Flotation Tire Size Decoder
Read a code like 33x12.50R15. Each part is explained. Use the calculator below. Results update as you type.
How the Flotation Tire Size Decoder Works
This decoder works by reading the 3 inch values in a flotation marking: overall diameter, section width and rim diameter. A marking such as 33x12.50R15 states the tire height first, not the width.
- Enter the flotation size, for example 35x12.50R17.
- Read the overall diameter: 35 in (889 mm).
- Read the section width: 12.5 in (318 mm) and the rim: 17 in.
- Read the computed sidewall height: (35 − 17) ÷ 2 = 9.0 in (229 mm).
The decoder adds the implied aspect ratio (72% in this case) so the size can be compared with metric tires.
The Formula
The formula is Sidewall Height = (Overall Diameter − Rim Diameter) ÷ 2, with all values in inches.
Sidewall (in) = (Overall Diameter − Rim) ÷ 2 Implied Aspect Ratio (%) = Sidewall ÷ Section Width × 100 Metric Width (mm) = Section Width (in) × 25.4Worked example for 31x10.50R15: sidewall is (31 − 15) ÷ 2 = 8.0 in (203 mm), the implied aspect ratio is 8.0 ÷ 10.5 × 100 = 76%, and the width is 267 mm.
Conversion Table
The conversion table decodes 8 flotation sizes from 31x10.50R15 to 40x13.50R17. Each row lists diameter, width, rim, sidewall and implied aspect ratio in both inches and millimeters.
The implied aspect ratio column lets you compare a flotation size with a metric size. A 37x12.50R17 has a 10.0 in sidewall and an implied ratio of 80%, which is taller than any common metric 17 in light truck tire.
| Marking | Overall Diameter | Section Width | Rim | Sidewall Height | Implied Aspect Ratio |
|---|---|---|---|---|---|
| 31x10.50R15 | 31 in (787 mm) | 10.5 in (267 mm) | 15 in | 8.00 in (203 mm) | 76% |
| 33x12.50R15 | 33 in (838 mm) | 12.5 in (318 mm) | 15 in | 9.00 in (229 mm) | 72% |
| 33x10.50R17 | 33 in (838 mm) | 10.5 in (267 mm) | 17 in | 8.00 in (203 mm) | 76% |
| 35x12.50R17 | 35 in (889 mm) | 12.5 in (318 mm) | 17 in | 9.00 in (229 mm) | 72% |
| 35x12.50R20 | 35 in (889 mm) | 12.5 in (318 mm) | 20 in | 7.50 in (190 mm) | 60% |
| 37x12.50R17 | 37 in (940 mm) | 12.5 in (318 mm) | 17 in | 10.00 in (254 mm) | 80% |
| 37x13.50R20 | 37 in (940 mm) | 13.5 in (343 mm) | 20 in | 8.50 in (216 mm) | 63% |
| 40x13.50R17 | 40 in (1016 mm) | 13.5 in (343 mm) | 17 in | 11.50 in (292 mm) | 85% |
What Is the Flotation Tire Size Decoder?
The Flotation Tire Size Decoder is a tool that explains inch-based light truck and off-road tire markings. Flotation sizes describe tires made for off-road vehicles, lifted pickups and Jeeps. The name refers to wide tires that "float" over sand and mud.
A flotation marking lists the tire's own height first, then the width, then the rim. That order makes flotation sizes easy to shop by height: a "35 inch tire" is a 35x12.50 on some rim. Most flotation tires carry an LT (light truck) suffix and a load range letter.
Common Uses
This decoder has 4 uses: lift-kit shopping, metric comparison, sidewall checks and wheel selection.
- Shop for lift kits: Match tire height to the lift height.
- Compare with metric sizes: Use the implied aspect ratio to find close metric options.
- Check sidewall height: See how much cushion a tire has for airing down.
- Select wheels: Pair a flotation width with a suitable rim width.