Metric to Imperial Tire Size Converter

Two notations, one tire

Metric, 285/70R17: section width in millimetres, then the sidewall as a percentage of that width, then the wheel in inches. The overall height is never printed and has to be worked out.

Flotation, or imperial, 33x12.50R17: overall diameter in inches, section width in inches, wheel in inches. The height is printed and the aspect ratio is the thing you have to derive.

Both describe the same object. Neither is more accurate; they are conventions from different markets, and light trucks in North America are sold in both, often on the same shelf.

Why the second half of the answer is the useful half

Converting 285/70R17 gives 32.71 in tall and 11.22 in wide — so the exact imperial code would be 32.71x11.22R17. Nobody makes that. The conversion is arithmetic, and arithmetic does not care whether the result is purchasable.

So the tool does the conversion and then names the sizes that genuinely exist on the same wheel diameter, with the difference of each. For 285/70R17 the list opens with 33x10.50R17, then 33x12.50R17, then 32x11.50R17.

The order is worth understanding, because the first two are a tie and the tie is broken on a dimension most cross-reference charts ignore. Both 33s stand 33.00 in, so both are 0.29 in taller than the 32.71 in the metric code computes to; nothing separates them on height. What separates them is width. The 285 is 11.22 in across, the narrow 33 is 10.50 in — 0.72 in away — and the wide 33 is 12.50 in, which is 1.28 in away. Height first, then width: the narrow 33 wins.

That is also why "is a 285 a 33?" keeps being asked and never quite resolves. On height it very nearly is. On width the answer depends entirely on which 33 the question means, and the two candidates sit on opposite sides of the metric size.

Where the notations disagree the most

Aspect ratio is where the two conventions diverge in a way that surprises people. 33x12.50R17 works out to a 64 aspect ratio, 35x12.50R17 to 72, and 37x12.50R17 to 80 — the same nominal width and wheel, three completely different sidewall proportions. In metric notation those would be obvious at a glance. In imperial notation they are invisible, because the notation prints the answer to a different question.

That is worth holding on to: a metric code tells you the shape and hides the height, and an imperial code tells you the height and hides the shape.

The rounding you should expect

Flotation sizes round hard. A tire marked 35 is often nearer 34.5 in measured, because the number on the sidewall is a class rather than a specification. Metric sizes round too, but less, because the code is a specification of the casing rather than a description of the finished tire.

So the arithmetic on this page is exact about the notation and approximate about the object the notation describes. Treat a converted figure as the specification it is, and a tape measure as the thing that settles arguments.