🛞 Tire Size Calculator
By ToolNimba Editorial Team · Updated 2026-06-19
Enter a tire size such as 225/45R17. The calculator finds the sidewall height, overall diameter, circumference and revolutions per mile and kilometre. Add a second size to see how much your speedometer reading would change.
A tire size like 225/45R17 packs three numbers into one code, and from them you can work out exactly how big the tire really is. This calculator turns that code into the overall diameter, the sidewall height, the rolling circumference, and the number of revolutions per mile and per kilometre. Enter a second size and it also shows how much your speedometer reading would shift if you fitted it, so you can plus-size or swap tires with confidence.
What is the Tire Size Calculator?
A metric tire size such as 225/45R17 is read in three parts. The first number, 225, is the section width in millimetres (the widest point of the tire). The second number, 45, is the aspect ratio, which is the sidewall height as a percentage of that width. The letter R means radial construction, and the last number, 17, is the wheel (rim) diameter in inches. So a 225/45R17 tire is 225 mm wide with a sidewall that is 45% of 225, which is 101.25 mm, mounted on a 17 inch wheel.
To find the overall diameter you add two sidewalls to the rim diameter, because the tire wraps the wheel top and bottom. The rim is given in inches, so convert it to millimetres first (multiply by 25.4), then add twice the sidewall height. For 225/45R17 that is 17 × 25.4 + 2 × 101.25 = 431.8 + 202.5 = 634.3 mm, or about 24.97 inches tall. The rolling circumference is simply the diameter times pi, and dividing a fixed distance by that circumference gives the revolutions per mile or per kilometre.
These figures matter because the speedometer and odometer are calibrated for the original tire's circumference. A taller tire travels farther per revolution, so the speedometer reads slower than your true speed, while a shorter tire reads faster. As a rule of thumb, keeping the new overall diameter within about 3% of the original keeps the speedometer accurate, preserves gearing and ride height, and avoids rubbing or clearance problems. The comparison mode here makes that 3% check easy.
When to use it
- Checking whether a plus-size or replacement tire keeps your speedometer accurate before you buy.
- Comparing two tire sizes side by side to see the difference in overall diameter and circumference.
- Working out revolutions per mile or per km for gearing, GPS, or odometer calculations.
- Confirming a staggered or upsized fitment stays within the safe 3% diameter window.
How to use the Tire Size Calculator
- Enter the section width in millimetres (the first number in the size, for example 225).
- Enter the aspect ratio as a percentage (the second number, for example 45).
- Enter the rim (wheel) diameter in inches (the last number, for example 17).
- Read off the overall diameter, sidewall height, circumference, and revolutions per mile and per km.
- Tick "Compare with a second size" to enter a new tire and see the diameter and speedometer difference.
Formula & method
Worked examples
Find the diameter and revs for a 225/45R17 tire.
- Sidewall = 225 × 45 ÷ 100 = 101.25 mm
- Rim in mm = 17 × 25.4 = 431.8 mm
- Overall diameter = 431.8 + 2 × 101.25 = 634.3 mm (634.3 ÷ 25.4 = 24.97 in)
- Circumference = π × 634.3 = 1992.7 mm
- Revolutions per mile = 1,609,344 ÷ 1992.7 = 807.6
- Revolutions per km = 1,000,000 ÷ 1992.7 = 501.8
Result: Diameter 634.3 mm (24.97 in), circumference 1992.7 mm, about 807.6 revs/mile.
Compare 225/45R17 (current) with 245/40R18 (new) for speedometer error.
- Tire A diameter = 634.3 mm (from the example above)
- Tire B sidewall = 245 × 40 ÷ 100 = 98 mm
- Tire B diameter = 18 × 25.4 + 2 × 98 = 457.2 + 196 = 653.2 mm
- Diameter change = (653.2 − 634.3) ÷ 634.3 × 100 = 2.98%
- True speed at an indicated 60 mph = 60 × 653.2 ÷ 634.3 = 61.8 mph
Result: Tire B is 2.98% taller, so at an indicated 60 mph you actually travel about 61.8 mph.
Overall diameter for common tire sizes (rim in mm = inches × 25.4)
| Tire size | Sidewall (mm) | Overall diameter (mm) | Overall diameter (in) |
|---|---|---|---|
| 195/65R15 | 126.75 | 634.5 | 24.98 |
| 205/55R16 | 112.75 | 631.9 | 24.88 |
| 225/45R17 | 101.25 | 634.3 | 24.97 |
| 245/40R18 | 98.00 | 653.2 | 25.72 |
| 275/35R19 | 96.25 | 675.1 | 26.58 |
Reading a metric tire code (example 225/45R17)
| Part | Value | Meaning |
|---|---|---|
| Width | 225 | Section width in millimetres |
| Aspect ratio | 45 | Sidewall height as a percent of width |
| Construction | R | Radial ply construction |
| Rim diameter | 17 | Wheel diameter in inches |
Common mistakes to avoid
- Treating the aspect ratio as a fixed millimetre value. The aspect ratio is a percentage of the width, not a length. A 45 sidewall on a 225 tire is 101.25 mm, but the same 45 on a 275 tire is 123.75 mm. Always multiply width by aspect and divide by 100.
- Forgetting to add two sidewalls. The tire wraps the wheel top and bottom, so the overall diameter is the rim diameter plus twice the sidewall, not just one. Adding a single sidewall undercounts the height badly.
- Mixing inches and millimetres. Width and sidewall are in millimetres while the rim is in inches. Convert the rim to millimetres (multiply by 25.4) before adding it to the sidewalls, or your diameter will be far too small.
- Ignoring the 3% diameter rule when changing size. A new tire whose overall diameter is more than about 3% different from stock throws off the speedometer, odometer and gearing, and can cause rubbing. Check the comparison before fitting an upsized tire.
Glossary
- Section width
- The widest point of the tire across the tread, given in millimetres (the first number in the size).
- Aspect ratio
- The sidewall height expressed as a percentage of the section width (the second number in the size).
- Sidewall height
- The vertical distance from the rim to the tread, equal to width × aspect ÷ 100, in millimetres.
- Overall diameter
- The full height of the mounted tire, equal to the rim diameter plus two sidewalls.
- Rolling circumference
- The distance the tire covers in one full revolution, equal to π times the overall diameter.
- Revolutions per mile
- How many times the tire turns to cover one mile, used for gearing and odometer calibration.
Frequently asked questions
How do I read a tire size like 225/45R17?
The 225 is the section width in millimetres, the 45 is the aspect ratio (sidewall height as a percent of width), the R means radial construction, and the 17 is the rim diameter in inches. So it is a 225 mm wide tire with a 101.25 mm sidewall on a 17 inch wheel.
How is the overall tire diameter calculated?
Convert the rim diameter to millimetres (multiply the inch value by 25.4), then add two sidewall heights, since the tire wraps the wheel top and bottom. The sidewall height is width × aspect ÷ 100. For 225/45R17 that is 431.8 + 2 × 101.25 = 634.3 mm.
What is revolutions per mile and why does it matter?
Revolutions per mile is how many times the tire turns to travel one mile, found by dividing 1,609,344 mm by the tire circumference in mm. It is used to calibrate the speedometer and odometer, and matters when you change tire size or set up gearing.
How much does tire size affect the speedometer?
The speedometer is calibrated for the original tire. A taller tire makes it read slower than your true speed and a shorter tire makes it read faster. The percentage error equals the percentage change in overall diameter, so a 3% taller tire means a 3% optimistic speedometer.
How much can I change tire size safely?
A common rule of thumb is to keep the new overall diameter within about 3% of the original. Staying inside that window preserves speedometer accuracy, gearing and ride height, and avoids the tire rubbing the bodywork or suspension.
Is tire size in millimetres or inches?
A metric size mixes both: the width and sidewall are in millimetres while the rim diameter is in inches. This calculator handles the conversion for you and reports the overall diameter in both millimetres and inches.