Quick Answer
- Overall diameter = rim + 2 x sidewall, where sidewall = width x aspect / 100. A 225/45R17 has a 24.97 in (634.3 mm) diameter, so the radius is 12.49 in.
- Circumference is pi x diameter: 78.45 in (1993 mm), which is 808 revolutions per mile (calculated).
- The sidewall is the tire section width in mm times the aspect ratio, divided by 25.4 for inches. Diameter is the nominal figure; a loaded tire is slightly shorter.
Enter any metric tire size to calculate overall diameter, sidewall height, circumference, and revolutions per mile and km. Works in both inches and metric.
Tire Size
→ 225/45R17
Diameter Formula Reference
Metric Tire (e.g. 225/45R17)
Sidewall (mm) = Width × Aspect ÷ 100
Diameter (mm) = Rim × 25.4 + 2 × Sidewall
Diameter (in) = Diameter (mm) ÷ 25.4
Circumference (in) = π × Diameter (in)
Revs/Mile = 63,360 ÷ Circumference (in)
Example: 225/45R17
Sidewall = 225 × 0.45 = 101.25 mm
Diameter = 431.8 + 2×101.25 = 634.3 mm = 24.97″
Circumference = π × 24.97 = 78.45″
Revs/Mile = 63,360 ÷ 78.45 = 808
Wheel + Tire (inches direct)
Total Diameter (in) = Rim + 2 × Sidewall
Circumference (in) = π × Total Diameter
Revs/Mile = 63,360 ÷ Circumference
Speedo Error (%) = (New OD ÷ Stock OD − 1) × 100
Continental guidance: within 3%
Speedometer error equals the diameter difference. Continental advises keeping rolling circumference within 3% of the original; it is not a vehicle maker specification.
Tire Diameter Chart
Nominal dimensions calculated from the size. Makers build each size with some spread.
| Size | Diameter in | Diameter mm | Radius in | Sidewall in | Circumference in | Circumference mm | Revs per mile |
|---|---|---|---|---|---|---|---|
| 195/65R15 | 24.98 | 635 | 12.49 | 4.99 | 78.5 | 1993 | 807 |
| 205/55R16 | 24.88 | 632 | 12.44 | 4.44 | 78.2 | 1985 | 811 |
| 215/60R16 | 26.16 | 664 | 13.08 | 5.08 | 82.2 | 2087 | 771 |
| 225/45R17 | 24.97 | 634 | 12.49 | 3.99 | 78.5 | 1993 | 808 |
| 225/65R17 | 28.52 | 724 | 14.26 | 5.76 | 89.6 | 2275 | 707 |
| 235/55R18 | 28.18 | 716 | 14.09 | 5.09 | 88.5 | 2248 | 716 |
| 245/45R18 | 26.68 | 678 | 13.34 | 4.34 | 83.8 | 2129 | 756 |
| 255/55R19 | 30.04 | 763 | 15.02 | 5.52 | 94.4 | 2397 | 671 |
| 265/65R17 | 30.56 | 776 | 15.28 | 6.78 | 96.0 | 2439 | 660 |
| 265/70R17 | 31.61 | 803 | 15.80 | 7.30 | 99.3 | 2522 | 638 |
| 275/65R18 | 32.07 | 815 | 16.04 | 7.04 | 100.8 | 2559 | 629 |
| 275/60R20 | 32.99 | 838 | 16.50 | 6.50 | 103.6 | 2633 | 611 |
| 285/70R17 | 32.71 | 831 | 16.35 | 7.85 | 102.8 | 2610 | 617 |
| 295/70R18 | 34.26 | 870 | 17.13 | 8.13 | 107.6 | 2734 | 589 |
| 305/55R20 | 33.21 | 844 | 16.60 | 6.60 | 104.3 | 2650 | 607 |
| 315/70R17 | 34.36 | 873 | 17.18 | 8.68 | 108.0 | 2742 | 587 |
Tire Circumference by Diameter in Inches and mm
For inch-size tires such as 33x12.50R17 or 37x12.50R17. Calculated as pi x diameter.
| Diameter in | Diameter mm | Radius in | Circumference in | Circumference mm | Revs per mile |
|---|---|---|---|---|---|
| 28 | 711 | 14.0 | 88.0 | 2234 | 720 |
| 30 | 762 | 15.0 | 94.2 | 2394 | 672 |
| 31 | 787 | 15.5 | 97.4 | 2474 | 651 |
| 32 | 813 | 16.0 | 100.5 | 2553 | 630 |
| 33 | 838 | 16.5 | 103.7 | 2633 | 611 |
| 34 | 864 | 17.0 | 106.8 | 2713 | 593 |
| 35 | 889 | 17.5 | 110.0 | 2793 | 576 |
| 36 | 914 | 18.0 | 113.1 | 2873 | 560 |
| 37 | 940 | 18.5 | 116.2 | 2952 | 545 |
| 38 | 965 | 19.0 | 119.4 | 3032 | 531 |
| 40 | 1016 | 20.0 | 125.7 | 3192 | 504 |
Tire Width and Sidewall in Inches (mm / 25.4)
Section width in mm divided by 25.4, and sidewall height at common aspect ratios.
| Width mm | Width in | Sidewall at 40 | At 50 | At 65 | At 75 |
|---|---|---|---|---|---|
| 185 | 7.28 | 2.91 | 3.64 | 4.73 | 5.46 |
| 195 | 7.68 | 3.07 | 3.84 | 4.99 | 5.76 |
| 205 | 8.07 | 3.23 | 4.04 | 5.25 | 6.05 |
| 215 | 8.46 | 3.39 | 4.23 | 5.50 | 6.35 |
| 225 | 8.86 | 3.54 | 4.43 | 5.76 | 6.64 |
| 235 | 9.25 | 3.70 | 4.63 | 6.01 | 6.94 |
| 245 | 9.65 | 3.86 | 4.82 | 6.27 | 7.23 |
| 255 | 10.04 | 4.02 | 5.02 | 6.53 | 7.53 |
| 265 | 10.43 | 4.17 | 5.22 | 6.78 | 7.82 |
| 275 | 10.83 | 4.33 | 5.41 | 7.04 | 8.12 |
| 285 | 11.22 | 4.49 | 5.61 | 7.29 | 8.42 |
| 295 | 11.61 | 4.65 | 5.81 | 7.55 | 8.71 |
| 305 | 12.01 | 4.80 | 6.00 | 7.81 | 9.01 |
| 315 | 12.40 | 4.96 | 6.20 | 8.06 | 9.30 |
What a Diameter Change Does
Calculated for common swaps. For any pair use Compare Two above, or the tire size difference calculator.
| From | To | Diameter change | Speedometer error | Clearance change | True speed at 60 mph |
|---|---|---|---|---|---|
| 225/45R17 | 235/45R17 | +0.35 in | +1.4% | +0.18 in | 60.9 mph |
| 225/65R17 | 245/65R17 | +1.02 in | +3.6% | +0.51 in | 62.2 mph |
| 265/70R17 | 285/70R17 | +1.10 in | +3.5% | +0.55 in | 62.1 mph |
| 265/70R17 | 315/70R17 | +2.76 in | +8.7% | +1.38 in | 65.2 mph |
| 275/60R20 | 275/55R20 | -1.08 in | -3.3% | -0.54 in | 58.0 mph |
Related: speedometer error calculator, revolutions per mile calculator and the rolling radius calculator.
Tire Diameter Formulas and Sources
Overall diameter
Rim + 2 x width x aspect / 100 / 25.4. For 225/45R17: 17 + 2 x 101.25 / 25.4 = 24.97 in.
Radius
Half the diameter: 12.49 in for the 225/45R17. A loaded tire has a smaller static loaded radius; see the rolling radius calculator.
Circumference
Pi x diameter: 78.45 in, or 1993 mm. For a size in mm, circumference = pi x diameter in mm.
Revolutions per mile
63,360 / circumference in inches = 808. Speedometer error is the diameter change in percent.
| Source | Statement (paraphrased) |
|---|---|
| Continental Tire, plus-sizing guidance | When changing tire size keep the overall rolling circumference within 3% of the original, using only approved size and rim width combinations. |
| USTMA, Care and Service of Passenger and Light Truck Tires | Acceptable load capacity or overall diameter does not imply acceptable vehicle clearance. Check clearance at all four wheel positions through full suspension travel and steering lock. |
| Bridgestone Truck Tire Data Book 2023 | Publishes a range of overall diameters for each size across models (for example 39.5 to 41.0 in for 295/75R22.5), because tread depth and design vary. The code gives a calculated nominal diameter, not an exact one. |
How to Use the Tire Diameter Calculator
- 1Enter the width, aspect ratio and rim from the tire sidewall, for example 225, 45 and 17.
- 2Read the overall diameter in inches and mm, the sidewall, circumference and revs per mile.
- 3Open Compare Two to see the diameter change, speedometer error and clearance change against a second size.
- 4Use By Diameter to find metric sizes near a target height, and Plus Sizing for taller or lower options on the same rim.