Every passenger tire carries a code moulded into its sidewall, and almost every code mixes units: the width is in millimetres, the height is a percentage, and the wheel is in inches. A tire size calculator removes the guesswork by translating that code into the dimensions you can actually measure with a tape — width, sidewall height, overall diameter and circumference — in both inches and millimetres.
How to use the tire size calculator
- Read the size from the sidewall, for example 225/45R17.
- Enter the width (225), the aspect ratio (45) and the rim diameter (17 in, or switch the unit to mm).
- Read the width, sidewall, overall diameter, circumference and revolutions per mile. Use the unit menus to see millimetres, centimetres or feet.
To compare an existing size with a candidate replacement, use the tire size comparison calculator; for a size already written in inches (such as 33x12.50R17) see the tire size conversion calculator.
Tire size formula
Sidewall = width × aspect ratio ÷ 100
Overall diameter = rim + 2 × sidewall
Circumference = π × diameter
Revolutions per mile = 63,360 ÷ circumference (in)
- 25.4 — millimetres per inch
- Aspect ratio — sidewall height as a percentage of the width
- 63,360 — inches in a mile
Worked example: 225/45R17
- Width: 225 ÷ 25.4 = 8.86 in.
- Sidewall: 225 × 45 ÷ 100 = 101.25 mm = 3.99 in.
- Overall diameter: 17 + 2 × 3.986 = 24.97 in (634 mm).
- Circumference: π × 24.97 = 78.5 in.
- Revolutions per mile: 63,360 ÷ 78.45 = 808.
Retailers list the same size at 24.97 in, so the formula matches published values.
Dimensions of popular tire sizes
All values below are calculated with the formulas above.
| Size | Width | Sidewall height | Overall diameter | Circumference | Revs per mile |
|---|---|---|---|---|---|
| 195/65R15 | 195 mm (7.7 in) | 4.99 in (127 mm) | 24.98 in (634 mm) | 78.5 in | 807 |
| 205/55R16 | 205 mm (8.1 in) | 4.44 in (113 mm) | 24.88 in (632 mm) | 78.2 in | 811 |
| 215/60R16 | 215 mm (8.5 in) | 5.08 in (129 mm) | 26.16 in (664 mm) | 82.2 in | 771 |
| 225/45R17 | 225 mm (8.9 in) | 3.99 in (101 mm) | 24.97 in (634 mm) | 78.5 in | 808 |
| 235/55R18 | 235 mm (9.3 in) | 5.09 in (129 mm) | 28.18 in (716 mm) | 88.5 in | 716 |
| 245/40R19 | 245 mm (9.6 in) | 3.86 in (98 mm) | 26.72 in (679 mm) | 83.9 in | 755 |
| 265/70R17 | 265 mm (10.4 in) | 7.30 in (186 mm) | 31.61 in (803 mm) | 99.3 in | 638 |
| 275/55R20 | 275 mm (10.8 in) | 5.95 in (151 mm) | 31.91 in (810 mm) | 100.2 in | 632 |
Using the numbers
Overall diameter is the figure that matters most when you change tires. It sets the ride height, how far the speedometer is off, and the engine speed at a given road speed. Sidewall height controls ride comfort and how much the wheel is protected from potholes: a 245/40R19 has a sidewall of only 3.86 inches, while a 215/60R16 has 5.08 inches. Width controls the contact patch and how much room is needed in the wheel well and on the rim.
When moving to a larger wheel, shops often look for a tire with a similar overall diameter, usually within about 3% of the original. A 17-inch wheel with a 45% tire becomes an 18-inch wheel with a 40% tire at the same width, and the diameter stays close. The speedometer error calculator turns the difference into a real-world speed error.
Remember that tire size is only part of fitment: wheel width, offset, load index and speed rating must also suit the vehicle. The dimensions here are nominal values; a real tire may be a little narrower or wider depending on the rim it is mounted on.
Sources
Size markings follow the ISO metric tire designation used by the Tire and Rim Association and the DOT tire label. The 24.97 in check value for 225/45R17 comes from the tire-size reference data collected from tiresize.com on 2026-10-09. Table values are computed from the formulas shown.
Frequently asked questions
What do the numbers on a tire sidewall mean?
In 225/45R17, 225 is the section width in millimetres, 45 is the sidewall height as a percentage of that width, R means radial construction, and 17 is the wheel diameter in inches.
How do I turn a metric tire size into inches?
Divide the width by 25.4 for the width in inches, multiply width by aspect ratio for the sidewall (then divide by 25.4), and add two sidewalls to the rim diameter for the overall diameter.
Why is the sidewall called a percentage?
The aspect ratio is sidewall height ÷ section width. That is why a wider tire with the same aspect ratio is also taller: 245/45R17 has a taller sidewall than 225/45R17.
Is the real tire exactly this size?
The result is the nominal size. A measured tire varies by roughly ±1–2% with brand, inflation, wheel width and tread wear, so use it to compare sizes, not to measure fender clearance to the millimetre.
Why are revolutions per mile useful?
It tells you how often the tire turns per mile (63,360 inches ÷ circumference). Speedometers and gear calculations assume a particular figure, so changing it changes the reading.