🛞 Tire Size Calculator
Enter a tire size like 225/45R17 and get its true measurements — overall diameter, sidewall height, circumference and revolutions per mile.
Overall diameter
24.97″
634 mm
Sidewall height
101.3 mm
3.99″
Circumference
1.993 m
78.45″
Revolutions per mile
808
Revolutions per km
502
Overall diameter = rim + 2 × sidewall, where sidewall = width × aspect ratio. All exact geometry. 🔒 In your browser.
How the tire size calculator works
A metric tire code packs three numbers: section width in millimetres (225), the aspect ratio as a percentage of that width (45), and the rim diameter in inches (17). The sidewall height is width × aspect ratio, and the overall diameter is the rim plus two sidewalls: 17″ × 25.4 + 2 × (225 × 0.45) ≈ 634 mm (25.0″). Circumference is π × diameter, and revolutions per mile is one mile divided by the circumference.
The rim diameter is in inches while the width and sidewall are in millimetres — a quirk of the standard. This tool handles the mixed units for you. It reads plain codes like 225/45R17 as well as P- and ZR-prefixed ones.
Frequently asked questions
What does 225/45R17 mean?
225 is the tire’s section width in millimetres, 45 is the aspect ratio (the sidewall height is 45% of the width), R means radial construction, and 17 is the wheel (rim) diameter in inches. Together they give a tire about 634 mm (25.0″) in overall diameter.
How do I calculate a tire’s overall diameter?
Overall diameter = rim diameter + 2 × sidewall height, where sidewall height = section width × aspect ratio. For 225/45R17: 17 × 25.4 + 2 × (225 × 0.45) = 431.8 + 202.5 = 634.3 mm (about 25.0 inches).
What is the sidewall height of a tire?
The sidewall (aspect) height is the section width times the aspect ratio. A 225/45 tire has a sidewall of 225 × 0.45 = 101.25 mm. A lower aspect ratio means a shorter, stiffer sidewall.
What is revolutions per mile?
How many full turns the tire makes in one mile — one mile (1,609,344 mm) divided by the tire’s circumference. A 225/45R17 turns about 808 times per mile. It’s used for gearing, speedometer and odometer calculations.
Why is part of the size in mm and part in inches?
It’s a historical quirk of the P-metric standard: width and sidewall are metric (millimetres), but the rim is still specified in inches. This calculator converts between them automatically.
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How to Read a Tire Size: 225/45R17 Decoded
A tire size like 225/45R17 packs four facts into one code: section width in millimetres (225), aspect ratio as a percentage (45), radial construction (R), and rim diameter in inches (17). What each part means, how to find the real overall diameter, and why part is metric and part is inches.