What do the numbers on a tire mean?

In 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 load index and the speed symbol.

How do I find a tire's overall diameter from the size?

Sidewall height is width times aspect ratio over 100. Overall diameter is the rim diameter in millimetres plus two sidewalls. Every size page here shows the calculation with the numbers filled in.

Where do the load index and speed rating figures come from?

Published tables, cross-checked against at least two independent publishers per row, with the sources listed on every page and the one cell they disagreed on printed rather than silently resolved. 26 sources in total.

Read a tire sidewall

Type the size and get every component named and every dimension worked out, with the formula printed beside the answer.

Decode a size

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.

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

What this reference holds

Size pages: 879
Load index rows: 151
Speed symbols: 30
Sources cited: 26

The load index, the speed symbols, the date code, the UTQG grades and the rest of the markings, each cross-checked against at least two published sources.

Sizes people look up most

All 879 sizes, across 15 rim diameters.

The rest of the sidewall

Load index chart - every published index in kilograms and pounds, and what each one means for a set of four.
Speed symbol chart - in speed order, because the table is not alphabetical, plus the ZR, W, Y and (Y) cases people get wrong.
Service type prefixes - P, LT, ST, T, C, and what an absent prefix actually means.
DOT date code - how old the tire is, and why a three digit code cannot tell you the decade.
UTQG grades - treadwear, traction and temperature, with the regulation's own thresholds.
M+S and the snowflake - one is awarded on tread shape, the other has to be earned in a test.

Changing size

Put two sizes side by side and see the diameter difference, the change in axle height and the speedometer and odometer error the swap implies - with the assumption behind the figure stated, because a speedometer error is a change from whatever the instrument did before.

Compare two sizes Build a fitting worksheet

Why the sources are on every page

A load index is not something you can derive - it is a code whose meaning is set in a published table, and a copy of that table is only worth as much as its provenance. Every published row here was read from at least two independent sources, every cell they disagreed on is printed rather than resolved in silence, and every figure this site works out itself carries its formula.

See every source

Common questions

What do the numbers on a tire mean?: In 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 load index and the speed symbol.
How do I find a tire's overall diameter from the size?: Sidewall height is width times aspect ratio over 100. Overall diameter is the rim diameter in millimetres plus two sidewalls. Every size page here shows the calculation with the numbers filled in.
Is this a fitment guide?: This is geometry, not fitment approval. It says what a size measures, never whether it fits a vehicle.
Where do the load index and speed rating figures come from?: Published tables, cross-checked against at least two independent publishers per row, with the sources listed on every page and the one cell they disagreed on printed rather than silently resolved. 26 sources in total.