LazyTools

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🛞 Tire Size Calculator

Enter a tire size like 225/45R17 and get its true measurements, overall diameter, sidewall height, circumference and revolutions per mile.

Width 225 mmAspect 45%Rim 17″

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.
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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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