Cycling & wheel building
Bike ERD calculator.
Work out the effective rim diameter with the two-spoke method, then convert hub and rim measurements to the numbers a spoke length calculator wants. Runs in your browser.
Measurements (mm)
Results
Fill the form and press Calculate.
Worked example
Two 266 mm spokes joined at their J-bends with a 35.7 mm gap give an ERD of 266 + 266 + 35.7 = 567.7 mm. A hub 135.4 mm over locknuts has a centre at 67.7 mm; with the flanges 35.2 mm and 49 mm from the locknuts, the flange-to-centre distances are 67.7 − 35.2 = 32.5 mm (left) and 67.7 − 49 = 18.7 mm (right). A 35 mm rim with the spoke hole 13.2 mm from the edge has a centre at 17.5 mm, so its offset is 17.5 − 13.2 = 4.3 mm. These are the figures used on Park Tool’s own wheel-building page.
How it works
ERD = spoke 1 + spoke 2 + J-bend gap. Park Tool’s method joins two spokes of known length through opposite spoke holes of the empty rim, measures the inside-to-inside gap between their J-bends and adds the three numbers. The hub flange-to-centre distance is half the overall locknut width minus the flange-to-locknut distance, and the rim offset is half the rim width minus the distance from the hole to the rim edge.
ERD sets the length of every spoke, so measure it on your own rim rather than using a catalogue figure, and check the result at more than one position round the rim.
Sources
Retrieved 2026-10-16. Park Tool gives the ERD, flange-to-centre and rim offset formulas used here, with the worked numbers reproduced above.
- Park Tool, Determining Spoke Length for Wheel Building
- Sheldon Brown, Damon Rinard’s Spoke Length Calculator
FAQ
How do I measure ERD on a bike rim?
Park Tool joins two spokes of known length in opposite holes of the bare rim, measures the gap between their J-bends with calipers and adds all three: spoke 1 + spoke 2 + gap. Two 266 mm spokes with a 35.7 mm gap give 567.7 mm.
What does ERD stand for?
Effective rim diameter: the diameter on which the ends of the spokes (the nipple seats) sit, used to work out spoke length.
How do I find the flange-to-centre distance of a hub?
Half the over-locknut width minus the distance from the flange to the locknut on that side. A 135.4 mm hub with the flange 35.2 mm from the locknut gives 32.5 mm.
What is rim offset?
How far the spoke holes sit from the centre line of the rim: half the rim width minus the distance from the hole to the rim edge. It is shown here for reference.
Why should I measure ERD instead of using the catalogue figure?
It is the most sensitive input to spoke length, and rims and nipples vary, so a measured figure is safer than a published one.
Disclaimer
Measure carefully and confirm against the rim and hub maker’s data. A wrong ERD gives wrong spoke lengths.