How to Measure Spoke Length: ERD, Hub and Calculator
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To get the right spoke length for a wheel build, you need four measurements: the rim’s effective rim diameter (ERD), the hub’s flange diameter, the distance from each hub flange to the hub center, and the spoke count and lacing pattern. Enter them in the calculator below. If you only need to replace one broken spoke, skip the math and measure the old spoke from the inside of the elbow to the end of the threads, which is how Sheldon Brown defines spoke length.
Spoke length calculator
Measure your parts first (steps are below), then enter the numbers in millimeters. This calculator assumes a symmetric rim; for an asymmetric rim, add the rim offset in a full calculator such as the one Park Tool describes.
The default values are Park Tool's worked example (ERD 568 mm, 61 mm flanges, 135.4 mm over-locknut width, 32 spokes, three-cross), which gives about 274.5 mm on the left and 273.2 mm on the right. The calculator uses the standard law-of-cosines spoke formula that tools like Damon Rinard's Spocalc are built on.
How to measure your rim, hub and lacing
Step 1: Count the spoke holes and choose a lacing pattern
Count the holes in the rim and confirm the hub has half as many holes per flange. Then pick the cross pattern, because each extra cross needs a longer spoke. Sheldon Brown calls three-cross the usual choice (36 spokes are typically three- or four-cross, 32 spokes three-cross), notes that 24 and 28 spoke wheels are often two-cross, and says drive-side and disc-brake wheels should never be radial.
Step 2: Measure the rim's effective rim diameter (ERD)
ERD is the diameter of the circle formed by the tips of the spokes inside the nipples. It is smaller than the rim's bead seat diameter and differs from rim to rim, so don't guess it from the tire size. Park Tool's method uses two "measuring stick" spokes with nipples:
- Measure the length of each measuring spoke with a spoke ruler or gauge (they should be slightly shorter than half the rim diameter).
- Thread a nipple on each spoke until the spoke end is even with the bottom of the nipple slot, and put one in a rim hole and the other in the hole directly opposite.
- Pull both spoke heads toward the center so the nipples seat, then measure the gap between the two J-bends with a digital caliper.
- Add them: ERD = spoke 1 length + spoke 2 length + J-bend-to-J-bend distance. Park Tool's ERD article rounds down to the whole millimeter (260 + 260 + 48.4 = 568.4, so 568 mm).
Step 3: Measure the hub flanges
Per Park Tool's spoke-length guide, you need three hub measurements:
| Measurement | How to take it | Park Tool example |
|---|---|---|
| Flange diameter (spoke pitch diameter) | Caliper from the inside edge of one spoke hole to the outside edge of the hole directly opposite. Measure each flange separately. | 61 mm |
| Flange-to-center distance, left and right | Find the hub center (half the over-locknut or end-cap-to-end-cap width). Measure from the middle of each flange to the face of the locknut or end cap, then subtract each from the hub center. | Left 32.5 mm, right 18.7 mm on a 135.4 mm hub |
| Spoke hole diameter | Usually 2.5 mm; measure it to be sure. | 2.5 mm |
A digital caliper handles every measurement above, and a spoke ruler or spoke gauge checks the length of the measuring spokes. Check the current price on Amazon.
How to measure an existing spoke to replace it
If one spoke broke and the rest of the wheel is fine, the quickest route is to match it. Take an intact spoke from the same side of the wheel (left and right can differ by a millimeter or two), and measure from the inside of the elbow to the very end of the threads. A spoke gauge or ruler gives a quick reading; a caliper works too. Buy the same length, same gauge and same head type. Sheldon Brown also points out that measuring an existing wheel with the same lacing gets you reasonably close when you are building a matching wheel.
Mistakes that give the wrong spoke length
| Mistake | What goes wrong |
|---|---|
| Using a rim's listed ERD from a database without checking it | Park Tool advises taking your own measurements, especially for a rim or hub you haven't built with before; not every database is current. |
| Mixing up the sides of a dished rear wheel | Drive-side and non-drive-side spokes need different lengths; use the matching flange measurements for each side. |
| Using a different cross pattern than the one you calculated | Every cross changes the length, so recalculate whenever the lacing changes. |
| Rounding early | Keep tenths of a millimeter through the calculation and round only the final spoke length. |
| Ignoring rim offset on asymmetric rims | An offset rim shifts the effective flange distances, so use a calculator that accepts rim offset. |
For wheel truing, tensioning and anything beyond a straight replacement, a spoke tension meter is the tool that tells you whether tension is even. If you are not confident building a wheel, have a shop lace and tension it; wheel failures are a safety issue.
Related guides
Working on the hub itself? See how to tighten a rear hub. Choosing a new rim width? Read how to choose bike tire width. Rim diameters are explained in are 26-inch mountain bikes obsolete.
Frequently asked questions
What do I need to measure spoke length for a new wheel?
A caliper, a ruler or spoke gauge, two spare spokes with nipples to measure the rim's ERD, and the hub's flange diameter and flange-to-center distances. Then use a spoke calculator with your lacing pattern.
Where do you measure a spoke from?
From the inside of the elbow (the J-bend) to the very end of the threads, which is the standard definition of spoke length.
Can I measure spokes while they are still on the bike?
You can compare an exposed spoke against a ruler or gauge, but a spoke under tension is hard to measure accurately. Removing one intact spoke from the same side of the wheel gives a reliable reading.
What happens if I order the wrong length?
Spokes that are too long can poke into the rim bed or tube, and spokes that are too short may not engage enough thread in the nipple to tension safely. Re-measure and reorder rather than forcing it.
Can I use an old spoke to size new ones?
Yes, if it is from the same wheel and side and the lacing is unchanged. If you are changing rims, hubs or the cross pattern, recalculate.
Suryashankar Dasgupta founded Wise Wheeling. He has been cycling for more than 20 years, on geared and single-speed bikes, across many kinds of terrain. His guides cover bike repair, fit and gear, and each one is checked against manufacturer manuals, Park Tool and other primary sources before it’s published. Read how guides are made: https://wisewheeling.com/editorial-policy/
