A tire marked “26 inch,” “28 inch,” or “700” still may not fit the wheel in front of you. The dependable starting point is the ISO/ETRTO size molded into the tire and, when available, printed on the rim. In a marking such as 37-622, 37 is the tire’s nominal width in millimeters and 622 is the bead-seat diameter. The second number must match the rim’s diameter exactly. After that, width, frame clearance, tube range, valve, tubeless approval, and pressure limits still need checking. Treat the familiar inch or French name as a useful label, not as the final fit decision.
What to bring
Most wheels can be identified without removing the tire. Bring only what helps you reveal and record the markings:
- A clean rag and flashlight
- A phone for clear photographs of both tire sidewalls and rim labels
- A metric ruler or caliper for approximate width and valve measurements
- The bicycle, wheel, rim, and tire manufacturers’ current fit information
Don’t deflate or unseat a tire unless you own the bicycle or have the seller’s permission. Opening an unknown tubeless system can spill sealant, disturb a sound bead, or reveal a setup you aren’t prepared to reinflate.
Start with the ISO/ETRTO marking on the tire
Rotate the wheel slowly and read both sides of the tire. The useful marking normally contains two numbers separated by a hyphen, such as 25-622, 47-584, or 54-559. It may appear beside a marketing size, pressure range, rotation arrow, casing name, and tubeless designation.
Read the two numbers separately:
| Marking element | What it describes | What it does not establish |
|---|---|---|
First number, such as 37 | Nominal inflated tire section width in millimeters | Exact mounted width on every rim |
Second number, such as 622 | Bead-seat diameter in millimeters | Tire volume, clearance, or intended use |
Marketing name, such as 700 × 35C | Familiar size family and nominal width | A safe fit without checking the ISO size |
The bead-seat diameter is the critical diameter because it describes where the tire beads sit in the rim. A 37-622 tire belongs on a 622 mm bead-seat rim. It does not belong on a 584, 590, or 635 mm rim, even if two of those sizes have historically shared a similar inch name.
The first number is less absolute. A nominal 37 mm tire can measure wider or narrower after installation because rim internal width, casing construction, and inflation pressure affect its shape. Use it to begin the width check, then verify the tire maker’s approved rim range and measure the installed tire if clearance is close.
Find the rim’s diameter and internal-width marking
Look along the rim exterior, inside the rim channel if the tire is already off, and near the valve or joint. A rim might be marked 622 × 19, 19-622, or with a model name that leads to a manufacturer specification. In this example, 622 is again the bead-seat diameter and 19 mm is the nominal internal rim width.
Wheel stickers often emphasize an outside diameter or marketing family. “700C,” “29er,” and “28 inch” commonly refer to wheels using a 622 mm bead-seat diameter, but that shared diameter doesn’t make every tire width suitable for every rim or frame. A narrow road rim, wide mountain rim, and city-bike rim can all be 622 mm and require different tire-width decisions.
If the tire and rim both carry ISO dimensions, their bead-seat diameters must be identical. If the tire reads 40-584 and the rim reads 622 × 21, stop. No tube, extra pressure, sealant, or force can correct that mismatch.
Translate legacy names without trusting them alone
Older bicycles, imported tires, and seller listings may use inch fractions, inch decimals, or French names. Those systems are useful for recognition, but some names describe several incompatible bead-seat diameters.
Common examples illustrate the problem:
700C, many “28 inch” tires, and modern29 × …mountain-bike tires normally use a 622 mm bead-seat diameter.650Bnormally corresponds to 584 mm, while650Cnormally corresponds to 571 mm. The letter matters.- “26 inch” can refer to several bead-seat diameters. Modern mountain-bike tires commonly use 559 mm, but older roadsters, cruisers, and specialty bicycles may not.
- Fractional and decimal inch labels that look mathematically equivalent may describe different, noninterchangeable tire systems.
Use a legacy marking to locate the likely family, then confirm the ISO/ETRTO number on the tire, rim, packaging, or manufacturer’s specification. Don’t convert a fraction to a decimal and assume the result names the same tire.
Identify an unmarked wheel carefully
An unreadable tire doesn’t make the wheel unidentifiable, but it changes the job. Clean the sidewall first. Search the rim and hub for model names, photograph distinctive decals, and compare those identifiers with the manufacturer’s archived documentation.
Measuring the tire’s outside diameter is a poor substitute. Outside diameter changes with tire width, tread, pressure, and wear. Directly measuring bead-seat diameter is also awkward while the tire is installed because the bead seat is hidden inside the rim.
With the tire removed, a mechanic can inspect the rim profile and take controlled measurements, but rare obsolete sizes deserve more than a quick tape-measure estimate. If no reliable marking or documentation survives, bring the wheel to a shop before ordering. A returnable guess may waste time; a tire that appears to mount but doesn’t share the correct bead seat can be dangerous.
Check tire width against the rim
Matching bead-seat diameter answers only the first compatibility question. The tire also needs an approved relationship with the rim’s internal width. A tire that is too narrow for a wide rim can have a poor bead and casing shape. A very wide tire on a narrow rim can become unstable, feel vague in corners, or place loads outside the intended combination.
Read the actual internal rim width, then consult the rim and tire makers’ fit guidance. Where their instructions differ, use the combination and limits accepted by both. A generic chart is useful for screening, but it cannot override a model-specific restriction.
Expect the mounted tire to differ from its nominal sidewall width. Wider rims generally spread the casing wider. That change can affect frame clearance, mud clearance, pressure choice, and whether the tire contacts a rim-brake caliper or fender.
If you are replacing a tire with the same printed size, still inspect the new model’s measured-width guidance. Two 40-622 tires can use different casings and tread shapes, so the tire that cleared the frame does not prove every other tire bearing that number will clear.
Check the space around the mounted tire
Inflate the proposed tire within the applicable limits, seat it evenly, and check clearance around the full rotation. Look at the fork crown and blades, chainstays, seatstays, seat tube, brake bridge, fenders, and brake hardware. Turn the handlebar on bikes where the tire can approach a mudguard or frame at steering lock.
Clearance must account for more than a stationary wheel in a stand. Wheels flex, tires pick up debris, rims can move slightly out of true, and mud occupies space. A tire that lightly polishes paint or leaves rubber marks is already too close. Manufacturer minimum-clearance instructions control where provided; don’t invent a universal millimeter allowance for every frame, discipline, and tire.
Rim-brake bicycles need one extra check: the wheel’s brake track must align with the pads, and the inflated tire must pass through the brake when the release is opened. Diameter compatibility does not prove the wheel is appropriate for the bicycle’s brake system.
Match the inner tube and valve
An inner-tube package should name both an approved bead-seat diameter and a tire-width range. A tube marked for 28/47-622 is intended for 622 mm tires within the listed width span. Stretching a tube across the wrong diameter or using one far outside its width range increases the chance of folds, excessive stretching, installation damage, or poor service life.
Match the valve as well:
- The valve type must suit the rim hole and the pump or inflator you intend to use. Presta and Schrader are the most common; some regional bicycles use Dunlop/Woods valves.
- The stem must be long enough to pass through the rim and leave usable thread or valve body for inflation.
- A narrow valve shouldn’t be treated as a safe way to adapt every large rim hole. Use the rim or valve maker’s specified hardware.
A deep-section rim may need a longer valve or a suitable extender. Confirm whether the extender must carry a removable valve core, especially if the wheel will use sealant.
Treat tubeless and hookless as separate approvals
Correct diameter and width do not prove a tire can run tubeless. Look for explicit tubeless approval on the exact rim and exact tire model, then follow the required tape, valve, sealant, pressure, and installation instructions. A “tubeless-ready” tire cannot make an unapproved rim tubeless-ready.
Hookless rims require another model-specific check. Confirm that the tire maker approves the exact tire size for hookless use, that the rim’s internal width is permitted, and that the operating pressure stays within both makers’ limits. If the rim and tire state different maximum pressures, the lower applicable limit governs.
Don’t infer hookless approval from a tight-fitting bead or a successful inflation. An unsupported combination may remain seated in the workshop yet fail under heat, impact, cornering load, or pressure.
Findings that change the purchase or installation plan
| Finding | Appropriate action |
| Tire and rim bead-seat diameters differ | Do not install or inflate the combination |
| Diameter cannot be established reliably | Pause the order and have the bare rim identified |
| Tire width falls outside rim or tire-maker guidance | Choose an approved width or different rim |
| Mounted tire rubs or lacks specified clearance | Fit a smaller approved tire or resolve the interfering component |
| Tube range or valve does not match | Buy the correct tube or approved valve hardware |
| Tubeless or hookless approval is missing | Use an explicitly supported setup; don’t treat fit as approval |
| Rim, bead, or tire casing is damaged | Replace or professionally assess the damaged part before riding |
For condition checks on a used wheel, continue with Checking Wheels and Tires on a Used Bike. Size compatibility and physical condition are separate decisions; a perfectly matched tire still isn’t safe on a cracked rim or damaged casing.
What Velopedia data can establish
A Velopedia Archive record can provide the bicycle’s original wheel designation, tire size, rim model, brake type, and sometimes rim width. That evidence is valuable for identifying the factory configuration and locating manufacturer documents.
It cannot prove that the wheel currently fitted is original, that an aftermarket rim has the same internal width, or that a replacement tire has adequate clearance. Treat an absent specification as unknown, not as evidence that a feature or standard is absent. Read the markings on the physical parts before ordering.
Where a self-check stops
Ask a qualified bicycle mechanic or the component manufacturer to resolve an unmarked rim, an obsolete size, a damaged bead seat, uncertain hookless approval, or a combination outside published fit information. Measuring tools help, but they don’t replace the profile dimensions and tolerances held by the manufacturer.
Before installation, retain photographs of the tire and rim markings and the relevant compatibility pages. If the numbers cannot be made to agree across diameter, width, pressure, and system approval, the responsible next action is to select a documented combination rather than test the uncertainty on the road.
Sources and technical review
Author: Jeff South
Technical review date: August 29, 2026
- Schwalbe: Tire Sizes — ISO/ETRTO dimensions, legacy naming, and bead-seat diameter relationships.
- Park Tool: Tire, Wheel and Inner Tube Fit Standards — bead-seat diameter, legacy sizes, inner tubes, valves, and fit checks.
- WTB: Tire & Rim Fit Chart — tire section width and rim internal-width guidance.
- Continental: Tire–Rim Combinations and ETRTO Standards — rim-width, tire-width, pressure, and frame-clearance considerations.
- Continental: Hookless vs. Hooked Rims — hookless identification, approved combinations, and pressure limits.