Most passenger tires use, and most car owners are familiar with, metric sizing: a three-part code like 245/55R19 that gives you section width in millimeters, aspect ratio as a percentage, and wheel diameter in inches. If you have read How to Read a Tire Sidewall, you know how to decode that system. The problem is that not all tires use metric sizing. Light truck and off-road tires frequently use an inch-based flotation sizing system. Trailer tires use the metric format but with a prefix that changes how load ratings are applied. This guide covers how to read and convert between these sizing systems, with reference tables for the most common conversions.
Looking for tire size compatibility tables? If you want to find passenger tire sizes that are interchangeable with your current fitment — alternatives within the ±3% diameter tolerance — see the Tire Size Compatibility Tables. This article covers conversion between metric, flotation, and trailer sizing systems, not replacement compatibility.
LT Flotation Sizing
LT flotation sizes appear on larger light truck, off-road, and all-terrain tires. When a Jeep or lifted truck owner says they want to run ”35s,” this is the sizing system they are referencing. Instead of the metric format, they use a three-number inch-based format:
[Overall Diameter]×[Section Width]R[Wheel Diameter]
A 35×12.50R17 tire is 35 inches tall, 12.50 inches wide, and mounts on a 17-inch wheel. The R still means radial construction as with metric sizing. This format is used because it communicates the dimensions that matter for truck and off-road fitment more directly than the metric format. When someone says they are running ”35s,” they mean the overall diameter is approximately 35 inches. In the metric format, you have to make calculations to know what you have.
Reading LT Flotation Sizes
The three numbers in an LT flotation size are:
- Overall diameter (inches): The full height of the mounted and inflated tire. This is the dimension most commonly cited when describing off-road fitment.
- Section width (inches): The widest point of the tire’s cross section.
- Wheel diameter (inches): The rim size the tire is designed to mount on.
LT flotation tires also carry a load range designation (C, D, E) that indicates their maximum inflation pressure and load capacity, which is more important for truck and trailer applications than the standard load / extra load distinction on passenger tires.
Converting Between LT Flotation and Metric
The conversion formulas are:
Metric to LT flotation:
- Overall diameter (in) =
((section_width_mm × aspect_ratio / 100) × 2 / 25.4) + wheel_diameter_in - Section width (in) =
section_width_mm / 25.4
LT flotation to metric:
- Section width (mm) =
section_width_in × 25.4 - Sidewall height (mm) =
((overall_diameter_in − wheel_diameter_in) / 2) × 25.4 - Aspect ratio (%) =
sidewall_height_mm / section_width_mm × 100
The result from LT-to-metric will almost never be a clean standard aspect ratio (35, 40, 45, etc.) because LT flotation sizes are specified to the nearest 0.25” and metric sizes use specific aspect ratio increments. The closest available metric tire is rarely a perfect match. You are looking for the metric size with the closest overall diameter and section width. Use the Flotation ↔ Metric Calculator to spare yourself the calculations. A quick-entry version is included below.
LT Flotation to Metric Conversion Table
The table below shows common LT flotation sizes and their closest metric equivalents. “Closest metric” means the metric size with the smallest combined difference in overall diameter and section width. In many cases there are two or three reasonable metric options. Actual fitment depends on specific manufacturer dimensions. Always verify with a tire size comparison calculator before purchasing.
| LT Flotation | OD (in) | Width (in) | Closest Metric | Metric OD (in) | Difference |
|---|---|---|---|---|---|
| 28×8.50R14 | 28.0” | 8.5” | 215/75R14 | 27.7” | −0.3” |
| 29×9.50R15 | 29.0” | 9.5” | 235/75R15 | 29.0” | 0.0” |
| 30×9.50R15 | 30.0” | 9.5” | 235/80R15 | 29.8” | −0.2” |
| 31×10.50R15 | 31.0” | 10.5” | 265/75R15 | 30.6” | −0.4” |
| 31×10.50R15 | 31.0” | 10.5” | 275/70R15 | 30.7” | −0.3” |
| 33×10.50R15 | 33.0” | 10.5” | 265/75R16 | 31.6” | −1.4” |
| 33×10.50R17 | 33.0” | 10.5” | 265/70R17 | 31.6” | −1.4” |
| 33×12.50R15 | 33.0” | 12.5” | 325/60R15 | 32.0” | −1.0” |
| 33×12.50R17 | 33.0” | 12.5” | 315/65R17 | 33.1” | +0.1” |
| 33×12.50R18 | 33.0” | 12.5” | 315/65R18 | 34.1” | +1.1” |
| 35×12.50R15 | 35.0” | 12.5” | 325/65R15 | 33.5” | −1.5” |
| 35×12.50R17 | 35.0” | 12.5” | 325/65R17 | 33.5” | −1.5” |
| 35×12.50R18 | 35.0” | 12.5” | 325/65R18 | 34.1” | −0.9” |
| 35×12.50R20 | 35.0” | 12.5” | 325/60R20 | 34.0” | −1.0” |
| 37×12.50R17 | 37.0” | 12.5” | 335/80R17 | 36.7” | −0.3” |
| 37×12.50R20 | 37.0” | 12.5” | 325/65R20 | 35.7” | −1.3” |
| 37×13.50R17 | 37.0” | 13.5” | 345/75R17 | 36.4” | −0.6” |
| 38×15.50R15 | 38.0” | 15.5” | 395/85R15 | 37.2” | −0.8” |
| 40×13.50R17 | 40.0” | 13.5” | 345/80R17 | 37.6” | −2.4” |
A note on this table: LT flotation overall diameters are nominal — a tire labeled 35” rarely measures exactly 35.0” when mounted and inflated. The differences between LT flotation and metric equivalents shown here reflect the label-to-label comparison, not the actual measured tire dimensions. For real applications where clearance and speedometer accuracy matter, measure the specific tires or consult manufacturer specifications. The table skips diameters like 34” and 36” intentionally. Those sizes exist but are primarily found in agricultural and specialty equipment, not in automotive light truck fitments where the standard increments are 31”, 33”, 35”, 37”, and 40”.
Why Perfect Metric Equivalents Often Do Not Exist
The LT flotation system specifies tires in 0.25” diameter increments. Standard metric aspect ratios are 50, 55, 60, 65, 70, 75, 80. Those two grids do not align cleanly. A 35×12.50 requires a 35” diameter from a 12.50” (317.5mm) section width, which means a sidewall height of (35" − 17") / 2 × 25.4 = 228.6mm for a 17” rim, and an aspect ratio of 228.6 / 317.5 = 72%.
The closest available metric sizes are a compromise in either diameter or width. When converting from flotation to metric for a direct replacement, prioritize matching overall diameter first (within ±3%) and section width second. A slightly wider tire on the same rim width typically fits within T&RA width guidelines.
P-Metric and Euro-Metric
Both P-metric and Euro-metric tires use the same 245/55R19-style format. The difference is in load capacity: for the same size and load index, a Euro-metric tire has a slightly higher load-carrying capacity than a P-metric tire because Euro-metric load ratings are calculated at a slightly higher maximum inflation pressure. How to Read a Tire Sidewall covers the exact load pressure differences by load range. For most replacement decisions on passenger cars, P-metric and Euro-metric tires in the same size and load index are interchangeable. Your vehicle’s door jamb specifies the recommended inflation pressure — use that figure regardless of which metric variant is installed.
Trailer Tire (ST) Sizing
Trailer tires use the same metric size format as passenger tires but are prefixed with ST (Special Trailer): ST235/80R16. Trailer tires are constructed differently from passenger tires, with stiffer sidewalls to handle trailer sway loads and no optimization for the steering or cornering forces that a driven or steered wheel experiences.
Never use ST tires on vehicles (cars, trucks, vans). Also do not use passenger tires on trailers. The construction and load ratings are not equivalent. The ST designation exists precisely because a passenger tire in a trailer application will fail in ways it would not fail under a vehicle.
ST tire sizes cover the same range as passenger and LT sizes for common trailer wheel diameters (13”, 14”, 15”, 16”). If you need to find an equivalent ST size for a given wheel diameter and load requirement, match the wheel diameter and verify that the load index meets or exceeds the per-tire requirement, calculated from the trailer’s GVWR divided by the number of tires. The specific section width and aspect ratio are secondary to the load rating in trailer tire selection.
Now that the sizing systems make sense, the practical work of choosing a replacement or modified size usually comes down to diameter math. The Tire Size Comparison Calculator handles that for any two metric sizes side by side. If you are changing wheel diameter as part of a tire swap, the Plus Sizing Guide covers how to maintain overall diameter across wheel sizes.