Travel, Auto & Lifestyle

How to determine your tire size?

How to determine your tire size: calculation method, required inputs, worked check, limitations and a link to Tire Size Calculator. Reviewed 25 September 2026.

By Uddhav Gopani, FounderPublished 25 September 2026Reviewed 25 September 2026

Your tire size is printed on the sidewall as a code like 265/70 R17: 265 is the section width in millimetres, 70 is the aspect ratio (sidewall height as 70% of width), R means radial construction, and 17 is the wheel diameter in inches. The driver's-door placard and the owner's manual repeat the factory specification.

What this specific question is asking

If you need the overall diameter rather than the code: diameter in inches = (width × aspect ratio × 2 ÷ 25.4) + wheel diameter. For 265/70 R17 that is (265 × 0.70 × 2 ÷ 25.4) + 17 ≈ 31.6 inches. Measure the actual tire mounted and loaded if precision matters — nominal and real diameters differ by a few percent.

Verify against two independent sources before ordering: the sidewall on the current tire and the door placard. If they disagree, the vehicle may be running a non-stock size — trust the placard for the OEM spec and the sidewall for what is actually fitted.

Inputs to verify

Verify tire code (for example 205/55r16) before calculation; if this value uses a different unit, period or definition, convert or restate it first.
Verify section width in mm (optional) before calculation; if this value uses a different unit, period or definition, convert or restate it first.
Verify aspect ratio in percent (optional) before calculation; if this value uses a different unit, period or definition, convert or restate it first.
Verify rim diameter in inches (optional) before calculation; if this value uses a different unit, period or definition, convert or restate it first.

Step-by-step check

Enter a tire code such as 205/55R16 in the first field.
Or fill the width, aspect ratio, and rim fields instead.
Leave the fields you are not using blank.
Press Calculate to see diameter, sidewall, and circumference.

Worked example and sanity check

As a worked reference for “How to determine your tire size”, Worked example A 205/55 R16 tyre has a sidewall of 205 × 0.55 = 112.75 mm and an overall diameter of 16″ (406.4 mm) + 2 × 112.75 = 631.9 mm .

Interpreting the result

Interpretation starts with magnitude. The worked example landed at 112; compare yours before trusting the decimals. If the two disagree wildly, the setup — usually tire code (for example 205/55r16)'s unit or period — is the suspect. Read the magnitude before the decimals: a result ten times too large or small almost always means a unit or period slipped, not that the formula failed.

Reviewed 25 September 2026

Verify any current tax threshold, lender rate, provider rule or health guidance with the authoritative source before acting.

Use the Tire Size Calculator

Open Tire Size Calculator

Common questions

Which inputs matter most for how to determine your tire size?

The key inputs are Tire code (for example 205/55R16), Section width in mm (optional), Aspect ratio in percent (optional), Rim diameter in inches (optional). Match their units and periods before using Tire Size Calculator.

How can I check an answer for how to determine your tire size?

Run the example exactly as written, then vary tire code (for example 205/55r16) alone. A result that tracks that change predictably tells you the setup — units, periods, definitions — is correct.

When should I recalculate how to determine your tire size?

Re-run it when the real-world inputs move — new measurements, changed rates or thresholds, or a different unit convention on the same quantity.