How to Measure Spoke Length: From Tire Sidewall Numbers to the Right Spoke

Measure a J-bend spoke from the inside of the elbow (the bend under the head) to the tip of the threaded end. Measure a straight-pull spoke from the underside of the head to the tip of the threaded end. Tire sidewall numbers cannot tell you what spoke length a new wheel needs. To find that length, measure the rim's effective rim diameter (ERD), the hub's flange diameter and center-to-flange distances, then count the spokes, note the lacing pattern and calculate. The tire marking only identifies the rim diameter family you are working with. This guide explains the markings first and then gives the measurement steps for bicycle and motorcycle spokes.

Which Tire Sidewall Numbers Matter for Spoke Length?

Only one number on the tire sidewall relates to spokes: the rim diameter, also called the bead seat diameter. It tells you which rim size the tire fits. The other markings describe tire width, sidewall proportion, construction, load capacity or speed capability, and none of them change spoke length. Spoke length depends on the rim and hub, not on the tire mounted on them.

Bicycle tire markings

Bicycle tires usually carry an ETRTO (ISO) size such as 25-622, often printed alongside a traditional size such as 700×25c.

• 25 is the nominal tire width in millimetres. This is a dimension.
• 622 is the bead seat diameter in millimetres, which is the rim diameter the tire fits. This is the number that identifies the rim family.
• 700×25c is the older French-style name for the same size. The 700 is a name, not a measured diameter.
• Inch sizes such as 26×2.1 or 29×2.2 are nominal names. A 29-inch mountain bike tire and a 700c road tire both use the 622 mm bead seat, and a 26-inch mountain bike tire uses 559 mm.

For spoke work, the bead seat diameter tells you which rims are in play. It is not the spoke circle. On most rims the spoke nipples sit below the bead seat, so the ERD is smaller than the bead seat diameter. The difference depends on rim depth.

Motorcycle tire markings

Motorcycle tires use either metric or inch sizing. A metric example is 120/70ZR17 58W. An inch example is 3.00-18 47P.

• 120 is the nominal section width in millimetres. This is a dimension.
• 70 is the aspect ratio. It is not a length but a percentage: the sidewall height is 70% of the width.
• ZR or R indicates radial construction, and the Z marks a high-speed category. 17 is the rim diameter in inches.
• 58 is the load index, a performance rating. Index 58 corresponds to 236 kg per tire.
• W is the speed rating, also a performance rating. W corresponds to 270 km/h.
• In 3.00-18, 3.00 is the nominal width in inches and 18 is the rim diameter in inches. No aspect ratio is printed.
• Motorcycle rims carry their own marking, such as 2.50×17 or MT2.50×17. This gives the rim width between the flanges and the rim diameter, both in inches.

How Tire Size Affects Diameter and Circumference, and Why Spoke Length Stays the Same

A tire's calculated overall diameter equals the rim diameter plus two sidewall heights. Circumference equals overall diameter multiplied by π (about 3.1416). Changing tire width or aspect ratio changes the outside diameter of the wheel. It does not change spoke length, because the spokes end at the rim, inside the tire.

Motorcycle example: metric sizes

1. Convert the rim diameter: 17 in × 25.4 = 431.8 mm.
2. Find the sidewall height for 120/70-17: 120 mm × 0.70 = 84 mm.
3. Find the overall diameter: 431.8 + 2 × 84 = 599.8 mm.
4. Find the circumference: 599.8 × 3.1416 ≈ 1884 mm.

Compare similar sizes on the same 17-inch rim:

Notice that a 10 mm increase in width at the same aspect ratio adds height, because the sidewall is a percentage of the width. Lowering the aspect ratio from 70 to 60 reduces height more. None of these changes touch the spokes, because all three tires fit the same 17-inch rim.

For inch-size tires such as 3.00-18, there is no printed aspect ratio. A rough estimate treats the tire as about as tall as it is wide: 18 in × 25.4 = 457.2 mm, plus 2 × 76.2 mm (3.00 in) gives about 610 mm. Treat this as an approximation only.

Bicycle example: ETRTO sizes

Bicycle tires have no printed aspect ratio. A common estimate treats tire height as roughly equal to tire width.

• 25-622: 622 + 2 × 25 = 672 mm diameter, about 2111 mm circumference.
• 32-622: 622 + 2 × 32 = 686 mm diameter, about 2155 mm circumference.

The wider tire adds about 14 mm of diameter and about 44 mm of circumference. Both tires fit the same 622 mm rim, so the spoke length is unchanged.

Calculated geometry versus real tires and fitment

These figures are calculated geometry, not measured tire dimensions. Real tire width and height vary with brand, rim width, inflation pressure and load. For an accurate circumference, for example to set a speedometer or cycle computer, measure the tire's rollout under load. Also note that a matching rim diameter does not guarantee fitment. Frame, fork or swingarm clearance, rim width range and the vehicle manufacturer's approved sizes all decide whether a tire actually fits.

How to Measure Bike Spoke Length on an Existing Spoke

Bicycle spoke length is the distance from the point where the spoke seats in the hub to the tip of the threaded end. For J-bend spokes, that point is the inside of the elbow. For straight-pull spokes, it is the underside of the head. Measure in millimetres.

Measuring J-bend spokes

1. Remove the spoke, or measure an unused spare from the same wheel side.
2. Lay the spoke flat and straight. A bent spoke reads short.
3. Put the zero of your ruler at the inside of the spoke elbow, on the inner curve of the bend just below the head.
4. Read the length at the very tip of the threaded end, not where the threads begin.
5. Round to the nearest millimetre and write down which side of the wheel the spoke came from.

Measuring straight-pull spokes

1. Lay the spoke straight against the ruler.
2. Place zero at the underside of the head, the face that bears against the hub.
3. Read the length at the tip of the threaded end.

Using a spoke ruler

A spoke ruler is a measuring scale with a hook, hole or notch at zero. The J-bend elbow hangs in it, so the measurement starts exactly at the inside of the bend. Many spoke rulers also have a gauge for checking spoke diameter. A steel rule or caliper works too, as long as zero sits at the correct reference point.

Spoke length varies between wheels and often between the two sides of the same wheel. Rear wheels and wheels with disc brakes commonly need different lengths left and right. Measure one spoke from each side, and from each group if the hub has different flange sizes.

How to Measure Motorcycle Spoke Length

Motorcycle spoke length is usually measured the same way as a bicycle J-bend spoke: from the inside of the bend under the head to the tip of the threaded end. Motorcycle spokes are thicker, and their heads are often bent at specific angles to suit the hub. Because suppliers do not all use the same reference point, confirm the measuring convention with the supplier before ordering.

1. Identify the spoke groups. Many motorcycle wheels have inner and outer spokes on each flange, and these often differ in length, head angle or both.
2. Remove one spoke from each group and label it.
3. Lay each spoke straight. Do not straighten the head bend, because its angle is part of the specification.
4. Measure from the inside of the bend to the tip of the threaded end.
5. Record the head angle (how sharply the head bends), the spoke diameter, the thread size and the nipple type for each group.
6. Record the rim offset as well. On motorcycles the rim is often deliberately off-center relative to the hub, and this offset affects the spoke lengths on each side.

A motorcycle spoke that matches in length but has the wrong head angle will not seat properly in the hub. Length is only one part of a correct motorcycle spoke.

How to Work Out the Spoke Length a New Wheel Needs

The required spoke length is calculated from rim and hub geometry, not from the tire size. You need six inputs, and the result is accurate only if they are measured carefully.

Measurements you need

• ERD (effective rim diameter): the diameter at which the spoke ends sit inside the nipples, in mm.
• Flange diameter: the diameter of the circle through the centers of the hub spoke holes, measured for each flange, in mm.
• Center-to-flange distance: the distance from the hub's center plane to the middle of each flange, in mm. Measure the locknut-to-locknut width, halve it, then subtract the distance from the locknut face to the flange center.
• Spoke count: the number of spoke holes in the rim and hub.
• Cross pattern: how many other spokes each spoke crosses, for example radial (0), 2-cross or 3-cross.
• Hub spoke hole diameter: in mm, typically measured with a caliper.

How to measure ERD with two spokes

1. Take two spokes of the same known length (L) and two nipples.
2. Thread each nipple onto its spoke until the spoke tip sits where you want it in the finished wheel, usually level with the bottom of the nipple's screwdriver slot.
3. Insert the nipples into two spoke holes directly opposite each other across the rim.
4. Pull both spokes toward the rim center so they lie side by side across the middle.
5. Measure the overlap between the two elbows, from the inside of one bend to the inside of the other.
6. Calculate ERD = 2 × L − overlap. If the spokes do not reach each other, ERD = 2 × L + gap.
7. Repeat at another pair of holes and average the results.

Spoke length formula and worked example

The standard spoke length formula for a J-bend spoke is:

Length = √(d² + r1² + r2² − 2 × r1 × r2 × cos θ) − s ÷ 2

• d = center-to-flange distance (mm)
• r1 = flange radius, which is flange diameter ÷ 2 (mm)
• r2 = ERD ÷ 2 (mm)
• θ = 720° × crosses ÷ spoke count
• s = hub spoke hole diameter (mm)

Here is a worked example with assumed inputs: ERD 600 mm, flange diameter 45 mm, center-to-flange 35 mm, 32 spokes, 3-cross, spoke hole 2.6 mm.

1. θ = 720 × 3 ÷ 32 = 67.5°, and cos 67.5° ≈ 0.3827.
2. r1 = 22.5 mm, so r1² = 506.25. r2 = 300 mm, so r2² = 90,000. d² = 1,225.
3. 2 × 22.5 × 300 × 0.3827 ≈ 5,166.
4. 506.25 + 90,000 + 1,225 − 5,166 ≈ 86,565, and √86,565 ≈ 294.2 mm.
5. 294.2 − 2.6 ÷ 2 ≈ 292.9 mm, so this side needs spokes of about 293 mm.

Run the calculation separately for each side of the wheel, because flange diameter and center-to-flange distance often differ left and right. For straight-pull hubs and motorcycle hubs with angled spoke holes, the geometry is different. Use the hub manufacturer's data or a calculator designed for that hub type.

Common Spoke Length Measurement Mistakes

• Using tire size instead of ERD. Rims with the same bead seat diameter can have very different ERDs because rim depths differ.
• Measuring from the outside of the elbow. J-bend length starts at the inside of the bend.
• Stopping where the threads start. Length runs to the tip of the threaded end.
• Measuring only one side. Dished rear wheels, disc hubs and offset motorcycle wheels usually need different lengths per side.
• Ignoring nipple length and seating. ERD should reflect where the spoke tip will sit inside the nipple you plan to use.
• Mixing up motorcycle spoke groups. Inner and outer spokes can differ in both length and head angle.

A spoke that is too long can bottom out in the nipple before the wheel reaches full tension. A spoke that is too short engages too few threads. Careful measurement of the existing spoke, or of the rim and hub for a new build, prevents both problems.