builds.
Tip: Use manufacturer dimensions whenever possible. If your wheel is dished (rear or disc front), left and right lengths will differ.
How this Park Tool spoke calculator works
A spoke length calculator turns hub and rim measurements into practical spoke lengths you can buy and build with. The geometry is straightforward: each spoke connects the rim radius to a hub flange hole while crossing a certain number of neighboring spokes. Change any dimension and the required length changes too.
This tool uses the classic wheel-building trigonometry approach and computes each side independently. That is essential for modern wheels, because left and right bracing angles are often different. Rear wheels with cassettes and many front disc wheels are asymmetrical by design.
Inputs explained (quick reference)
ERD (Effective Rim Diameter)
ERD is the single most critical measurement. It is not the outer rim diameter; it is the diameter at which nipple heads seat in the rim. If your ERD is off by 2 mm, your spoke length will be off by about 1 mm.
Flange diameter (left and right)
Flange diameter is measured from spoke hole center to opposite spoke hole center on each flange. Many hubs are symmetrical, but not all. Measure both sides.
Center-to-flange (left and right)
This is the distance from the hub centerline to each flange. It sets bracing angle and has a direct impact on spoke length. Wider center-to-flange dimensions generally increase spoke length slightly.
Spoke count and cross pattern
Spoke count is total wheel count (e.g., 24, 28, 32, 36). Cross pattern is how many times each spoke crosses others: radial = 0x, one-cross = 1x, two-cross = 2x, three-cross = 3x.
Practical wheel-building workflow
- Get manufacturer hub dimensions and published ERD first.
- Verify ERD with two old spokes and nipples if possible.
- Run the calculator and record left/right results separately.
- Round down conservatively (often to nearest 0.5 mm).
- Build and check thread engagement during tensioning.
Formula used
For each side, the calculator uses:
L = √(R² + r² + d² − 2Rr cos(θ))
- R = ERD / 2
- r = flange diameter / 2
- d = center-to-flange
- θ = (360 / spokes per side) × cross pattern
This gives a theoretical spoke length before optional adjustment and rounding.
Common mistakes to avoid
1) Mixing units
Keep everything in millimeters. A single inch-based measurement will ruin results.
2) Using marketing rim diameter instead of ERD
A 700c label does not provide ERD. Always use real ERD data.
3) Assuming both sides are equal
Many wheels need different spoke lengths side-to-side. Ordering one size for both sides is a common and expensive error.
4) Rounding up aggressively
Slightly short spokes are usually safer than too long spokes that bottom out in nipples or protrude past the slot.
Final notes
This calculator is ideal for planning and verification. For critical race wheels or unusual rims/hubs, cross-check against manufacturer calculators and a trusted wheel-building reference. Small differences between calculators can occur because of assumptions about nipple seat, spoke hole geometry, and rounding methods.