What do the numbers on a tire mean?

In a size like 225/65R17, 225 is the section width in millimetres, 65 is the aspect ratio (the sidewall is 65 percent of the width), R is radial construction and 17 is the rim diameter in inches. A trailing 102H is the service description: load index 102 and speed symbol H.

How do I work out a tire's overall diameter from its size?

Sidewall height is width times aspect ratio over 100. Overall diameter is the rim diameter converted to millimetres plus two sidewalls. For 225/65R17: 225 x 65 / 100 = 146.25 mm sidewall, then 17 x 25.4 = 431.8 mm rim, so 431.8 + 292.5 = 724.3 mm, or 28.52 inches.

What is revolutions per mile and how is it calculated?

There are 63360 inches in a mile, so revolutions per mile is 63360 divided by the tire's circumference in inches. Circumference is pi times the overall diameter. A loaded tire deflects and turns slightly more often than this geometric figure, so a manufacturer's published value usually runs a few percent higher.

Why will this decoder not read my tire size?

It reads metric sizes (225/65R17), sizes with no printed aspect ratio (195R14C) and flotation sizes (33x12.50R15). Alphanumeric sizes such as FR78-14 and the old numeric sizes such as 7.50-16 are not covered. Reporting a size as unreadable is deliberate - approximating one of those formats would produce a number that looks right and is not.

Does the first number of 33x12.50R15 mean the width?

No. In a flotation size the first number is the tire's overall diameter in inches. 33x12.50R15 is a 33 inch tall tire, 12.5 inches wide, on a 15 inch rim. Reading the 33 as a width is the commonest misreading of a tire size there is.

Tire size decoder

Type the size off the sidewall. Every character is named, every dimension is worked out, and the arithmetic is printed next to the answer.

Tire size, as printed on the sidewall

Metric (225/65R17), no aspect ratio (195R14C) and flotation (33x12.50R15) sizes. Include the load index and speed symbol if you want those translated too.

How the arithmetic works

Sidewall height (one side): section width (mm) x aspect ratio / 100
Overall diameter: (rim diameter in inches x 25.4) + 2 x sidewall height
Circumference: pi x overall diameter
Revolutions per mile: 63360 / circumference in inches
Revolutions per kilometre: 1000000 / circumference in millimetres

Every dimension on a size page is computed from the size code, not copied from a catalog. The formulas are printed with the figures so any of them can be checked by hand.

Common questions

What do the numbers on a tire mean?: In a size like 225/65R17, 225 is the section width in millimetres, 65 is the aspect ratio (the sidewall is 65 percent of the width), R is radial construction and 17 is the rim diameter in inches. A trailing 102H is the service description: load index 102 and speed symbol H.
How do I work out a tire's overall diameter from its size?: Sidewall height is width times aspect ratio over 100. Overall diameter is the rim diameter converted to millimetres plus two sidewalls. For 225/65R17: 225 x 65 / 100 = 146.25 mm sidewall, then 17 x 25.4 = 431.8 mm rim, so 431.8 + 292.5 = 724.3 mm, or 28.52 inches.
What is revolutions per mile and how is it calculated?: There are 63360 inches in a mile, so revolutions per mile is 63360 divided by the tire's circumference in inches. Circumference is pi times the overall diameter. A loaded tire deflects and turns slightly more often than this geometric figure, so a manufacturer's published value usually runs a few percent higher.
Why will this decoder not read my tire size?: It reads metric sizes (225/65R17), sizes with no printed aspect ratio (195R14C) and flotation sizes (33x12.50R15). Alphanumeric sizes such as FR78-14 and the old numeric sizes such as 7.50-16 are not covered. Reporting a size as unreadable is deliberate - approximating one of those formats would produce a number that looks right and is not.
Does the first number of 33x12.50R15 mean the width?: No. In a flotation size the first number is the tire's overall diameter in inches. 33x12.50R15 is a 33 inch tall tire, 12.5 inches wide, on a 15 inch rim. Reading the 33 as a width is the commonest misreading of a tire size there is.

Or start from a size page

879 sizes have their own page, with the alternatives nearest to them in rolling diameter already worked out.

Browse every size Compare two sizes

Every number on this page, and where it comes from

Computed on this page, not looked up. Each figure and the arithmetic behind it:

Sidewall height section width (mm) x aspect ratio / 100
Overall diameter (rim diameter in x 25.4) + 2 x sidewall height
Circumference pi x overall diameter
Revolutions per mile 63360 / circumference in inches
Revolutions per kilometre 1000000 / circumference in mm

Before you fit anything

This is geometry, not fitment approval. It says what a size measures, never whether it fits a vehicle.

SidewallRef calculates the dimensions a size code implies and reproduces the published load index and speed symbol tables. It does not know your vehicle.

Load index and speed rating must meet or exceed what the vehicle placard specifies. The placard is in the driver's door jamb on most vehicles and is the authority for your vehicle, not this site.

Clearance at full lock and full suspension travel, wheel width and offset, brake and suspension component clearance, axle load capacity, inflation pressure and any legal requirement where you drive are the owner's and the fitter's responsibility.

Nothing here approves a fitment. Confirm any change with a qualified tire fitter and with the vehicle manufacturer's own specification before driving on it.