Tire Size Calculator — diameter, sidewall & circumference

How it works

A P-metric tire code such as 225/45R17 packs three measurements (Wikipedia, Tire code):

• 225 — the section width in millimetres (sidewall to sidewall)
• 45 — the aspect ratio: the sidewall height as a percentage of the width
• 17 — the wheel (rim) diameter in inches

From these the geometry follows directly:

Sidewall height = width × aspect ratio ÷ 100

Overall diameter = wheel diameter × 25.4 + 2 × sidewall height

Circumference = π × overall diameter

The overall diameter matters because it sets your rolling circumference, which in turn affects speedometer accuracy, gearing and clearance. Fitting a tire with a different diameter makes the speedometer read high or low and changes how far the car travels per wheel revolution. A common rule when changing sizes is to keep the overall diameter within about 3% of the original so these effects stay negligible — this calculator lets you check two sizes side by side.

Worked example

For 225/45R17:

• Sidewall: 225 × 45 ÷ 100 = 101.25 mm
• Overall diameter: 17 × 25.4 + 2 × 101.25 = 431.8 + 202.5 = 634.3 mm (≈ 24.98 in)
• Circumference: π × 634.3 ≈ 1,992.7 mm, giving about 502 revolutions per km

Frequently asked questions

What do the numbers in a tire size mean?

In 225/45R17, 225 is the width in millimetres, 45 is the aspect ratio (sidewall height as a percent of width), R means radial construction, and 17 is the wheel diameter in inches.

How do I calculate a tire's overall diameter?

Find the sidewall height (width × aspect ÷ 100), then add twice that to the wheel diameter converted to millimetres (inches × 25.4). For 225/45R17 that is 431.8 + 2 × 101.25 = 634.3 mm.

What is the aspect ratio?

It is the sidewall height expressed as a percentage of the section width. A 45 aspect ratio means the sidewall is 45% as tall as the tire is wide, so a wider tire with the same ratio has a taller sidewall.

Why does tire size affect my speedometer?

The speedometer is calibrated to the original tire's rolling circumference. A larger-diameter tire travels farther per revolution, so the speedometer reads slower than you are actually going; a smaller tire does the opposite.

How much can I change tire size safely?

A common guideline is to keep the new overall diameter within about 3% of the original to limit speedometer error and clearance issues. Compare the diameters here, and check load rating and clearance before fitting a different size.

Does a lower aspect ratio mean a shorter sidewall?

Yes. A lower aspect ratio (say 40 instead of 55) gives a shorter, stiffer sidewall for sharper handling, while a higher ratio gives a taller sidewall and a more cushioned ride.