Make sure the new tire’s load index is at least the original and that the wheel’s width, offset and bolt pattern suit the vehicle.
How the tire size calculator works
For a metric size W/A R D, the sidewall height is W × A ÷ 100 millimetres and the overall diameter is D × 25.4 + 2 × sidewall. Circumference = π × diameter, revolutions per mile = 63,360 ÷ circumference in inches, and revolutions per km = 1,000,000 ÷ circumference in mm. With a new tire fitted, actual speed = speedometer reading × new diameter ÷ old diameter.
Common tire sizes in inches
| Size | Diameter | Width | Sidewall | Revs per mile |
|---|---|---|---|---|
| 195/65R15 | 24.98″ | 7.68″ | 4.99″ | 807 |
| 205/55R16 | 24.88″ | 8.07″ | 4.44″ | 811 |
| 215/55R17 | 26.31″ | 8.46″ | 4.66″ | 767 |
| 225/45R17 | 24.97″ | 8.86″ | 3.99″ | 808 |
| 225/65R17 | 28.52″ | 8.86″ | 5.76″ | 707 |
| 235/55R18 | 28.18″ | 9.25″ | 5.09″ | 716 |
| 245/40R18 | 25.72″ | 9.65″ | 3.86″ | 784 |
| 265/70R17 | 31.61″ | 10.43″ | 7.3″ | 638 |
| 275/55R20 | 31.91″ | 10.83″ | 5.95″ | 632 |
| 285/75R16 | 32.83″ | 11.22″ | 8.42″ | 614 |
TireSizeCalc calculates from the nominal size code (ISO / ETRTO metric and Tire and Rim Association P-metric and LT conventions). Actual dimensions differ by brand, tread depth, wheel width and load; manufacturers' spec sheets list the measured diameter and revolutions per mile. The ±3% guideline is a common rule of thumb, not a legal limit: check your vehicle's placard, load index, speed rating and clearance before changing size.
Frequently asked questions
What is plus-one tire sizing?
One inch more wheel diameter with a lower-profile tire of similar overall diameter, for example 205/55R16 to 225/45R17.
Why go minus one for winter?
Smaller wheels with taller sidewalls are cheaper, ride better on rough winter roads and often allow a narrower tire that bites into snow.
How close should the diameter be?
Within about 3% is a common guideline; closer is better for speedometer accuracy and stability systems.
Is TireSizeCalc free?
Yes, with no sign-up or ads.
Are my sizes saved?
Yes, in this browser only.