How to Measure Effective Rim Diameter (ERD): From Tire Markings to Spoke Length

Effective rim diameter (ERD) is the diameter of a bicycle rim measured where the spoke ends sit inside the seated spoke nipples. It is not printed on the tire sidewall, and it cannot be calculated from tire markings. The tire tells you the rim's bead seat diameter, which identifies the wheel size family. ERD must come from the rim manufacturer's specification or from your own measurement. The most reliable home method uses two spokes of known length with nipples, placed through opposite spoke holes. You then calculate: ERD = 2 × spoke length − overlap.

This article starts with the numbers on the tire sidewall. It shows what they say about the rim and what they do not. It then explains how to measure ERD step by step and why accuracy matters for spoke length.

What do the numbers on a bicycle tire sidewall mean?

A bicycle tire sidewall usually carries one or more size designations and several ratings. Size designations are dimensions. Pressure figures and construction details are ratings that describe use and performance, not size.

ETRTO (ISO) size: the most precise marking

The ETRTO or ISO marking is written as two numbers, for example 37-622. It is the most useful sidewall marking for wheel building.

• 37 is the nominal tire width in millimetres.
• 622 is the bead seat diameter (BSD) in millimetres. This is the diameter of the rim surface where the tire bead sits.

The BSD tells you which rims the tire fits. It is a different number from the ERD and is not interchangeable with it.

Inch and French designations: traditional size names

Traditional designations such as 700x35C, 29x2.2 or 26x1.95 are size names rather than exact measurements.

• In 700x35C, "700" is a nominal outer diameter class in millimetres and "35" is an approximate width in millimetres.
• In 29x2.2, "29" is a nominal outer diameter in inches and "2.2" is an approximate width in inches.

Different traditional names can share one BSD. For example, 700C and 29-inch tires both fit 622 mm rims. In other cases, one inch name covers several BSDs; "26 inch" tires exist in 559 mm and 590 mm versions, among others. Always check the ETRTO number when matching tires to rims.

Ratings: pressure, construction and other markings

Ratings describe how the tire should be used, not how big it is.

• Pressure range is given in bar, psi or kPa, for example "4.5–6.5 bar / 65–95 psi". It states the inflation limits the tire maker recommends. Your rim may have its own pressure limit.
• TPI (threads per inch) describes casing construction.
• Tubeless or TLR markings describe tire type and mounting compatibility.
• E-bike ratings describe intended use.

None of these ratings affects ERD or spoke length.

Aspect ratio on bicycle tires

Most bicycle tires carry no aspect ratio, unlike car tires. A bicycle tire's inflated cross-section is roughly round, so for rough geometry, tire height is often treated as about equal to tire width. Actual height varies with rim width, casing design and pressure.

Common bicycle tire markings compared

Marking	Example	Type	Unit	What it tells you
ETRTO / ISO size	37-622	Dimension	mm	Nominal width and exact bead seat diameter
French designation	700x35C	Dimension (nominal)	mm	Size class and approximate width
Inch designation	29x2.2	Dimension (nominal)	inch	Size class and approximate width
Pressure range	4.5–6.5 bar	Rating	bar, psi, kPa	Recommended inflation limits
TPI	60 TPI	Construction rating	threads per inch	Casing thread density

How does the tire bead seat diameter relate to ERD?

The bead seat diameter and the effective rim diameter both describe the same rim, but at different points.

• The BSD is measured where the tire bead sits. It is standardised for each wheel size.
• The ERD is measured lower in the rim, where the spokes end inside the nipples. It varies from one rim model to another.

Two rims with the same 622 mm BSD can have very different ERDs. A deep-section rim places the nipple seats much closer to the hub than a shallow rim does, so it has a smaller ERD and needs shorter spokes. This is why the tire size can identify the wheel size family but cannot give you a spoke length.

Rims often carry their own ETRTO marking, such as 622x19. Here 622 is the BSD in millimetres and 19 is the inner rim width between the bead hooks or walls, also in millimetres. Some rims also have the ERD printed on a sticker or listed in the manufacturer's specifications. If that value is available, check which measuring convention it uses before you rely on it.

Common bead seat diameters

BSD (mm)	Common names
630	27 inch (older road bikes)
622	700C, 29 inch, 28 inch
590	26 x 1 3/8 (older utility bikes)
584	650B, 27.5 inch
559	26 inch mountain bike
406	20 inch (BMX and many folding bikes)
305	16 inch

How do tire width and rim diameter affect overall wheel size?

You can estimate the overall diameter of a bicycle wheel as BSD + 2 × tire height. Because bicycle tires are roughly round in section, tire width is often used in place of height. Circumference is then π × overall diameter. These calculations describe the tire and the overall wheel size; they do not change the rim's ERD or the spoke length.

Worked examples

• 37-622 tire: 622 + 2 × 37 = 696 mm estimated diameter. Circumference ≈ 3.1416 × 696 ≈ 2187 mm.
• 28-622 tire: 622 + 2 × 28 = 678 mm. Circumference ≈ 2130 mm.
• 57-584 tire (27.5 x 2.25): 584 + 2 × 57 = 698 mm. Circumference ≈ 2193 mm.

These examples show that a wide tire on a smaller rim can give almost the same overall diameter as a narrower tire on a larger rim. Going from 28 mm to 37 mm on the same 622 mm rim adds about 18 mm to the diameter and about 57 mm to the circumference.

Limits of calculated geometry

Calculated diameter and circumference are estimates, not measured tire dimensions. Actual tire height depends on the tire model, the inner rim width, inflation pressure and rider load. For accurate speedometer or cycle-computer settings, measure the rolling circumference of the inflated tire under load.

The calculation also says nothing about fitment. Clearance to the frame, fork, brakes and mudguards must be checked on the actual bicycle, and the tire must suit the rim's inner width.

What exactly is effective rim diameter?

Effective rim diameter is the diameter of a circle that passes through the ends of the spokes when the spoke nipples are seated in the rim. Spoke length calculators use ERD as the outer reference point for each spoke, together with the hub dimensions and the lacing pattern.

Not every manufacturer and calculator defines the spoke end position in exactly the same way. The two common reference points are:

• Spoke ends flush with the outer end of the nipple head, for example the bottom of the screwdriver slot.
• A set distance measured from the nipple seat.

Before you combine a published ERD with a spoke calculator, confirm that both use the same convention. A mismatch can make the calculated spokes slightly too long or too short.

Terms used in ERD measurement

• Spoke nipples: threaded nuts that pass through the rim and screw onto the spoke threads. They are used to tension and true the wheel. Common lengths include 12 mm, 14 mm and 16 mm.
• Nipple seat: the surface inside the rim where the nipple head bears against the rim. This may be the rim wall itself, an eyelet or a washer.
• Spoke length: for J-bend spokes, conventionally measured from the inside of the elbow bend to the threaded end. For straight-pull spokes, it is measured from the underside of the head.

How to measure ERD with two spokes

The two-spoke method measures ERD directly with parts you already need for the wheel. You need two spokes of identical, known length, two nipples of the type you will use, and a ruler or tape measure.

1. Measure the spoke length. Check the length L of both spokes in millimetres using the standard convention: from the inside of the elbow bend to the threaded end. Do not rely only on the packaging.
2. Thread on the nipples. Screw a nipple onto each spoke until the spoke end sits at your chosen reference point, such as flush with the bottom of the nipple slot. Use the same position on both spokes.
3. Insert the spokes into opposite holes. Push each spoke and nipple through spoke holes that are directly opposite each other, 180° apart. The nipple heads should sit fully in the nipple seats.
4. Pull the spokes toward the centre. Pull both spokes straight toward the middle of the rim so they lie along one diameter. Keep the nipples seated.
5. Measure the overlap or gap. Where the two spokes meet, measure how far the elbow bends overlap each other, or how far apart they are if they do not reach.
6. Calculate ERD. If the spokes overlap: ERD = 2 × L − overlap. If there is a gap: ERD = 2 × L + gap.
7. Repeat in other positions. Measure across at least two or three different pairs of holes, including one pair near the rim joint. Use the average.

Example: two 300 mm spokes, with elbows overlapping by 2 mm. ERD = 2 × 300 − 2 = 598 mm.

Why the two-spoke formula works

Each spoke reaches from the spoke end at the rim to its elbow, a distance L. When two spokes start from opposite ends of a diameter, together they cover 2 × L. If that total is longer than the ERD, the excess shows up as overlap in the middle. If it is shorter, the shortfall shows up as a gap.

How to measure ERD with a ruler or caliper

You can also measure ERD by measuring across the rim between the nipple seats and adding the depth the spoke end sits above each seat.

1. Measure seat to seat. Measure the distance across the rim between the nipple seats of two opposite spoke holes. Use a long caliper, a rim measuring tool or a stiff rule. On many rims the seats are hidden inside a hollow section, so this step can be difficult.
2. Find the spoke end position inside the nipple. Measure from the underside of the nipple head (the part that rests on the seat) to the point where the spoke end will sit. Use the reference point that matches your spoke calculator.
3. Add both sides. ERD = seat-to-seat distance + 2 × spoke-end distance.
4. Check several positions. Repeat across other pairs of holes and average the results.

This method relies on reaching the nipple seats accurately. When the seats are hard to access, the two-spoke method is usually easier and more consistent.

What affects an ERD measurement?

ERD depends on the complete spoke-end setup, not only the rim profile. Re-measure, or adjust a published value, if any of these differ from the reference setup:

• Nipple type and length: internal or hidden nipples seat differently from standard external nipples.
• Washers and eyelets: nipple washers, single eyelets and double eyelets can raise the nipple head relative to the rim wall.
• Measuring convention: the reference point for the spoke end, such as the nipple slot or the nipple head face, changes the result.
• Rim roundness and the joint: rims are not perfectly round, especially at a welded or pinned joint. Averaging several readings reduces this error.
• Offset spoke holes: on asymmetric rims, measure through the holes as they will actually be used.

How much does ERD accuracy matter for spoke length?

An ERD error of 1 mm changes the calculated spoke length by roughly 0.5 mm. This is because ERD is a diameter and each spoke spans approximately the radius. A 4 mm ERD error therefore shifts spoke length by about 2 mm, which is enough to cause problems.

• Spokes too long can bottom out in the nipples before reaching full tension, or protrude into the rim bed.
• Spokes too short may leave too few threads engaged inside the nipple.

Spoke lengths are usually sold in steps, often 1 mm or 2 mm. A careful ERD measurement lets you choose the correct step with confidence.

Key points for measuring ERD

• The tire's ETRTO marking, such as 37-622, gives the bead seat diameter. It identifies compatible rims but does not give the ERD.
• Tire width and BSD let you estimate overall wheel diameter and circumference. These figures are approximations that do not affect spoke length and do not confirm frame fitment.
• ERD is the diameter at the spoke ends with the nipples seated. It varies with rim depth, nipple type, washers and measuring convention.
• With the two-spoke method, ERD = 2 × spoke length − overlap (or + gap). Measure in several positions and average.
• Each 1 mm of ERD error shifts the calculated spoke length by about 0.5 mm.