Tire Size Calculator: Diameter, Sidewall and Revs
Turn a tire size such as 225/45R17 into its diameter, width and sidewall in mm and inches, circumference and revolutions per mile. Compare two sizes and see the speedometer error.
- Diameter
- 634.3 mm (24.97 in)
- Width
- 225 mm (8.86 in)
- Sidewall
- 101.3 mm (3.99 in)
- Circumference
- 1,992.7 mm
- Revolutions per mile
- 807.6
- Revolutions per km
- 501.8
Dimensions are nominal values from the size code. Real tires vary by brand, rim width, pressure and wear.
About the Tire Size Calculator
The Tire Size Calculator turns the size code on a tire's sidewall into real dimensions. Type a metric size such as 225/45R17 and the calculator shows the tread width, the sidewall height and the overall diameter in millimetres and inches, the circumference, and how many times the tire turns per mile and per kilometre. Switch to Compare two sizes to see how much taller or shorter a new size is and how far it throws your speedometer off.
The calculator accepts the size the way it is printed or typed: an optional P (passenger) or LT (light truck) prefix, ZR for high-speed tires in place of R, spaces and lower case, so "225/45 zr 17" works as well as "225/45R17". Widths from 105 to 455 mm, aspect ratios from 20 to 95 and rims from 10 to 30 inches are supported.
All figures are nominal values from the size code. Real tires vary by brand, rim width, pressure, load and wear, so a measured tire can be a few millimetres off.
Reading a tire size
225
The section width in millimetres, from sidewall to sidewall.
45
The aspect ratio: the sidewall height as a percentage of the width. 45 means the sidewall is 45 percent of 225 mm.
R17
R stands for radial construction, and 17 is the rim diameter in inches.
The formulas
Revolutions per mile are 1,609,344 mm divided by the circumference, and revolutions per kilometre are 1,000,000 mm divided by it.
Worked examples
- 225/45R17. The sidewall is 225 × 0.45 = 101.25 mm and the diameter is 17 × 25.4 + 2 × 101.25 = 634.3 mm, or 24.97 in. The circumference is 1,992.7 mm, so the tire turns 807.6 times per mile.
- LT265/70R16. The sidewall is 185.5 mm (7.30 in) and the diameter 777.4 mm (30.61 in), for 659.0 revolutions per mile.
Changing tire sizes
When you fit a different size, try to keep the overall diameter within about 3 percent of the original. A bigger difference changes the speedometer and odometer readings, the gearing and how the ABS and stability control see wheel speed. Use the comparison mode to check before you buy.
Privacy
Nothing is uploaded; every calculation runs in your browser and never leaves your device.
Related tools
Compare two sizes on the Tire Size Comparison Calculator, cost a trip with the Fuel Cost Calculator, or convert millimetres and inches with the Length Converter.
Frequently asked questions
What do the numbers in 225/45R17 mean?
225 is the tread width in millimetres, 45 is the sidewall height as a percentage of that width, R means radial construction and 17 is the rim diameter in inches.
How do I work out a tire's diameter?
Multiply the width by the aspect ratio and divide by 100 to get the sidewall, double it, and add the rim diameter converted to millimetres at 25.4 mm per inch.
What are revolutions per mile used for?
Revolutions per mile tell you how many times the tire turns in one mile. Speedometers, odometers and some trip computers rely on it, and truck fleets use it to set up tachographs.
Does the calculator accept LT and ZR sizes?
Yes. It ignores a P or LT prefix and treats ZR like R, and it accepts spaces and lower case, so lt265/70 zr16 reads the same as LT265/70R16.
Why does my tire measure differently?
The results are nominal values from the size code. Real tires vary by brand, the width of the rim they are mounted on, inflation pressure, the load on them and tread wear.