Inner width versus outer width
Rim width is usually discussed as internal rim width: the distance between the inner walls of the rim, where the tire bead sits. Outer width is measured across the outside of the rim and is not what tire charts use. Some brands name their rims by internal width, such as WTB’s “i25”, but others list outer width or both, so check the product’s specifications.
Why rim width matters
The rim shapes the tire. A narrow rim can pinch a wide tire into a more rounded profile, while a wider rim spreads the tire out. Tire makers and standards bodies such as ETRTO publish ranges of rim widths that are approved for each tire width, mainly for safety and stable handling. Wider rims can also change actual tire width after mounting.
How to measure it
- Remove the tire, or look for the internal width in the rim maker’s specifications.
- If measuring yourself, use calipers across the inside of the rim between the bead hooks (or the equivalent point on hookless rims).
- Write down the number in millimeters.
Measuring with the tire still on is difficult and not reliable for inner width. It is better to rely on the rim specs or measure once the tire is off.
Using manufacturer charts
Both rim and tire makers publish compatibility charts. Find your tire’s ETRTO width (the first number, such as 28 in 28-622), then find your internal rim width and see whether the combination is listed as approved or recommended. Continental advises that if the rim and tire makers’ limits differ, you follow the lower value, and the same logic applies to maximum pressure (see the bike tire pressure chart for typical working ranges). Schwalbe and WTB provide similar tables.
Hooked and hookless rims
Hookless rims have straight sides without a bead hook. Schwalbe states that hookless or straight-sided rims may only be used with tires approved for them (marked tubeless easy or tubeless ready, for example), and the rim maker’s maximum pressure and minimum tire width must also be followed. If your wheels are hookless, read both the rim and tire documentation before mounting.
Rules of thumb
You may see a “3% rule” or other ratios in forums. Sources describe these differently and they are not substitutes for the charts, so treat them as rough ideas only. A rule of thumb cannot account for hookless limits or specific tire models.
Checklist
- Know your tire’s ETRTO size and your rim’s internal width.
- Compare against both manufacturers’ charts.
- Follow the lower pressure and the stricter width limit.
- Confirm clearance in the frame; see road bike tire sizing.
- If you are unsure, ask the wheel or tire manufacturer or a bike shop.
For width choices themselves, see 25mm vs 28mm tires.