Jun 20, 2026 · updated Jul 3, 2026 · Ger

How to measure ERD: the #1 spoke-length error

Get every hub and rim number perfect, and a wrong ERD will still leave you with spokes a millimetre or two off: long enough to bottom out in the nipple, or short enough to leave threads showing. More spoke-length errors trace back to ERD than to anything else, and almost every free calculator quietly assumes you measured it the same way it expects. We don’t. The calculator asks you which convention you used.

What ERD actually is

Effective Rim Diameter is the diameter of the circle the spoke ends reach. It is not the rim’s outer diameter, and not the bead seat. You measure it to where the spoke actually terminates inside the nipple. Because that endpoint depends on the nipple and how deep the spoke seats, ERD is a property of the rim and the nipples you build with. That is why you should measure it yourself rather than trust a spec sheet.

The two conventions

There is no single “ERD”. Two reference points are in common use, and they differ by roughly a nipple’s worth of length:

1. Two-spoke method (Park Tool style). Thread two old spokes into opposite nipple holes until their ends sit flush with where a spoke would seat, then measure tip-to-tip with a caliper:

ERD = 2 × (length of one reference spoke) + (caliper reading tip-to-tip)

2. Nipple-head / tape method. Measure to the top of the nipple head on each side:

ERD = A + 2 × B

where A is the diameter measured across the rim to the seat of the nipple heads, and B is the length the spoke must reach past that seat (often a fixed nipple offset). Different references give a different number, typically with a 2 to 4 mm gap between the two.

The two typically sit 2–4 mm apart. Neither is wrong — what wrecks wheels is feeding one convention’s number into a calculator while assuming the other.

Worked example

Say you thread two 300.0 mm reference spokes and read 2.0 mm tip-to-tip:

ERD = 2 × 300.0 + 2.0 = 602.0 mm

Now feed that into the calculator. A 1 mm error in ERD moves each side’s spoke length by very nearly 0.5 mm. Spokes are sold in 2 mm steps, so a careless ERD can push you to the wrong length on both sides at once.

Why it dominates accuracy

Hub flange diameter and centre-to-flange shift the result a little, and so does the cross pattern. ERD sits under a square root multiplied by the rim radius, so its leverage on the final number is large and direct. A few rules that save wheels:

1. Measure your own rim with your own nipples. Listed ERDs are starting points, not gospel.
2. Know which convention your number is. A spec sheet that doesn’t say is a guess.
3. Round each side down independently. A short spoke that buries in the nipple beats one that leaves threads proud. And never average two asymmetric sides into one length.

In the length calculator the ERD field is explicit and front-and-centre, and the convention is shown rather than assumed. Take the measurement carefully, then take it again to confirm before you order spokes.

FAQ

Can I just use the ERD from the rim maker or this site’s database?

As a starting point, yes — every record in the rim database carries its source and a confidence score, so you can see what you’re trusting. But ERD depends on the nipples you build with, and listed values rarely say which convention they used. For a wheel you’re about to order spokes for: measure.

My measurement is 3 mm off the listed ERD. Which number wins?

Yours — if it repeats. Measure across a second pair of holes; if both readings agree, trust your caliper. A stable 2–4 mm gap against a published figure usually just means the other number was taken with the other convention.

For the rest of the jargon, see the glossary — or put the number straight to work: build a wheel from start to finish.