Tire size conversion chart: flotation to metric

33x12.50R15 and other flotation sizes next to the LT metric sizes closest in diameter, with every metric diameter worked out so you can check it.

Flotation to metric, in one line

A flotation size like 33x12.50R15 is read as overall diameter, section width and rim diameter, all in inches. The nearest metric size on the same 15-inch rim in the chart below is LT315/70R15, which works out to 32.4 inches tall, so it sits close in diameter to a 33 without being the same tire.

That second half is the part that matters at the counter. A customer with a lifted pickup asks for "a thirty-three in metric," and what he wants is a tire that fills the wheel well the same way, clears the same fender and keeps the speedometer near where it was. Diameter gets you most of the way. Width, load range and the vehicle maker's own size list get you the rest.

Tire size conversion chart

Every flotation number below comes straight off the size itself. Every metric diameter and width is computed with the method in the next section and rounded to one decimal inch, so any row can be checked with a phone calculator. Each pair shares a rim size, because a close diameter on a different wheel is a different job.

The gap column is the metric diameter minus the nominal flotation diameter. Read it before you read the match. LT285/70R15 comes out at 30.7 inches against a nominal 31, and LT285/60R20 lands half an inch taller than a 33. Both get called "the same size" in parking lots. Neither is.

The arithmetic, shown once

This is the same method the guide to reading a size code uses for P225/65R17. Michelin describes the first number of a metric size as the section width in millimeters and the second as the aspect ratio, the sidewall height as a percentage of that width. The rim number is already in inches.

1. Sidewall height in millimeters: width times aspect ratio, divided by 100.
2. Both sidewalls: multiply by 2, since the tire stands above and below the wheel.
3. Convert to inches: divide by 25.4.
4. Add the rim diameter.

Worked through for LT315/70R15: 315 × 0.70 = 220.5 mm. Doubled, that is 441 mm. Divided by 25.4 it becomes 17.36 inches, and adding the 15-inch rim gives 32.36, which rounds to 32.4. Width is simpler. Tire Rack converts a metric width to inches by dividing the millimeters by 25.4, so 315 mm is 12.4 inches.

Run it on the original P225/65R17 and you get 28.5 inches, the same answer the size guide reaches in millimeters. Same method, same result.

The method also handles a plain tire size comparison between two metric sizes, which is most of what people type into a size calculator. Take a truck on 265/70R17 whose owner wants 285/70R17. The first is 265 × 0.70 = 185.5 mm of sidewall, doubled to 371 mm, or 14.6 inches, plus 17 for 31.6 inches overall. The second is the 32.7 inches already worked out in the chart. That is 1.1 inches taller, or a little over half an inch more on each side of the hub. Small on paper. Not small next to a lower control arm.

Close in diameter is not the same tire

Three things keep a metric match from being a straight swap.

The flotation number is nominal. Michelin's FAQ reads 31x10.50R15LT as "31 = Tire Overall Diameter in Inches," yet Tire Rack's light-truck chart puts a 33x12.50R15LT at about 32-1/2 inches typical and notes that tread design and molded tread depth shift the real figure. So a computed 32.4 for LT315/70R15 and an actual 33x12.50R15 can end up nearer each other than the nominal numbers suggest. Or further apart. The spec sheet for the exact tire settles it.

Width moves too. LT285/70R17 is 11.2 inches wide on paper, more than an inch narrower than a 12.50. On a truck running spacers, or one with a tight fender liner, that inch decides whether it rubs at full lock. The wheel matters too. Going between a 12.50 flotation width and a 285 to 325 mm metric width, check the rim-width range the tire maker lists for that size; Michelin calls that range "extremely important."

Load is the third, and the one that gets people hurt. Michelin warns that fitting P-metric tires where the factory tires were LT-rated "will leave you short on load capacity." Every metric size in the chart carries the LT prefix for that reason. Match the load range, not only the height: the LT load range chart covers the letters, and the load index chart covers the numbers.

What the placard and NHTSA say about changing size

NHTSA's guidance is plain. Buy new tires that are the same size as the vehicle's original tires or another size recommended by the vehicle manufacturer, and look for it in the owner's manual or on the Tire and Loading Information label on the driver's door edge or post. Federal standard 571.110 requires that placard to list the size designation of the tires installed at first sale, plus the spare, along with the maker's recommended cold inflation pressure for front, rear and spare.

So the placard tells you what the truck left the factory on. It does not tell you a 35x12.50R17 will clear. When a customer wants a size that is on neither the placard nor the manual's list, you have left the rule behind and moved into judgment, and the chart above is where that conversation starts rather than where it ends.

Pressure changes with size as well. Michelin's guidance is that a replacement in a different size "should be inflated to provide the same load capability of the original tire size at the manufacturer's recommended pressure." The placard number was written for the original size, not for whatever you just mounted. The door placard and TPMS guide walks through that part.

Writing the size down

Write the full size on the ticket, both formats if the customer compared them. "33x12.50R15LT, close in diameter to LT315/70R15" is a note a coworker can act on next week. "33s" is not. In DuroBay each tire has notes, so the metric size a customer cross-shopped can sit right next to the flotation size you actually sold.

A used flotation tire still gets graded like any other. Diameter tells you what it replaces, not what it is worth, so run it through the used tire grading checklist before it gets a price.