What width tire fits my rim?
The one that falls inside the ETRTO chart for your rim's inner width, the distance between the bead seats inside the rim. ETRTO, the European standards body for tires and rims, publishes which tire widths are approved on which rims, and the 2024 table is generous: a 21 mm rim, the modern road standard, is approved for tires from 25 to 65 mm. The limits are at the edges. A 25 mm tire is approved on rims from 15 to 22 mm and not wider; a 28 mm tire from 16 to 23. The calculator reads the table both ways: which tires your rim takes, and which rims your tire takes.
| Rim inner width | Approved tires (mm) | In inches |
|---|---|---|
| 15 mm | 20–27 | 0.79–1.06″ |
| 17 mm | 20–57 | 0.79–2.24″ |
| 19 mm | 22–57 | 0.87–2.24″ |
| 21 mm | 25–65 | 0.98–2.56″ |
| 23 mm | 28–65 | 1.10–2.56″ |
| 25 mm | 29–71 | 1.14–2.80″ |
| 30 mm | 47–71 | 1.85–2.80″ |
| 35 mm | 58–83 | 2.28–3.27″ |
ETRTO 2024. Labeled tire widths. Your rim maker's own list, if it has one, wins, especially on hookless rims.
The same 21 mm rim is ETRTO-approved for a 25 mm road tire and a 65 mm (2.6-inch) mountain bike tire. "Is my tire too wide for my rim?" is rarely the real problem any more. The one that bites is the other direction: a narrow tire on a wide modern rim, especially a hookless one.
Tire too narrow for the rim, or too wide?
Too narrow a tire on a wide rim is the combination the table's lower limits exist for. The sidewalls stand nearly vertical, the rim edge sits exposed to rocks and curbs, and on a hookless rim nothing but the bead seat holds the tire, which is why the hookless rules sit on those same limits. Too wide a tire on a narrow rim gives a lightbulb profile that squirms in corners, and tubeless setups are more likely to burp under hard cornering at low pressure. Inside the table, pick by what you want: a wider rim for a given tire gives a squarer, more stable shape and lets you run less pressure; a narrower one saves a little weight.
How wide will my tire actually measure?
Wider than its label on a wide rim. The casing spreads by about 0.4 mm for every millimeter of extra rim width. That figure is the slope in Continental's own published measurements: its 25 mm GP5000 measures 24.7 mm on a 13 mm rim and 27.9 mm on a 21 mm rim, 3.2 mm over 8 mm of rim. It's the same model our tire pressure calculator uses to set pressure by the width the tire really stands at.
| Labeled tire | 17 mm rim | 19 mm rim | 21 mm rim | 23 mm rim | 25 mm rim |
|---|---|---|---|---|---|
| 25 mm | 24.2 mm | 25.0 mm | 25.8 mm | — | — |
| 28 mm | 27.2 mm | 28.0 mm | 28.8 mm | 29.6 mm | — |
| 32 mm | 31.2 mm | 32.0 mm | 32.8 mm | 33.6 mm | 34.4 mm |
| 40 mm | 37.2 mm | 38.0 mm | 38.8 mm | 39.6 mm | 40.4 mm |
Taking each tire's label as its width on the rim its category is measured on (19 mm road, 24 mm gravel), plus 0.4 mm per mm of rim beyond that. "—" = outside the ETRTO table.
Every tire widens by about the same 0.4 mm per millimeter of rim, but brands label from different reference rims. By Continental's own numbers, its "25 mm" measures about 27.1 mm on a 19 mm rim, while a tire labeled true to that rim measures 25.0. For frame clearance, measure the tire you have on the rim you have. For the pressure and handling difference between two rims, the slope is what matters, and that part is the same for everyone.
Hookless rims: what are the rules?
Three, and the table above is one of them. Tubeless-ready tires only (marked TLE or TLR), even if you run a tube: a hookless rim has no hook to hold the bead, so only a bead built to tubeless tolerances is approved on it. 72.5 psi (5 bar) maximum, the ETRTO hookless limit, which the tire pressure calculator enforces automatically. And stay inside the width table, whose lower limits settle the forum argument about hookless minimums: a 23 mm hookless rim takes 28 mm tires and up, and a 25 mm rim takes 29 mm and up. Some rim makers publish stricter minimums for their own rims, and theirs win.
Does tubeless or a tube change which tire fits?
Not on a hooked rim: the width table is the same either way, and a tube-type tire fits a hooked rim whether or not the rim is tubeless- compatible. On a hookless rim, tubes are fine, but the tire itself must still be tubeless-ready. The rim's own maximum pressure applies on top of all of this and is often lower than the tire's, so check the rim sticker before pumping to the number on the tire sidewall. For size names that don't match (700c, 29-inch, 650b), the tire size converter translates them.