Flotation sizes are the imperial, inch-based tyre sizes seen on off-road and modified 4x4s, such as 31x10.50 R15. They look unlike a normal metric size because they are read entirely in inches, and they state the tyre's overall height directly.
How to read it
A flotation size has three parts, all in inches:
| Part | Example | Meaning |
|---|---|---|
| First number | 31 | Overall tyre diameter (height) |
| Second number | 10.50 | Section width |
| Third number | 15 | Wheel (rim) diameter |
So 31x10.50 R15 describes a tyre roughly 31 inches tall and 10.5 inches wide, fitted to a 15-inch wheel. The R means radial construction, the same as in a metric size. Many flotation tyres also carry an LT marking and a load range letter, reflecting their light-truck, load-carrying build.
What "flotation" means
The name describes what the tyre is for. A flotation tyre has a wide footprint that spreads the vehicle's weight over a larger area, helping it float over soft ground, sand, mud, loose soil, instead of sinking in and digging down. That makes these sizes popular on off-road 4x4s and pickups built to leave the tarmac. They typically come in an aggressive all-terrain or mud-terrain tread to match that off-road purpose.
Flotation versus metric
The big practical difference from a standard metric size is how height is expressed. A metric size gives width in millimetres and height as a percentage of that width, so the overall diameter has to be calculated. A flotation size states the overall diameter directly as its first number, which makes it easy to compare tyre heights when judging ground clearance or how a larger tyre will fit.
Fitting them
A flotation tyre still has to suit the vehicle: it needs a load and speed rating appropriate for the weight and use, and it must physically fit without fouling the bodywork or suspension on full lock and over bumps. Because flotation tyres are often part of a larger off-road set-up, fitment is best checked with a fitter who can confirm the size and rating are right for the vehicle; once the spec is settled, the right size can be ordered from a tyre retailer like Tyres.co.uk.
From the workshop: the handy thing about a flotation size is that the first number is the height in inches, so a customer can see at a glance that a 33 is taller than a 31. With metric off-road sizes we have to work the height out, which is why a lot of the off-road crowd stick with the inch sizes.
Sources and accuracy. The way a flotation size is read, in inches as diameter, width and rim, reflects the standard high-flotation sizing system at the time of writing. The right size and rating for a specific vehicle should be confirmed against its placard and with a fitter. If anything here looks wrong, get in touch and we will check it and put it right.
Common questions
How do you read a flotation tyre size like 31x10.50 R15?+
In inches, in three parts. 31 is the overall tyre diameter, 10.50 is the section width, and 15 is the wheel diameter. So 31x10.50 R15 is a 31-inch tall, 10.5-inch wide radial tyre for a 15-inch wheel.
What is a flotation tyre?+
A flotation tyre has a wide footprint designed to spread the vehicle's weight and float over soft ground such as sand and mud, rather than digging in. The sizes are given in inches and are common on off-road and modified 4x4s.
How is a flotation size different from a metric size?+
A metric size like 235/85 R16 gives width in millimetres and height as a percentage. A flotation size like 31x10.50 R15 gives everything in inches and states the overall diameter directly, which makes the tyre's height easy to read for clearance.
Are flotation tyres road legal in the UK?+
Road-legal flotation tyres carry the required markings and a load and speed rating suitable for the vehicle. As with any size change, the tyre must suit the vehicle and not foul the bodywork, so fitment should be checked with a specialist.