Trailer Tire Size Charts: How to Read, Convert and Compare Trailer Tire Sizes

A trailer tire size such as ST205/75R15 tells you four things. ST means Special Trailer service. 205 is the section width in millimetres. 75 is the aspect ratio, meaning the sidewall height is 75 percent of the width. R15 means radial construction on a 15-inch rim. From these numbers you can calculate the sidewall height, overall diameter and circumference. You can then convert the result to inches and compare sizes. Load range, load index and speed symbol are separate ratings. They describe how much weight the tire carries and how fast it may run. They do not describe its dimensions.

Calculated sizes are a useful starting point. Real tires differ slightly, and a tire only fits a trailer if load capacity, wheel and clearance requirements are also met. The sections below explain each marking, show how to calculate dimensions, and give calculated trailer tire size charts for comparison.

How do you read the numbers on a trailer tire sidewall?

Read a trailer tire size marking from left to right. It starts with the service type, then lists width, aspect ratio, construction and rim diameter. The ratings that follow are not dimensions.

The ST metric size format

Most modern ST trailer tire sizes follow this pattern. Using ST205/75R15 as the example:

• ST: the tire is designed for trailer service only. It is not intended for passenger cars or trucks.
• 205: the nominal section width, in millimetres. This is the width from sidewall to sidewall, not the tread width. 205 mm is about 8.07 inches.
• 75: the aspect ratio. This is a percentage, not a unit of length. The sidewall height is 75 percent of 205 mm, or about 154 mm.
• R: radial construction. A D in this position means bias-ply construction, as in ST175/80D13. A B means belted bias.
• 15: the rim diameter, in inches. The tire mounts on a 15-inch wheel.

The units are mixed: the width is in millimetres but the rim diameter is in inches. Every size conversion has to deal with this mix.

Load range, load index and speed symbol

These markings are performance ratings. They show what the tire can carry and how fast it can run, not how big it is.

• Load range is a letter showing carcass strength. B corresponds to a 4-ply rating, C to 6-ply, D to 8-ply and E to 10-ply. The ply rating is a strength equivalent, not a count of actual layers. Two ST205/75R15 tires can have identical dimensions but different load ranges, and so different load capacities.
• Load index is a number that stands for a maximum load per tire. Some tires show two numbers separated by a slash. The first is the rating for single fitment and the second is for dual fitment.
• Speed symbol is a letter showing the maximum rated speed. Common ratings on trailer tires are L (75 mph, 120 km/h), M (81 mph, 130 km/h) and N (87 mph, 140 km/h). Older ST tires without a speed symbol were commonly limited to 65 mph.

Other trailer tire size markings worth knowing

• Maximum load and maximum pressure are printed together on the sidewall. The stated load capacity applies only at the stated inflation pressure.
• DOT code ends in four digits that give the manufacturing week and year. For example, 2324 means the 23rd week of 2024.
• C on caravan tyres, as in 185R14C, marks a commercial or reinforced tyre built for higher loads than a standard passenger tyre of the same size.

What trailer tire size formats exist?

Trailer tires use three main size formats: ST metric, metric commercial (C-type), and older inch-based formats including flotation sizes. Each format expresses dimensions differently, so you need to read each one in its own way.

Numeric inch sizes such as 4.80-8, 4.80-12 and 5.30-12 do not state an aspect ratio. The size marking alone cannot give you an exact overall diameter. For these tires, use the manufacturer's published dimensions.

Caravan tyre sizes such as 185R14C

A caravan tyre marked 185R14C has a 185 mm section width, radial construction, a 14-inch rim and commercial (reinforced) construction. When no aspect ratio appears, the tyre has a tall sidewall. For calculation purposes, sizes like this are roughly comparable to an 80 percent profile.

Assuming an 80 percent ratio, a 185R14C has a sidewall of about 148 mm. Its overall diameter works out to about 652 mm, or 25.7 inches. Treat this as an approximation. The manufacturer's specification sheet gives the actual figure.

Caravan tyre sizes and camper tire sizes in metric C format are common on European-built trailers. ST sizes are more common on North American trailers. Both formats can describe similar dimensions, but their load ratings are expressed differently. Compare load capacities directly rather than assuming that equal dimensions mean equal capacity.

Mobile home tire sizes

Mobile home tire sizes such as 7-14.5 MH and 8-14.5 MH use a 14.5-inch rim diameter. A 14.5-inch tire does not interchange with 14-inch or 15-inch wheels. The first number is the approximate section width in inches.

MH tires are built for transporting manufactured homes and are not general-purpose trailer tires. When comparing them with ST sizes, check both the rim diameter and the intended service.

How do width, aspect ratio and rim diameter affect tire dimensions?

Width and aspect ratio together set the sidewall height. The overall diameter equals the rim diameter plus two sidewall heights. If you increase the width while keeping the same aspect ratio, the sidewall gets taller. If you raise the aspect ratio, the sidewall also gets taller. If you increase the rim diameter, the overall diameter grows by the same amount, assuming the sidewall stays the same.

How to calculate trailer tire diameter from the size

A trailer tire size calculator uses these steps. You can do them by hand for any metric size that states an aspect ratio:

1. Find the sidewall height in millimetres: multiply the section width by the aspect ratio divided by 100. For ST205/75R15: 205 × 0.75 = 153.75 mm.
2. Convert the sidewall height to inches: divide by 25.4. 153.75 ÷ 25.4 = 6.05 in.
3. Add both sidewalls to the rim diameter: 15 + (2 × 6.05) = 27.1 in overall diameter.
4. Find the circumference: multiply the diameter by 3.1416. 27.1 × 3.1416 = about 85.2 in.
5. Find revolutions per mile (optional): divide 63,360 (inches in a mile) by the circumference. 63,360 ÷ 85.2 = about 744 revolutions per mile.

To work in millimetres, convert the rim diameter (15 × 25.4 = 381 mm) and add twice the sidewall height. The result is 381 + 307.5 = 688.5 mm.

Trailer tire size chart in inches

The trailer tire size chart below shows calculated dimensions for frequently seen trailer sizes. The figures come from the size markings alone, using the steps above. Actual tires usually measure slightly differently.

How do you compare two trailer tire sizes?

To compare two trailer tire sizes, calculate the overall diameter of each and find the difference. The ride height at the axle changes by half the diameter difference, because the axle sits at the centre of the wheel.

Practical trailer tire size comparison examples

• ST205/75R14 vs ST215/75R14: the width grows by 10 mm. At the same 75 percent ratio, each sidewall grows by 7.5 mm. The overall diameter rises from about 26.1 in to 26.7 in, a gain of about 0.6 in or roughly 2.3 percent. The axle rises by about 0.3 in.
• ST205/75R14 vs ST205/75R15: width and ratio are identical, so the sidewalls match. The 1-inch larger rim adds exactly 1 in to the calculated diameter and about 3.1 in to the circumference. The two tires need different wheels.
• ST205/75R15 vs ST225/75R15: the diameter grows from about 27.1 in to 28.3 in, a difference of about 1.2 in. The axle centre rises by about 0.6 in, and the tire is about 0.8 in wider. Both changes reduce fender clearance.
• ST175/80R13 vs ST185/80R13: the 10 mm extra width adds about 0.6 in to the overall diameter on the same 13-inch rim.

What a dimensional difference does and does not tell you

A calculated difference shows how much taller, wider and longer-rolling one size is than another. It does not show whether the larger tire clears the fenders, frame or the other tire on a tandem axle. It also does not show whether the tire suits the wheel width or carries the required load.

On tandem and triple axles, tires of noticeably different diameters do not share the load evenly. All tires on a multi-axle trailer are therefore normally kept to the same size.

How do you convert trailer tire sizes between metric and inch?

Convert millimetres to inches by dividing by 25.4. Convert inches to millimetres by multiplying by 25.4. Metric and inch sizing describe the same physical quantities in different units and formats. A converted size gives you comparable dimensions, but it is not automatically an interchangeable tire.

Trailer tire size conversion chart

Rim diameters stay in inches in almost every format, including metric sizes. A metric tire and an inch tire can only share a wheel if they have the same rim diameter. Matching width alone is not enough.

What are the common boat trailer tire sizes?

Boat trailer tire sizes range from small 8-inch tires on light boat trailers to 15-inch ST radials on larger boat and pontoon trailers. The right boat trailer tire size depends on the trailer's original specification and the load it carries. Boat length alone does not determine it.

Small boat trailer tire sizes

Small boat trailers for dinghies, jon boats and personal watercraft often use 8-inch or 12-inch wheels. Sizes such as 4.80-8, 5.70-8, 4.80-12 and 5.30-12 are typical. These smaller diameters keep the trailer low, which helps with launching. They also turn faster at highway speed than larger tires.

Boat trailer tire size: 13-inch and 14-inch

13-inch sizes such as ST175/80R13 and ST185/80R13 are frequently seen on mid-size boat trailers. The calculated overall diameter is about 24.0 to 24.7 inches.

Boat trailer tires in size 14, such as ST205/75R14 and ST215/75R14, are taller at about 26.1 to 26.7 inches. They are used where more load capacity or ground clearance is needed.

Pontoon boat trailer tire sizes

Pontoon trailers carry wide, long loads and commonly use 13-, 14- or 15-inch ST tires. ST205/75R14 and ST205/75R15 are among the sizes seen on them. Because pontoon trailers are built in many configurations, the correct size comes from the trailer's tire placard or the existing tires.

Boat trailer tire diameter and launching

Boat trailer tire diameter affects the height of the hubs and bunks. Moving from a 13-inch to a 14-inch size of similar profile raises the trailer by roughly half the diameter difference. That changes how deep the trailer must go into the water to float the boat. It also changes how the boat sits on its supports.

Utility, camper and motorcycle trailer tire sizes

Utility trailers, campers and motorcycle trailers use the same size formats as boat trailers. The sizes differ with weight and wheel size.

• Utility trailer tire size chart: light utility trailers often use 4.80-12, 5.30-12 or ST175/80R13 tires. Heavier utility and equipment trailers move up to ST205/75R15, ST225/75R15 or 16-inch ST sizes with higher load ranges.
• Camper tire sizes: travel trailers and campers commonly use 14-, 15- and 16-inch ST radials. Caravans built to European specifications often use C-type sizes such as 185R14C.
• Motorcycle trailer tire size: small towable trailers behind motorcycles typically use small-diameter tires such as 8-inch or 12-inch sizes. Lightweight construction and low height matter for these trailers. The trailer maker's specification sets the size.

What is the difference between trailer wheel size and tire size?

Trailer wheel size describes the rim, and tire size describes the tire mounted on it. A wheel marked 15x6 has a 15-inch diameter and a 6-inch rim width. Its bolt pattern, such as 5 on 4.5 inches, is a separate measurement.

A trailer wheel size chart and a trailer tire size chart share only one value: rim diameter. The tire's rim diameter must match the wheel diameter. Each tire is also designed for a range of approved rim widths. Bolt pattern and wheel load rating belong to the wheel, not the tire.

Calculated geometry vs actual dimensions vs fitment

Calculated tire dimensions come from the size marking. Actual dimensions come from measuring a specific tire. Fitment depends on the trailer. These three things are related, but they are not the same.

• Calculated geometry uses nominal numbers. It is ideal for comparing one size with another on equal terms.
• Actual dimensions vary by manufacturer, tread design and the rim width the tire is mounted on. Published specifications list measured overall diameter and section width on a stated measuring rim. When a trailer is loaded, the tire flattens at the bottom, so the loaded radius is smaller than half the overall diameter.
• Fitment compatibility requires more than matching dimensions. The tire must have sufficient load capacity at its rated pressure and the correct rim diameter. Its width must suit the wheel width. It needs clearance from fenders, frame and adjacent tires, and its speed rating must suit how the trailer is used.

Use the size markings and calculations to understand what a tire is and how it compares with another. Then check the trailer's tire placard, the tire manufacturer's published dimensions and its load ratings before choosing a different size.