Spoke Length Calculator

Frequently Asked Questions

How do I measure rim ERD accurately?

Thread two standard spokes into opposite holes in the rim with nipples. Measure the total distance from the outer face of one nipple to the outer face of the opposite nipple. Subtract the two nipple protrusion depths (typically 1 mm per side). The result is the ERD. Rim manufacturers also list ERD in their spec sheets - use that value if available.

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

A rear wheel is dished: the cassette-side flange is closer to the rim centre line than the non-drive-side flange. The closer flange has a larger centre-to-flange distance, which reduces the angle and shortens the spoke. Drive-side spokes are typically 1-3 mm shorter than non-drive-side spokes on a standard rear wheel.

What is the most common spoke pattern?

Cross 3 (three-cross) is the most common pattern for road, gravel, and mountain bikes. It provides good stiffness, strength, and stress distribution. Cross 2 is used on lighter road wheels where stiffness is prioritised. Radial (cross 0) is suitable only for the non-drive front wheel and provides a stiff, aerodynamic look but concentrates stress at the flange.

Can I use this for both alloy and carbon rims?

Yes - the formula uses ERD regardless of rim material. Carbon rims often have different ERD values than alloy rims of the same nominal size, and many manufacturers provide spoke drilling guides. Always use the manufacturer-specified ERD for carbon rims since spoke tension and length are more critical with carbon.

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This calculator provides estimates for informational purposes only. Results are based on assumptions and may not reflect actual outcomes. Consult qualified professionals in relevant fields before making important decisions based on these results.

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How This Calculator Works

Spoke length depends on the geometry of the rim and hub and on the lacing pattern. This calculator uses the standard three-dimensional formula that treats each spoke as the diagonal of a triangle running from the hub flange hole to the rim. It combines the rim Effective Rim Diameter (ERD), the hub flange Pitch Circle Diameter (PCD), the sideways offset from the hub centre to the flange, and the angle set by your cross pattern to return spoke length in millimetres, already adjusted for the nipple seat.

Formula and Methodology

The 3D spoke length formula is: L = √(r² + fl² + d² − 2 × r × fl × cos(α)) − 1.3, where:

r = ERD / 2 (rim radius to nipple seat, mm)

fl = PCD / 2 (hub flange radius, mm)

d = centre-to-flange distance (mm)

α = (4π × cross) / spokes (the angle between adjacent spoke holes around the flange)

The −1.3 subtracts the spoke head diameter from the outside of the flange hole to the bend point

The formula accounts for the three-dimensional path a crossed spoke takes: lateral displacement (d), circumferential angle (α), and radial span from flange to rim (r and fl).

How to Use the Results

Enter the rim ERD, which is the effective rim diameter measured to where the spoke nipple seats, not the bare rim diameter. Enter the hub flange PCD (the diameter of the circle formed by the spoke holes), the centre-to-flange distance (from the hub axle centre to the spoke flange), the spoke count, and the cross pattern (typically 2 or 3).

Important: Most wheels are dished, so the left and right flanges sit at different distances from the hub centre. Run the calculation twice - once for the drive side and once for the non-drive side - using each side's offset. The two results often differ by 1-3 mm.

Key Concepts: ERD, PCD, and Cross Patterns

ERD (Effective Rim Diameter): The diameter measured to the bottom of the nipple seat, not the outer rim diameter. Measuring ERD: thread two spokes into opposite holes with nipples, measure from nipple end to nipple end and add 2 × the nipple protrusion depth.

PCD (Pitch Circle Diameter): The circle formed by the spoke hole centres on the hub flange. Measure with calipers from the centre of one hole to the centre of the opposite hole.

Centre-to-flange distance: From the hub axle centre to the flange face, measured along the axle.

Cross pattern: The number of times each spoke crosses adjacent spokes from the same flange. Cross 3 is the most common for road and mountain bikes; cross 2 is used on lighter wheels; radial (cross 0) is only suitable for the front wheel on road bikes.

Common Mistakes

Using bare rim diameter instead of ERD - a 700c rim has a bare diameter of about 622 mm but ERD can range from 595-620 mm depending on rim depth and spoke-bed design.

Measuring only one side of a dished hub and applying it to both; drive and non-drive flanges have different offsets.

Ignoring the cross pattern and the resulting angle change - radial and cross-3 spokes for the same wheel differ by about 2-4 mm in length.

Forgetting to round to the nearest available commercial spoke length (spokes are sold in 1 mm or 2 mm increments).

Using flange OD instead of PCD - the measurement should be to the spoke hole centres, not the flange edge.

Tips and FAQ

Should I round up or down when the result falls between sizes? Round to the nearest available length. A 1 mm error is usually acceptable; 2 mm may cause the nipple to be too shallow in the rim (too long) or pull through too far (too short). When in doubt, order both lengths and test tension. Can I use radial lacing front and rear? Radial lacing is only suitable for non-drive front wheels; it concentrates stress at the hub flange and can crack flanges on rear and disc wheels under torque. What is a normal spoke count? Road bikes typically use 24-32 spokes, mountain bikes 28-36, touring and cargo bikes 36-40. Higher spoke count improves reliability and wheel stiffness at the expense of weight.

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