Tire Size Calculator
Compare tire sizes for diameter, sidewall, circumference and speedometer error.
About Tire Size Calculator
A metric tire like 225/45R17 means: 225 mm section width, 45% aspect ratio (sidewall = 45% of 225 = 101.25 mm), and 17-inch rim diameter. The overall diameter is twice the sidewall plus the rim. Circumference = π × diameter. Revolutions per mile = 63,360 in ÷ circumference. If you switch to a different size, your speedometer reads based on the old rev count; a larger tire makes the speedo under-read.
Formulas
Overall diameter
Diameter (in) = 2 × (width × aspect/100) / 25.4 + rim diameter
- W
- Section width (mm)
- A
- Aspect ratio (%)
- R
- Rim diameter (in)
Revolutions per mile
Revs/mile = 63,360 ÷ (π × diameter in inches)
- 63360
- Inches per mile
Worked example
With the default values below, Tire Size Calculator returns Tire 1 Diameter of 24.972. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Tire 1 Size
- 225/45R17
- Compare with a second tire?
- No
- Tire 2 Size
- 235/40R18
- Displayed Speedaccepts 1–300 mph
- 60 mph
Results
- Tire 1 Diameter
- 24.972 in
- Tire 1 Diameter
- 634.3 mm
- Sidewall Height
- 101.3 mm
- Circumference
- 78.452 in
- Revolutions per Mile
- 807.63
- Revolutions per km
- 501.84
Step by step
- 1Sidewall height= 101.3 mm
- 2Overall diameter= 24.972 in (634.3 mm)
- 3Circumference= 78.452 in
- 4Revolutions per mile= 807.63 rev/mi
Assumptions and limits
- The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
What does the 'R' mean in a tire size?
The R stands for Radial construction — the internal cords run perpendicular to the direction of travel, from bead to bead. Virtually all modern passenger tires are radial.
How much speedometer error is acceptable?
In most countries, regulations permit speedometers to read up to ~10% high but never below actual speed. An error of 1–2% is essentially imperceptible. Beyond ±3% you may want to consider a speedometer recalibration or a closer size match.
Will a plus-one tire fit my car?
A plus-one fitment (larger rim, lower profile tire) should maintain nearly the same overall diameter to avoid clearance issues. This calculator shows the diameter change so you can stay within the ±3% guideline.
How do I read a tire size?
225/45R17: 225 = section width in mm; 45 = aspect ratio (sidewall height as % of width); R = radial; 17 = rim diameter in inches. You can find this on the tire sidewall, usually preceded by the load index and speed rating.