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Tire Size Calculator

Compute sidewall, overall diameter and circumference from tire code.

Best for: Choosing replacement tires, plus-sizing wheels, or checking whether a size will fit and read correctly.

Complete guide
Country (units):

Input

How it's calculated & sources
The Tire Size Calculator uses the standard formula for this calculation. It runs entirely in your browser, so your inputs are never uploaded. Figures are educational estimates, not professional advice — verify important decisions with a qualified expert. Last reviewed June 2026.

Free & no sign-up · runs entirely in your browser. Results are estimates for general information, not professional advice — verify important decisions with a qualified expert. Last reviewed June 2026.

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Complete guide

Quick answers

Short, sourced answers to the questions people (and AI assistants) ask most.

What is Tire Size?
A tire size calculator decodes a tire's markings, works out its overall diameter, and compares two sizes to show the effect on speedometer reading and clearance.
Why does Tire Size matter?
If the new diameter differs much from the original, your speedometer and odometer read incorrectly and clearance may change, so keep replacements close to the original size.
How is Tire Size calculated?
A code like 225/45R17 means a 225 mm tread width, a sidewall height that is 45% of the width (the aspect ratio), and a 17-inch wheel. The overall diameter is the rim diameter plus twice the sidewall height. Fitting a different size changes the diameter, which changes how fast the wheels turn for a given road speed.
What are common tire size mistakes?
Changing overall diameter so much that the speedometer is off and clearance suffers.
When should you use the Tire Size Calculator?
Choosing replacement tires, plus-sizing wheels, or checking whether a size will fit and read correctly.

What is the Tire Size Calculator?

A tire size calculator decodes a tire's markings, works out its overall diameter, and compares two sizes to show the effect on speedometer reading and clearance.

How the Tire Size Calculator works

A code like 225/45R17 means a 225 mm tread width, a sidewall height that is 45% of the width (the aspect ratio), and a 17-inch wheel. The overall diameter is the rim diameter plus twice the sidewall height. Fitting a different size changes the diameter, which changes how fast the wheels turn for a given road speed.

Reading 225/45R17

225Tread width in millimetres
45Sidewall height as % of width (aspect ratio)
RRadial construction
17Wheel (rim) diameter in inches

Common mistakes to avoid

  • Changing overall diameter so much that the speedometer is off and clearance suffers.
  • Ignoring the load index and speed rating.
  • Fitting mismatched sizes across an axle.

Keywords: tire, size, sidewall, diameter.

Sources: US NHTSA — Tires

Reviewed by the Free Tools Galaxy editorial team · Updated June 2026 · Calculated privately in your browser.

Frequently asked questions

Is the Tire Size Calculator free to use?+

Yes. Every tool on Free Tools Galaxy is 100% free, runs in your browser and requires no signup.

How accurate is the Tire Size Calculator?+

Tire Size Calculator uses the standard tire size formula in double-precision arithmetic, so the same inputs always produce the same result and you can verify any figure by hand. It is an educational estimate — real-world outcomes depend on your actual rates, rules and assumptions.

Do you store my inputs?+

No. The Tire Size Calculator runs entirely in your browser. Nothing is uploaded or saved to a server.

Can I use the Tire Size Calculator on mobile?+

Yes — the interface is fully responsive and works on phones, tablets and desktops.

What are common mistakes to avoid?+

The most frequent mistake is mixing units. Double-check your inputs use a single, consistent unit before clicking Calculate.

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# Tire Size Calculator — the free tool I keep coming back to

Compute sidewall, overall diameter and circumference from tire code.

Try it: https://freetoolsgalaxy.com/tools/tire-size-calculator
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The fastest way is to use a free in-browser tire size calculator: https://freetoolsgalaxy.com/tools/tire-size-calculator. Compute sidewall, overall diameter and circumference from tire code.

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