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Tire Size Explainer

What does 245/40R18 mean?

Type the size printed on your tire and see what each part means and how big the tire is.

One Click Answers · Tire Size Explainer
oneclickanswers.com/tire-size-explainer · Printed
About this tool & how to use it

What it does

Reads a tire size such as 245/40R18 97Y and explains each part in plain words, with the sidewall height, overall diameter and circumference worked out. It does not say whether a size fits a vehicle.

How to use it

  1. Find the size on the side of the tire, for example 245/40R18 97Y, and type it in.
  2. Read what each part means: width, aspect ratio, construction, rim diameter and, if you typed them, the load index and speed rating.
  3. Use the diameter and circumference to compare with another size, or open the Tire Size Comparison.

It is printed on the side of the tire, for example 245/40R18 97Y.

One click answer

Your answer will appear here.

Fill in the form and your answer appears here as you type.

Assumptions

  • This page does arithmetic about the size. It does not say whether a size fits or is safe for your vehicle: check the vehicle's tire placard or owner's manual, or ask a tire professional.
  • A tire in use is slightly smaller than the figures here because it carries a load.
  • A size must be a real size: widths from 125 to 405 mm, aspect ratios from 25 to 95 and rim diameters from 10 to 26 inches. Anything else is refused with a reason instead of guessed.

Printed from oneclickanswers.com. Results are estimates based on what was entered; see the assumptions above.

How it works

In 245/40R18, the 245 is the width of the tire in millimetres. The 40 is the aspect ratio: the sidewall is 40% as tall as the tire is wide, so 245 x 0.40 = 98 mm. The R means radial construction, and the 18 is the rim diameter in inches.

The overall diameter is the rim diameter plus two sidewalls: 18 x 25.4 = 457.2 mm, plus 2 x 98 = 196 mm, gives 653.2 mm (25.7 inches). The circumference is pi times the diameter, 2,052 mm, and one mile is 1,609,344 mm, which is 784 turns.

The load index (97) and the speed rating (Y) are codes. This page names them but does not carry the tables that turn them into a weight or a speed, so it does not guess at either.

Assumptions

  • This page does arithmetic about the size. It does not say whether a size fits or is safe for your vehicle: check the vehicle's tire placard or owner's manual, or ask a tire professional.
  • A tire in use is slightly smaller than the figures here because it carries a load.
  • A size must be a real size: widths from 125 to 405 mm, aspect ratios from 25 to 95 and rim diameters from 10 to 26 inches. Anything else is refused with a reason instead of guessed.

Example

Input: 245/40R18 97Y

Result: 245/40R18: 245 mm wide, 98 mm sidewall, 18-inch rim. Overall diameter about 653 mm (25.7 in), circumference 2,052 mm, 784 turns in one mile.

Questions

What do the numbers on a tire mean?

For 245/40R18: 245 is the width in millimetres, 40 is the sidewall height as a percent of the width, R means radial, and 18 is the rim diameter in inches.

How tall is the sidewall?

Multiply the width by the aspect ratio: 245 x 0.40 = 98 mm, about 3.9 inches.

What do 97 and Y mean?

They are the load index and the speed rating, codes you look up in a table. This page does not carry those tables, so use the tire maker's data or a tire professional.

Does this tell me what size fits my car?

No. It reads the size and works out its dimensions. What fits your vehicle is shown on its tire placard and in the owner's manual.

Last reviewed 2026-10-04.

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