Data sheet TS-03 · Calculator
Tire size converter
A tire size converter reads 285/75R16 as a nominal 32.8 in (834 mm) overall diameter. It also converts inch sizes such as 33x12.50R20 to nearby metric sizes on the same rim. These are formula-based comparisons, not measured tire dimensions.
Fig. 1 — Cross-section
Table 1 — Inches and millimeters
285/75R16 is 32.83″ tall and 11.22″ wide.
| Item | in | mm |
|---|---|---|
| DOverall diameter | 32.83 | 834 |
| ASection width | 11.22 | 285 |
| BSidewall | 8.42 | 213.8 |
| CRim | 16 | 406.4 |
| Circumference | 103.1 | 2,620 |
| Aspect ratio | 75% | |
| Revs per mile · revs per km | 614 | 382 |
As a flotation size: 32.8x11.2R16
Diameter × width R rim, in inches, rounded to a tenth.
§ 1
How do metric sizes become inches?
Start with the section width in 285/75R16: 285 mm. Divide by 25.4 mm per inch to get a nominal width of 11.2 in.
Next, multiply the width by the aspect ratio expressed as a percentage. By the formula, the sidewall height is 8.4 in (214 mm).
Add both sidewall heights to the 16 in (406 mm) rim diameter. Using unrounded values, the formula gives an overall diameter of 32.8 in (834 mm), a circumference of 103.1 in (2,620 mm), and 614 revolutions per mile (382 per km).
§ 2
What does an inch size mean?
In 33x12.50R20, the first number is the nominal overall diameter: 33.0 in (838 mm). The next number is the nominal section width: 12.50 in (318 mm). The final number identifies the 20 in rim diameter.
To convert an inch dimension to millimeters, multiply by 25.4. To find nominal sidewall height, subtract the rim diameter from the overall diameter and divide the remainder equally between the sidewalls. For 33x12.50R20, that gives 6.50 in.
§ 3
Which metric sizes are closest?
Compare overall diameter while keeping the rim diameter unchanged. The tire size conversion chart identifies nearby nominal diameters; it does not establish equal width.
Maker-listed equivalents are a separate reference. Mickey Thompson lists 265/70R16 (P) as equivalent to 31X10.50R16, with a published diameter of 30.8 in. It lists 265/75R16 (P) as equivalent to 32X10.50R16, with a published diameter of 31.7 in.
- 33x12.50R20: 275/60R20 is nominally 33.0 in; 305/55R20 is 33.2 in; 295/55R20 is 32.8 in.
- 35x12.50R17: 295/75R17 and 315/70R17 are nominally 34.4 in, a −1.7% and −1.8% diameter change.
- 37x12.50R17: 315/80R17 is nominally 36.8 in, a −0.4% diameter change.
- 31x10.50R15: 265/75R15 is nominally 30.6 in, a −1.1% diameter change.
- 33x10.50R15: no metric candidate in the conversion chart is within 3% by the formula.
§ 4
Why is conversion not exact?
The formula produces unloaded nominal dimensions, not a measurement of a finished tire. A metric size described as a “33-inch” tire can round to that label without measuring exactly that diameter. Compare the 33-inch tire sizes by their actual nominal or maker-listed dimensions.
A conversion does not determine rim-width or load-range suitability. Fit depends on the vehicle, wheels, and suspension. The ±3% diameter window is a tire-shop rule of thumb, not a law.
Index
Truck and SUV sizes with their own sheet
38 sizes, by rim. Overall diameter by the formula.
15″ rims
Q&A
Common questions
What is 275/65R20 in inches?
By the formula, 275/65R20 measures 34.1 in (866 mm) tall and 10.8 in (275 mm) wide. Its nominal sidewall height is 7.0 in (179 mm).
What metric size is a 33-inch tire?
On a 20 in rim, 275/60R20 has a nominal diameter of 33.0 in. That is a diameter comparison, not an exact width equivalent to 33x12.50R20.
Is 315/70R17 a 35-inch tire?
Its nominal diameter is 34.4 in (873 mm) by the formula. That is below the 35.0 in (889 mm) nominal diameter of 35x12.50R17.
Can equivalent tire sizes measure differently?
Yes. Mickey Thompson lists 265/75R16 (P) as equivalent to 32X10.50R16 with a published diameter of 31.7 in, while its LT version is listed as equivalent to 32X10.50R16LT with a published diameter of 32 in.
Notes
Method and sources
Each number and rule comes from a source with the date it was checked.
- Sidewall = width × aspect ratio ÷ 100
- Diameter = rim × 25.4 + 2 × sidewall (mm)
- Revs per mile = 63,360 ÷ (π × diameter, in)
- True speed = dial × new diameter ÷ old diameter
- Clearance change = half the diameter change
Numbers are nominal: the tire unloaded, on its measuring rim. Revs per mile published by makers run higher than the formula. “Within 3%” is a rule of thumb tire shops use, not a law. Tire pressure: the placard on your door jamb.
- Mickey Thompson: size table (mickeythompsontires.com)checked Oct 15, 2026
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