Tire Size Conversion Calculator

How to use this converter

Select your input format from the dropdown, enter the three numbers that define your tire size, and click Convert. The calculator will output the equivalent size in all three formats and generate a table of the nearest real-world metric equivalents for the target rim diameter.

The three formats all convey the same physical information from different angles. Once you understand the math, converting between them is straightforward, and this tool makes it instant.

History of tire sizing standards

Tire sizing has changed three times in the modern automotive era, which is why three formats coexist today.

Early 20th century: Tires were described by their overall diameter and section width in inches, such as 7.50-16. The dash separated section width from rim diameter. Aspect ratios were assumed to be close to 100 percent, meaning the sidewall height equalled the section width. This worked for the high-profile, bias-ply tires of the era.

Mid-20th century flotation format: As tire profiles dropped and designs became more varied, the inch format evolved into the flotation format. Instead of just section width and rim diameter, flotation tires list overall diameter explicitly. A 31x10.50R15 tire is 31 inches in overall diameter, 10.5 inches wide, on a 15-inch rim. This format is unambiguous about the actual tire height, which matters for clearance-critical off-road applications.

1970s metric transition: The global oil crisis of 1973-1974 pushed the automotive industry toward metric standardization and fuel efficiency engineering. European tire manufacturers had been using metric sizing since the 1960s. The ISO metric format (width/aspect R rim) was adopted internationally. Passenger cars switched to metric sizing during the late 1970s and early 1980s. The metric format does not directly communicate overall diameter, requiring calculation, but it precisely specifies the aspect ratio that earlier formats ignored.

Today, metric is the global passenger car standard. Flotation remains common in North American truck and off-road markets. The old inch format survives in agricultural, industrial, and vintage vehicle applications.

Metric format breakdown

The metric tire size format follows this pattern: Width / Aspect R Rim

For 265/75R16:

• 265: Section width in millimetres. This is the widest point of the tire when properly mounted and inflated, measured from sidewall to sidewall. It is not the contact patch width, which is narrower.
• 75: Aspect ratio as a percentage. The sidewall height is 75 percent of the section width. 265 × 0.75 = 198.75mm sidewall height.
• R: Radial construction.
• 16: Rim diameter in inches.

For 265/75R16: 16 × 25.4 + 2 × 265 × 0.75 = 406.4 + 397.5 = 803.9mm = 31.65 inches.

The metric format has no single number for overall diameter. You always need to calculate it from the three components. This is the main reason the flotation format (which leads with overall diameter) remains popular for off-road applications where clearance is a primary concern.

Flotation format explained

The flotation format follows this pattern: OD x Width R Rim

For 31.5x10.50R15:

• 31.5: Overall diameter in inches. This is the most practically useful number for anyone fitting tires into a vehicle with clearance constraints.
• 10.50: Section width in inches (10.5 inches = 266.7mm).
• R: Radial.
• 15: Rim diameter in inches.

To find the aspect ratio implied by a flotation size, you work backward:

For 31.5x10.50R15: sidewall = (31.5 - 15) / 2 = 8.25 inches. Aspect = 8.25 / 10.5 × 100 = 78.6%. The equivalent metric size would be approximately 267/79R15, which does not correspond to a standard metric series, so the closest real-world equivalent is typically 265/75R15.

The flotation format is used by manufacturers who want customers to quickly understand what they are buying. A Jeep owner looking for 35-inch tires can immediately identify 35x12.50R17 as the right size without any calculations.

Inch-based format explained

The oldest format, also called the numeric or high flotation format, looks like: 31x10.50-15

The hyphen (instead of R) can indicate bias-ply construction, though some radial tires also use this format. The numbers have the same meaning as flotation format: overall diameter x section width - rim diameter, all in inches.

You will find this format on:

• Vintage military and collector vehicles
• Agricultural tires for tractors and implements
• Some trailer and industrial equipment tires
• Specialty off-road tires for rock crawling (which often still use this notation interchangeably with flotation)

Conversion from inch to metric format uses the same math as flotation to metric. The only practical difference is the separator character, which indicates construction type on older tires but is now inconsistently applied.

Converting metric to flotation step by step

Here is the full conversion process for 265/75R16 to flotation:

Step 1: Find section width in inches

Section width = 265mm / 25.4 = 10.433 inches

Step 2: Calculate sidewall height in inches

Sidewall = 10.433 × 0.75 = 7.825 inches

Step 3: Calculate overall diameter

OD = Rim diameter + 2 × sidewall = 16 + 2 × 7.825 = 31.65 inches

Step 4: Write flotation format

Round OD to one decimal place: 31.6 × 10.43R16. In common notation this is typically written as 31.5x10.50R16 (rounding section width to the nearest 0.50 inch standard increment).

The nearest standard flotation size that would be sold as an off-road tire is 31.5x10.50R16, which has a true OD of 31.62 inches versus the calculated 31.65 inches for the 265/75R16. This 0.03-inch (0.76mm) difference is negligible.

Converting flotation to metric step by step

Here is the reverse process for 35x12.50R17 to metric:

Step 1: Find sidewall height in inches

Sidewall = (35 - 17) / 2 = 9.0 inches

Step 2: Convert section width to mm

Section width = 12.5 × 25.4 = 317.5mm. Round to nearest standard metric width: 315mm or 325mm.

Step 3: Calculate aspect ratio

Aspect = (9.0 / 12.5) × 100 = 72.0%

Round to nearest standard aspect ratio: 70%.

Step 4: Verify OD of the rounded metric size

For 315/70R17: OD = 17 × 25.4 + 2 × (315 × 0.70) = 431.8 + 441 = 872.8mm = 34.36 inches. For 325/70R17: OD = 17 × 25.4 + 2 × (325 × 0.70) = 431.8 + 455 = 886.8mm = 34.91 inches.

The 35x12.50R17 at 35 inches is closest to the 325/70R17 at 34.91 inches.

Why equivalent sizes matter for rim fitment

When converting between formats for the purpose of finding alternative tires, the rim diameter must be an exact match. A tire marked R17 in any format fits only a 17-inch rim. The conversion does not change this.

What conversion can change is the effective rim width requirement. A 12.50-inch wide flotation tire needs a rim at least 8.5 inches wide. The metric equivalent 315/70R17 also needs a minimum of 8.5 inches of rim width. But if you were looking at the narrower metric equivalent 265/70R17, that tire is approved for rims from 7 to 8.5 inches.

When using the equivalents table to find replacement tires, always:

1. Confirm the rim diameter matches exactly
2. Check your current rim width against the approved range for each candidate size
3. Compare the overall diameter to ensure the change does not create speedometer errors above your acceptable threshold
4. Check the load index of the candidate tire meets your vehicle’s minimum requirement

Common conversion scenarios

Converting from stock metric for an off-road upgrade: A vehicle running 265/65R17 (30.6 inches) wants the largest tire that will clear stock fenders. Converting 30.6 inches to flotation gives approximately 30.6x10.43R17, suggesting 31x10.50R17 as a step up and 33x12.50R17 as a larger lift-required size.

Finding winter tire equivalents: A vehicle with 225/60R17 summer tires needs winter tires. Going 5mm narrower gives a 215/60R17 with the same OD (27.2 vs 27.0 inches). Alternatively, stepping down to 215/65R17 gives 28.0 inches, which is within 3 percent of stock and cuts better in snow due to the higher aspect ratio.

Finding metric equivalent for old truck tires: A restored 1970s pickup has 7.50-16 tires. Converting: 7.50 inches wide = 190.5mm. Estimating original OD for 7.50-16 at approximately 30 inches. Sidewall = (30-16)/2 = 7 inches. Aspect = 7/7.5 × 100 = 93%. Closest modern metric: 215/85R16 (OD: 30.4 in) or 225/75R16 (OD: 29.9 in).

Load range and speed rating differences across formats

Flotation and LT-metric tires use load ranges instead of individual load indexes. Load ranges are designated by letters: C (6-ply rating), D (8-ply), E (10-ply). Load range E is the strongest commonly available, used on heavy-duty trucks and serious off-road vehicles.

P-metric tires use load indexes. The two systems require careful translation when comparing:

When converting from LT flotation to LT-metric or vice versa, match the load range letter, not just the physical size. An LT265/70R17 load range E and a 31.5x10.50R17 load range E are comparable in load capacity even if the ODs differ slightly.

Speed ratings in metric format (T, H, V, W, Y) apply to both metric and LT-metric tires. Flotation tires sold for off-road use often do not carry speed ratings above S (180 km/h) because they are not designed for sustained highway speeds. For a dual-use vehicle driven at highway speeds, verify the speed rating of any replacement tire meets your country’s requirements and the vehicle manufacturer’s specification.

Practical tips for finding exact matches

When you have done the math and have a target metric size, the next step is verifying whether that size is actually manufactured and available for purchase. Not every theoretically valid combination of width, aspect ratio, and rim diameter exists as a production tire.

Tire manufacturers concentrate production on popular combinations. Common sizes like 265/70R17 or 235/65R17 are available from dozens of manufacturers in multiple tread patterns and performance tiers. Unusual combinations like 275/60R18 or 305/55R19 may be available only from a few manufacturers in limited tread styles.

Check the load index before committing: When the conversion produces a metric size, that size string may exist in P-metric and LT-metric variants with significantly different load ratings. A P245/70R16 and an LT245/70R16 look identical in size notation but differ substantially in construction. Always buy the type (P or LT) that matches your vehicle’s door jamb specification.

Use a tire size chart to find valid increments: Section widths come in 5 and 10mm increments. Common sizes: 155, 165, 175, 185, 195, 205, 215, 225, 235, 245, 255, 265, 275, 285, 295, 305, 315, 325, 335mm. Aspect ratios come in 5-percent increments from 30 to 85. Not all width-aspect combinations are produced, particularly at the extremes.

Verify OD before ordering: After identifying candidate sizes, calculate and compare the OD of each to your original or target size. The goal is to stay within 3 percent of original OD for street use. Off-road builds may accept larger differences if the vehicle has been lifted to accommodate them.

Cross-reference with manufacturer fitment guides: Michelin, Bridgestone, Goodyear, and Continental all publish online tire selector tools that filter by vehicle. These tools only show sizes that have been tested and approved for specific vehicles. If your converted size does not appear for your vehicle, the manufacturer may not recommend it even if it physically fits.

Tread pattern selection across formats

When converting between tire size formats for an off-road application, the tread pattern is as important as the size. Three main categories exist:

All-terrain (A/T): Moderate open tread pattern, usable on road and light off-road. These tires convert well between formats because they are made in a wide range of sizes in both metric and flotation formats. A/T tires sacrifice some on-road noise and highway fuel economy compared to highway tires but provide noticeably better off-road traction.

Mud-terrain (M/T): Aggressive tread with large open voids for mud self-cleaning. Available primarily in flotation sizes (33x12.50R17, 35x12.50R17 and similar) and some LT-metric sizes. When converting from metric to find mud-terrain options, the equivalents table in the calculator will show you which flotation sizes are closest. Most serious mud-terrain tires are not available in low-profile metric sizes because the taller sidewall of a higher aspect ratio provides better puncture resistance and bead retention on rocky terrain.

Highway (H/T): Optimized for on-road use. Available across all formats for common sizes. Highway tires generally have higher speed ratings and lower rolling resistance than all-terrain or mud-terrain options of equivalent size.

When converting formats specifically to shop for off-road tires, the flotation format opens up a much larger selection of A/T and M/T options than equivalent metric sizes. This is one practical reason the conversion calculator is useful: your vehicle may have come with metric sizes, but the off-road aftermarket is still largely organized around flotation format sizes.