TireDial › Guides › Measuring tire width
Guide
Measuring tire width
The number on your tire is a nominal size. How wide it really is depends mostly on your rim, and the actual width is what sets the right pressure.
Rule of thumb: +0.5 mm per mm of rim
For every extra millimeter of internal rim width, a tire measures about half a millimeter wider. A 28 mm tire on a 25 mm rim measures around 31 mm.
What the number on the tire means
Tire sizes are usually printed twice, e.g. 28-622 (ETRTO) and 700x28C. In ETRTO the first number is the nominal width in millimeters and the second the bead seat diameter (622 mm for 700C/29″, 584 mm for 650B/27.5″). MTB tires usually use inches; 2.4″ is about 61 mm.
The nominal width refers to a particular reference rim. Modern rims are often wider than that reference, which is why tires today usually measure wider than labeled.
How rim width changes the real width
A wider rim spreads the tire’s sidewalls apart. The tire gets wider and a bit flatter, and its air volume grows. More volume means less pressure for the same load.
| Internal rim | Effective width | Front / rear (PSI) |
|---|---|---|
| 17 mm | 27.0 mm | 64 / 73 |
| 19 mm | 28.0 mm | 61 / 70 |
| 21 mm | 29.0 mm | 58 / 67 |
| 23 mm | 30.0 mm | 56 / 64 |
| 25 mm | 31.0 mm | 54 / 61 |
Calculated with the TireDial model: 70 kg (154 lb) rider, 8 kg (18 lb) bike, average tarmac, dry. “Effective width” is the model’s estimate when you don’t measure yourself.
Between the narrowest and widest rim that’s about 12 PSI at the rear, with the same tire.
How to measure properly
- Let the tire settle. Tires stretch a little in the first days. Measure after a night or a first ride at your normal pressure.
- Inflate to riding pressure. Width depends slightly on pressure.
- Use calipers at the widest point of the casing, across the tire. A ruler isn’t accurate enough.
- On knobby tires (gravel, MTB), measure the casing, not the side knobs, which often stick out further.
- Measure in two or three places and take the average.
Finding your internal rim width
This is the distance between the rim walls on the inside, where the tire bead sits. It’s usually in the wheel’s spec sheet or on a sticker on the rim, often as an ETRTO size like 622x21 (21 mm internal). Typical today: about 19–23 mm for road, 23–25 mm for gravel and 28–35 mm for MTB.
What about frame clearance?
A tire that measures wider than expected can get tight in the frame or fork. Leave a few millimeters on all sides so mud and a little wheel flex don’t cause rubbing. The frame maker usually lists the maximum tire width.
How TireDial uses your measurement
If you enter your measured width in TireDial, the app uses it directly. Without a measurement it estimates the effective width from the printed size and your internal rim width, and without a rim width it assumes a typical modern rim (21 mm road, 25 mm gravel, 30 mm MTB).
Your exact pressure in seconds. TireDial uses your weight, tires and rims, tubeless or tubes, surface and wet conditions, front and rear separately, and always respects your maximum pressure. Free for iPhone.
FAQ
Why is my 28 mm tire wider than 28 mm?
Because your rim is wider than the reference rim the nominal width is based on. For every millimeter of internal rim width, the tire measures about 0.5 mm wider.
Should I set pressure by measured width?
Yes. A wider tire needs less pressure for the same load. In pressure charts, use the column closest to your measured width.
What do I measure tire width with?
With calipers at the widest point of the casing, at riding pressure, after the tire has settled for a night.
What does 28-622 on a tire mean?
28 mm nominal width and 622 mm bead seat diameter, so a 700C (29″) tire that is 28 mm wide.
Keep reading
TireDial’s values and the ones on this page are well-founded starting points, not manufacturer specifications. Always stay within your tire and rim limits. See also the safety notice.