Going from metric to inches is one calculation with one answer. Going the other way has no single answer, because inch-sized tires and metric-sized tires are not two notations for the same set of products.
The full sidewall explanation is on the tire size conversion page.
Metric to inches
overall diameter (in) = wheel diameter (in) + 2 × (section width mm × aspect ratio ÷ 100) ÷ 25.4
For 265/70R17:
- Sidewall height: 265 × 0.70 = 185.5 mm
- To inches: 185.5 ÷ 25.4 = 7.30 in
- Overall: 17 + (2 × 7.30) = 31.61 in
Section width converts on its own: 265 ÷ 25.4 = 10.43 in.
The one thing to get right is that the wheel number is already in inches. It is the single imperial value in a metric string, and it does not get divided by 25.4.
| Size | Section width | Sidewall | Overall diameter |
|---|---|---|---|
| 225/45R17 | 8.86″ | 3.99″ | 24.97″ |
| 235/45R18 | 9.25″ | 4.16″ | 26.33″ |
| 265/70R17 | 10.43″ | 7.30″ | 31.61″ |
| 275/55R20 | 10.83″ | 5.95″ | 31.91″ |
| 285/70R17 | 11.22″ | 7.85″ | 32.71″ |
| 315/70R17 | 12.40″ | 8.68″ | 34.36″ |
Flotation sizes need no conversion
Some tires — mostly light truck and off-road — are sold in flotation notation: 35×12.50R17. That reads directly as overall diameter × section width × wheel diameter, all in inches.
35 inches tall, 12.5 inches wide, on a 17-inch wheel. Nothing to calculate.
This is why off-roaders talk in inches and street tire discussions happen in millimetres: they are literally different labelling systems, and only one of them states the number everybody actually wants.
Inches to metric: why there is no single answer
Ask "what metric size is a 33-inch tire" and the honest answer is that several sizes land near 33 inches and none lands exactly on it:
| Metric size | Overall diameter | Off 33″ by |
|---|---|---|
| 255/80R17 | 33.06″ | +0.06″ |
| 255/85R16 | 33.07″ | +0.07″ |
| 275/70R18 | 33.16″ | +0.16″ |
| 285/75R16 | 32.83″ | −0.17″ |
| 305/70R16 | 32.81″ | −0.19″ |
| 295/70R17 | 33.26″ | +0.26″ |
| 285/70R17 | 32.71″ | −0.29″ |
All seven are commonly called "33s." They differ by more than half an inch top to bottom, and by nearly two inches in section width — which matters far more for fitment than the diameter difference does.
So the reverse conversion is a shortlist, not an answer. Pick from the shortlist by wheel size you already have, by width you can clear, and by what is actually available in the load rating you need.
Nominal is not measured
Every figure above is calculated from the nominal size. Real tires vary from it, typically by a few tenths of an inch, because:
- The moulded size is a nominal designation, not a guaranteed measurement.
- Section width is specified on a particular measuring rim, and mounting on a narrower or wider rim changes the actual width and slightly changes the height.
- Tread pattern and depth affect the measured overall diameter, and a worn tire is measurably smaller than a new one.
Manufacturers publish measured specifications for each tire in their range. For anything where a fraction of an inch matters — a marginal clearance decision — use the published measured figure rather than the arithmetic.
Quick reference
- 1 inch = 25.4 mm, exactly.
- Section width mm ÷ 25.4 = section width in inches.
- Sidewall = width × aspect ÷ 100, in whatever unit the width is in.
- Overall diameter = wheel + 2 sidewalls. The wheel is already inches.
- Circumference = π × overall diameter, which is what the speedometer cares about.
What this page does not cover
- Speedometer error from a size change, which follows from the diameter ratio — see the main conversion page.
- Fitment. Diameter says nothing about whether the tire clears the fender, suits the rim width, or carries the load.
- Load index and speed rating, which are safety specifications and must satisfy the vehicle placard.
- Rim width ranges, which constrain which tires may legally and safely mount on your wheels.
One direction is arithmetic. The other is a shopping list.