Tire Size Calculator — P-Metric Tire Dimensions
Decode any P-metric tire code (e.g. 225/55R17): enter the section width (mm), aspect ratio (%), and rim diameter (inches) to get the tire's outer diameter, sidewall height, circumference, and revolutions per km and mile.
mm
%
in
Total tire outside diameter (rim + 2 × sidewall)
- 1
Sidewall height
225 × (55 ÷ 100) = 123.8The aspect ratio is the sidewall height as a percentage of section width. - 2
Rim diameter in mm
17 × 25.4 = 431.8 - 3
Outer diameter
431.8 + 2 × 123.8 = 679.3
How does this calculator work?
For a tire code W/A/R: Sidewall = W × A/100 mm; Outer diameter = R × 25.4 + 2 × Sidewall mm; Circumference = π × OD. For 225/55R17: OD ≈ 679 mm (26.7 in), circumference ≈ 2,133 mm (84 in), ~472 rev/km. A ±3% OD change from stock is the common fitment tolerance.
Formula
How this is calculated
A standard P-metric tire code like 225/55R17 encodes three key dimensions. The first number (225) is the section width in millimetres — the widest point of the inflated tire from sidewall to sidewall. The second number (55) is the aspect ratio — the sidewall height expressed as a percentage of the section width, so a 55-aspect-ratio tire on a 225 mm wide tire has a sidewall of 225 × 0.55 = 123.75 mm. The third number (17) is the rim diameter in inches — always in inches regardless of country.
The total outer diameter of the assembled wheel is the rim diameter converted to millimetres (×25.4) plus two sidewall heights (top and bottom). Outer diameter = 17 × 25.4 + 2 × 123.75 = 431.8 + 247.5 = 679.3 mm in this example. The circumference is π × OD. Dividing the distance per revolution into the standard distances (1 km = 1,000,000 mm; 1 mile = 63,360 inches) gives revolutions per unit distance — a key figure for speedometer calibration when changing tire sizes.
This calculator assumes a standard P-metric (passenger-car) tire. LT (light-truck) and ST (trailer) tires use the same formula. The result assumes correct inflation pressure; an under- or over-inflated tire will have a slightly different effective radius.
Frequently asked questions
Rim diameters are measured in inches due to the long historical dominance of US manufacturing standards for wheels. The ISO metric system adopted millimetres for tire cross-sections but kept inches for the rim diameter to maintain backward compatibility. Most modern tire codes mix both: width and aspect ratio in metric, rim in imperial.
The key constraints are outer diameter (must stay within about ±3% to avoid speedometer error and clearance issues), section width (must fit inside the wheel arch and not rub the suspension), and load/speed ratings (must meet or exceed OEM specifications). Use this calculator to compare OD values for your current and candidate tire sizes.
Plus-sizing means fitting a larger-diameter rim (e.g. going from 17" to 18") while reducing the aspect ratio to keep the overall outer diameter approximately the same. As a rough guide, each 1" increase in rim diameter should be paired with roughly a 10-percentage-point reduction in aspect ratio on the same width tire. Always verify the resulting OD with a tire size calculator.
Also known as
TG we-Calculate Editorial Team. (2026). Tire Size Calculator — P-Metric Tire Dimensions [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/tire-size-calculator
TG we-Calculate Editorial Team. "Tire Size Calculator — P-Metric Tire Dimensions." TG we-Calculate. 2026. https://we-calculate.com/calculator/tire-size-calculator.
TG we-Calculate Editorial Team, "Tire Size Calculator — P-Metric Tire Dimensions," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/tire-size-calculator
@misc{wecalculate_tire_size_calculator, title = {Tire Size Calculator — P-Metric Tire Dimensions}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/tire-size-calculator}}, year = {2026}, note = {TG we-Calculate} }
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