What a spoke calculator does and why you need one
A spoke calculator is a tool that figures out the correct spoke length for a wheel you are building. When you lace a rim to a hub, the spokes must be the exact right length — too short and they will not reach the nipple, too long and they will stick out dangerously or bend under tension. A spoke calculator takes measurements from your specific rim and hub and tells you the length you need, down to the millimetre.
The calculation itself is geometry: the spoke runs from a hole in the hub flange to a hole in the rim, and the path it takes depends on how many spokes you are using, how far the hub flanges sit from the hub centre, and the rim's diameter. You cannot guess this. A wheel built with the wrong spoke length will either not tension properly or will have spokes that protrude into the tyre.
Most spoke calculators are free online tools. The most widely used is the Spoke Calc at wheelbuild.com, though others exist. You enter your hub and rim specifications, and the calculator returns the spoke length in millimetres for each side of the wheel (drive side and non-drive side are often different).
Key Takeaways
- A spoke calculator requires the flange diameter, flange offset, and centre-to-flange distance for your hub, plus the effective rim diameter (ERD) for your rim.
- The same rim and hub can need different spoke lengths depending on whether you are building a drive-side or non-drive-side wheel, because the flanges sit at different distances from the hub centre.
- Spoke length varies by lacing pattern (radial, two-cross, three-cross) because the angle at which the spoke approaches the rim changes.
- If you do not have the specifications for your hub or rim, the calculator cannot work — you will need to measure them yourself or contact the manufacturer.
What measurements you need to gather
Before you open a spoke calculator, you need four pieces of information: two about your hub and two about your rim.
For the hub, you need the flange diameter (the distance across the holes where spokes attach) and the distance from the hub centre to each flange. These are not the same on both sides — a rear wheel hub has the cassette body on one side, which pushes the drive-side flange outward. Measure from the hub centre to the middle of each flange hole. You also need to know whether your hub is 32-hole, 36-hole, or another spoke count, because the calculator uses this to find the angle between adjacent holes.
For the rim, you need the effective rim diameter (ERD), which is the diameter measured at the point where the spoke nipple sits — not the outer edge of the rim. This is usually printed on the rim itself or in the manufacturer's specifications. If it is not, you can measure it by laying a spoke across the rim at the nipple seat and measuring the distance from one side to the other, then multiplying by π (3.14159).
You also need to know your lacing pattern: radial (spokes go straight from hub to rim), two-cross (each spoke crosses two others), or three-cross (each spoke crosses three others). The pattern changes the angle at which the spoke meets the rim, which changes the length you need.
How to enter data into a spoke calculator
Open the calculator and enter your hub specifications first. Most calculators ask for flange diameter in millimetres, then the distance from the hub centre to the drive-side flange and the non-drive-side flange separately. Some calculators call this the "offset" or "dish" — it is the same thing.
Next, enter the rim ERD. Then select your spoke count (32, 36, 40, etc.) and your lacing pattern. Some calculators let you choose the cross number (radial, 1-cross, 2-cross, 3-cross); others assume a standard pattern.
When you click calculate, the tool returns two numbers: the spoke length for the drive side and the spoke length for the non-drive side. Write both down. These are the lengths you will order from a spoke supplier. Spoke lengths are sold in half-millimetre increments (e.g., 290 mm, 290.5 mm, 291 mm), so if the calculator returns 290.3 mm, you would order 290.5 mm.
Why spoke length differs between wheel sides
On a rear wheel, the drive-side flange (where the cassette sits) is further from the hub centre than the non-drive-side flange. This means spokes on the drive side have a longer path to travel from the hub to the rim, so they must be longer. The difference is usually 2 to 4 millimetres.
On a front wheel, both flanges are usually the same distance from the centre, so both sides use the same spoke length. However, if your hub is asymmetrical (some modern hubs are), the calculator will still return two different lengths.
This is why you cannot use a single spoke length for both sides of a rear wheel. If you order the drive-side length for the non-drive side, the spokes will be too long and will not tension properly. If you order the non-drive-side length for the drive side, the spokes will be too short and will not reach the nipple.
What to do if you do not have hub or rim specifications
If your hub or rim did not come with a spec sheet and you cannot find one online, you will need to measure. For hub flange diameter, place the hub on a flat surface and measure across the holes with a ruler or calliper. For the distance from hub centre to flange, measure from the centre of the axle to the middle of a spoke hole, perpendicular to the axle.
For rim ERD, the easiest method is to place a straight spoke across the rim at the nipple seat, measure the distance from one side to the other, and multiply by 3.14159. Alternatively, some rim manufacturers print the ERD on the rim sidewall or in their online catalogues.
If you cannot find or measure these numbers, contact the hub or rim manufacturer directly. Most will provide specifications if you give them the model name and year. This is worth doing before you order spokes, because ordering the wrong length means wasted money and a wheel that does not work.
Common mistakes when using a spoke calculator
The most common error is confusing rim diameter with ERD. The rim diameter printed on the tyre (e.g., 700c, 26") is not the same as ERD. ERD is always smaller and is measured at the nipple seat, not the tyre bead. If you use the wrong number, your spokes will be off by several millimetres.
Another mistake is entering the same flange distance for both sides of a rear wheel. Rear hubs are dished — the flanges are at different distances from the centre. If you enter the same number twice, the calculator cannot account for this, and you will get one correct length and one wrong length.
Some builders also forget to account for lacing pattern. A radial wheel (no crosses) needs shorter spokes than a three-cross wheel with the same hub and rim, because the spoke angle is different. If you do not tell the calculator which pattern you are using, it may assume a default that does not match your plan.
Spoke calculator tools and where to find them
The most widely used free spoke calculator is at wheelbuild.com. It accepts hub and rim data and returns spoke lengths for both sides. It also has a database of common hubs and rims, so if your components are in the database, you can select them by name instead of entering measurements.
Other calculators exist, including tools built into some wheel-building software and calculators on cycling forums. The math is the same across all of them, so the result should be identical if you enter the same data. If two calculators give you different answers, double-check that you entered the same measurements into both.
Some bike shops and wheel builders will calculate spoke length for you if you bring them your hub and rim. This is useful if you are uncomfortable measuring or if you want a second opinion before ordering spokes.
Frequently Asked Questions
Can I use the same spoke length for both sides of a rear wheel?
No. Rear wheel hubs are dished, meaning the drive-side flange is further from the centre than the non-drive-side flange. Spokes on each side must be different lengths to account for this. Using the same length on both sides will result in uneven tension and a wheel that does not hold true.
What if my hub or rim is not in the calculator's database?
You can enter the measurements manually. You need the flange diameter, distance from hub centre to each flange, rim ERD, spoke count, and lacing pattern. If you do not have these numbers, measure them yourself or contact the manufacturer.
Does lacing pattern affect spoke length?
Yes. Radial spokes (no crosses) are shorter than two-cross or three-cross spokes because they approach the rim at a different angle. You must tell the calculator which pattern you plan to use, or it cannot give you an accurate length.
What if the calculator gives me a length that is not a standard size?
Spokes are sold in half-millimetre increments. If the calculator returns 290.3 mm, round up to the nearest half-millimetre (290.5 mm). If it returns 290.7 mm, round up to 291 mm. Rounding up slightly is safer than rounding down, because spokes that are slightly too long can be trimmed, but spokes that are too short cannot be lengthened.
Can I use a spoke calculator for a wheel I am rebuilding?
Yes, if you are using the same hub and rim. If you are swapping either component, you must recalculate. Even a different rim with the same nominal size (e.g., two different 700c rims) can have different ERDs, which changes the spoke length you need.