How to Convert Inch and Flotation Tire Sizes to Metric

A flotation tire size such as 33x12.50R17 gives three measurements in inches: overall diameter (33 in), section width (12.50 in) and rim diameter (17 in). A metric size such as 315/70R17 gives section width in millimeters (315 mm), aspect ratio as a percentage (70%) and rim diameter in inches (17 in). To convert a flotation size to metric, follow three steps:

1. Multiply the width by 25.4 to get millimeters.
2. Subtract the rim diameter from the overall diameter and halve the result to get sidewall height.
3. Divide sidewall height by width to get the aspect ratio.

The result is a calculated, nominal equivalent. It is not a guarantee of identical real-world dimensions or of fitment on a particular vehicle.

How to Read the Numbers on the Tire Sidewall

The tire size code on the sidewall describes the tire's nominal dimensions. Separate codes printed next to it describe performance ratings. Flotation tire sizes and metric tire sizes use different notation for the same basic geometry.

Flotation size notation

Flotation tire sizing is common on light-truck and off-road tires. Take 33x12.50R17LT 120Q as an example:

• 33 is the nominal overall diameter in inches. This is the height of the mounted, inflated tire from the ground to the top of the tread.
• 12.50 is the nominal section width in inches. Section width is the widest point of the tire between the sidewalls, not the tread width.
• R means radial construction.
• 17 is the rim diameter in inches, the wheel size the tire fits.
• LT means the tire is built to light-truck standards.
• 120Q is the service description: a load index and a speed rating. These are performance ratings, not dimensions.

A flotation size does not state an aspect ratio. Sidewall height can be calculated from it, because overall diameter and rim diameter are both given.

Metric tire size notation

Metric tire sizing mixes units. Take LT315/70R17 as an example:

• LT (or P on some passenger tires, or no prefix on many European-metric sizes) indicates the tire's service type.
• 315 is the nominal section width in millimeters.
• 70 is the aspect ratio. The sidewall height is 70% of the section width.
• R means radial construction.
• 17 is the rim diameter in inches.

A metric size does not state overall diameter directly. You calculate it from the width, aspect ratio and rim diameter.

Older numeric inch sizes

Some older or specialty tires use a numeric inch size such as 7.50R16. This gives a nominal section width in inches (7.50 in) and a rim diameter (16 in), but no aspect ratio or overall diameter. Converting a numeric size like this to metric requires the overall diameter from the manufacturer's data. The width alone is not enough.

Dimensions versus performance ratings

Performance ratings appear beside the size but do not change the geometry:

• Load index is a number that corresponds to a maximum load per tire. For example, load index 120 corresponds to 1,400 kg (3,086 lb). Light-truck tires may show two numbers, such as 120/117, for single and dual-wheel use.
• Speed rating is a letter for the maximum rated speed. For example, Q is 160 km/h (99 mph), S is 180 km/h (112 mph) and T is 190 km/h (118 mph).
• Load range (C, D, E and so on) appears on LT tires and indicates load-carrying construction and inflation capability.

Two tires can share the same size and still have different load and speed ratings. A size conversion matches dimensions only. You must check the ratings separately.

How to Convert Inch Tire Sizes to Metric

Use this method to convert a flotation tire size, written as diameter x width R rim, to a metric size. It works for any flotation size.

1. Convert the width to millimeters. Multiply the section width in inches by 25.4.
2. Find the sidewall height. Subtract the rim diameter from the overall diameter, then divide by 2. This works because the tire has one sidewall above the rim and one below.
3. Calculate the aspect ratio. Divide the sidewall height by the section width, using the same units for both, then multiply by 100.
4. Round to standard metric notation. Metric section widths usually end in 5 (265, 275, 285, 295, 305, 315 mm). Aspect ratios are usually written in steps of 5 (60, 65, 70, 75).
5. Check the result. Calculate the overall diameter of the rounded metric size and compare it with the original flotation diameter.

Worked example: 35x12.50R17 to metric

• Width: 12.50 in × 25.4 = 317.5 mm, which rounds to 315 mm.
• Sidewall height: (35 − 17) ÷ 2 = 9.0 in, or 228.6 mm.
• Aspect ratio: 228.6 ÷ 317.5 = 0.72, or 72%. The nearest standard steps are 70 and 75.
• Candidate: 315/70R17. Check: 315 × 0.70 = 220.5 mm = 8.68 in. Then 8.68 × 2 + 17 = 34.36 in overall diameter.
• Alternative: 315/75R17 calculates to 35.60 in.

The flotation size 35x12.50R17 therefore falls between 315/70R17 and 315/75R17. The 70-series size is about 0.6 in shorter on paper, and the 75-series size is about 0.6 in taller. Rounding is why inch-to-metric conversion rarely produces an exact match.

How to Convert Metric Tire Sizes to Flotation Sizes

To convert a metric size to flotation notation, calculate the overall diameter and convert the width to inches.

1. Calculate the sidewall height in millimeters. Multiply the section width by the aspect ratio divided by 100.
2. Convert the sidewall height to inches. Divide by 25.4.
3. Calculate the overall diameter. Multiply the sidewall height in inches by 2 and add the rim diameter.
4. Convert the width to inches. Divide the section width in millimeters by 25.4.
5. Write the result in flotation form: diameter x width R rim.

Worked example: 285/70R17 to flotation size

• Sidewall height: 285 × 0.70 = 199.5 mm.
• In inches: 199.5 ÷ 25.4 = 7.85 in.
• Overall diameter: 7.85 × 2 + 17 = 32.71 in.
• Width: 285 ÷ 25.4 = 11.22 in.

The calculated flotation-style equivalent of 285/70R17 is about 32.7x11.2R17. This is why 285/70R17 is often grouped with 33-inch tires, even though its nominal diameter is slightly under 33 in. That exact flotation size may not be produced. The calculation describes the shape, not a product listing.

Flotation to Metric Tire Conversion Chart

The flotation tire size chart below converts common flotation sizes using the method above. The "closest metric by calculation" column shows standard metric notation nearest to the computed aspect ratio. A size listed here is not necessarily manufactured or available.

A pattern shows up in the chart. For the same flotation diameter and width, a larger rim leaves less room for sidewall, so the computed aspect ratio drops. A 33x12.50 tire works out to 72% on a 15-inch rim, 64% on a 17-inch rim and 52% on a 20-inch rim.

Metric to Flotation Tire Size Chart for 33, 34 and 35 Inch Tires

Common metric sizes can be grouped by calculated overall diameter. In everyday use, a "33-inch tire" is a tire whose overall diameter is close to 33 in. Several metric tire sizes on different rim diameters fall into each class. All diameters below are calculated from the nominal size, and circumference is π × diameter.

What is a 33 inch tire size in metric?

Metric sizes that calculate to roughly 32.7–33.2 in overall diameter are commonly treated as 33-inch tires.

What are 34 inch tires in metric?

Metric sizes that calculate to roughly 33.8–34.6 in overall diameter are commonly grouped as 34-inch tires.

What is a 35 inch tire size in metric?

Metric sizes that calculate to roughly 34.4–35.1 in overall diameter are usually marketed or described as 35-inch tires. Many common metric "35s" calculate slightly under 35 in.

These charts show that sizes in the same diameter class are not interchangeable. A 285/75R18 and a 315/70R17 both calculate to about 34.5 in tall, but they differ in width by 30 mm and need different rim diameters.

How Width, Aspect Ratio and Rim Diameter Change Tire Size

The overall diameter of a metric tire is the rim diameter plus two sidewall heights. Sidewall height equals section width × aspect ratio. A change to any of the three numbers changes the tire's height. The formula is:

Overall diameter (in) = rim diameter (in) + 2 × (section width (mm) × aspect ratio ÷ 100) ÷ 25.4

Changing section width

In metric notation, a wider tire with the same aspect ratio is also taller, because aspect ratio is a percentage of width. Compare 265/70R17 and 285/70R17:

• 265/70R17: sidewall 185.5 mm (7.30 in), overall diameter 31.6 in.
• 285/70R17: sidewall 199.5 mm (7.85 in), overall diameter 32.7 in.

The 20 mm increase in width adds about 1.1 in of overall diameter.

Changing aspect ratio

A lower aspect ratio means a shorter sidewall and a smaller overall diameter, if width and rim stay the same. Compare 285/70R17 (32.7 in) with 285/65R17. The 65-series sidewall is 185.3 mm (7.29 in), giving 31.6 in overall. A five-point drop in aspect ratio on a 285 mm tire removes about 1.1 in of diameter.

Changing rim diameter

A larger rim with the same width and aspect ratio increases overall diameter by exactly the rim increase, because the sidewall height does not change. Compare 285/70R17 (32.7 in) with 285/70R18 (33.7 in).

To keep the overall diameter close when moving to a larger rim, the aspect ratio usually has to drop. For example, 285/65R18 calculates to 32.6 in, almost the same as 285/70R17 at 32.7 in, but with about 0.6 in less sidewall on each side.

Circumference, revolutions and speedometer readings

Circumference is the distance a tire travels in one revolution, calculated as π × overall diameter. Taller tires turn fewer times per mile. Revolutions per mile equal 63,360 divided by the circumference in inches:

• A 33.0 in tire has a circumference of about 103.7 in and turns about 611 times per mile.
• A 35.0 in tire has a circumference of about 110.0 in and turns about 576 times per mile.

A speedometer calibrated for one tire diameter reads differently on another. The ratio of the new diameter to the old one shows the size of the error. Suppose a vehicle calibrated for 265/70R17 (31.6 in) switches to 285/70R17 (32.7 in). The ratio is about 1.035, so an indicated 60 mph corresponds to roughly 62 mph actual speed, based on calculated diameters.

What a Flotation Tire Size Calculator Does

A flotation tire size calculator applies the same formulas shown above. It converts flotation sizes, metric sizes or both into section width, sidewall height, overall diameter, circumference and revolutions per mile. Many calculators also compare two sizes and show the percentage difference and the resulting speedometer error.

A calculator works only from nominal dimensions. It cannot account for the specific tire model, its tread depth or the wheel it is mounted on. Its results are useful for comparing sizes, not for confirming fitment.

Calculated Geometry Versus Actual Dimensions and Fitment

Every number in a tire size code is nominal. Every conversion in this article is calculated geometry. Real tires differ from the calculated figures for several reasons:

• Manufacturer and tread design. Two tires with the same size code can have different actual overall diameters and section widths. Aggressive off-road treads are often built differently from highway treads.
• Rim width. Section width is specified on a particular measuring rim width. A wider or narrower wheel changes the measured width slightly.
• Inflation, load and wear. Loaded radius under vehicle weight is smaller than the unloaded diameter. Tread wear reduces diameter over time.
• Labeling conventions. A tire labeled with a flotation diameter such as 35 in can measure less than that figure when mounted. Manufacturers publish the actual specified diameter and width in their tire data.

Matching sizes on paper also does not confirm that a tire will fit a vehicle. Fitment depends on factors outside the size code, including:

• clearance at full steering lock and suspension travel
• wheel offset and backspacing
• the approved rim width range for the tire
• the required load and speed ratings
• effects on gearing and on electronic systems that rely on wheel speed

Use the inch-to-metric conversion to understand and compare tire sizes. Use the manufacturer's published dimensions and the vehicle's specifications to decide what actually fits.