Tire Conversion Calculator

Q: How do I convert an inch tire size to metric?

Switch the input format to Inch , then type in the overall height, section width, and rim diameter. The calculator will display the closest metric equivalent. For example, a 33X12.5R17 is close to a 315/70R17 in metric format.

Q: How is overall tire diameter calculated?

For a metric tire, the formula is: Overall Diameter = (Rim Diameter × 25.4) + (2 × Sidewall Height) . The sidewall height equals the section width times the aspect ratio divided by 100. Everything is converted to the same unit before adding. The calculator shows this math in the Step-by-Step Solution section.

Q: How close in size should my new tires be to my original tires?

A common fitment guideline is to stay within 3% of your original tire's overall diameter . Going beyond 3% can cause noticeable speedometer error, affect your braking distance, and put extra stress on your drivetrain. The comparison tab shows the exact percentage difference between two tire sizes so you can check this easily.

Tire Conversion Calculator

Updated Sep 1, 2026 By Jehan Wadia

Introduction

Our tire size calculator shows you the exact specs of any tire. Just type in a metric size like 265/70R17 or an inch size like 33X12.5R17, and the tool calculates the specs. It shows you the overall diameter, sidewall height, section width, circumference, and revolutions per mile. You also get a clear visual diagram of how the tire looks from the front and side.

Need to switch to a bigger or smaller wheel? The built-in tire conversion tool finds sizes that fit your new rim while keeping a similar overall diameter. This matters because the wrong tire size can throw off your speedometer, hurt fuel economy, and cause rubbing inside your fender.

Use the Tire Comparison tab to put two tire sizes side by side. You will see how they differ in diameter, width, and revolutions per mile. The tool also shows a speedometer correction chart so you know your true speed, plus a gear ratio table that estimates engine RPM at highway speed. This calculator gives you the numbers you need to choose the right tire size with confidence.

How to Use Our Tire Conversion Calculator

Enter your tire size details below to get the overall diameter, sidewall height, circumference, revolutions per mile, and equivalent tire sizes in both metric and inch formats. You can also compare two tire sizes side by side.

Input Format: Pick "Metric" if your tire size looks like 265/70R17. Pick "Inch" if it looks like 33X12.5R17. You can also paste a full tire size into the first field and the calculator will fill in the rest.

Section Width: In metric mode, type the tire width in millimeters. This is the first number in a metric tire size. In inch mode, type the total tire height in inches instead.

Aspect Ratio: In metric mode, type the aspect ratio as a percentage. This tells you how tall the sidewall is compared to the width. In inch mode, type the tire's section width in inches instead.

Rim Diameter: Type the wheel diameter in inches. This is the last number in any tire size, such as 17 in 265/70R17.

Tire Comparison Tab: To compare two tires, click the "Tire Comparison" tab. Enter the current tire size on the left and the new tire size on the right. Press "Compare Tires" to see differences in size, speedometer readings, and gear ratio effects.

What Is a Tire Size Conversion Calculator?

A tire size conversion calculator helps you understand the real measurements of any tire. Every tire has a size printed on its sidewall, like 265/70R17. These numbers tell you three things: the width of the tire in millimeters, the height of the sidewall as a percentage of the width, and the diameter of the wheel it fits. This calculator turns those numbers into useful measurements like overall diameter, circumference, and revolutions per mile.

How to Read a Tire Size

Tire sizes come in two common formats: metric and inch. A metric size like 265/70R17 means the tire is 265 mm wide, the sidewall height is 70% of that width, and it fits a 17-inch wheel. An inch size like 33X12.5R17 means the tire is 33 inches tall, 12.5 inches wide, and fits a 17-inch wheel. Both formats describe the same kind of tire, just in different ways. This tool converts between them instantly.

Why Tire Size Matters

Choosing the right tire size affects how your vehicle drives. A larger tire covers more ground per rotation, which makes your speedometer read lower than your actual speed. A smaller tire does the opposite. If you switch to a tire that is too different from your original size, it can cause problems with your speedometer, braking, fuel economy, and even transmission wear. A common fitment guideline is to stay within 3% of your original tire's overall diameter when picking a new size.

When to Use a Tire Comparison

Use the tire comparison tab when you want to swap to a different tire size or move to a bigger or smaller wheel. It shows you the exact difference in diameter, width, sidewall height, and circumference between two tires. It also calculates how much your speedometer reading will be off and how engine RPM changes at highway speed based on your gear ratio. This is helpful when upgrading wheels, fitting off-road tires, or finding a cheaper replacement tire that still fits your vehicle safely.

Formulas used

Sidewall Height

SH = W \times \frac{AR}{100}

Overall Diameter

OD = (D_r \times 25.4) + 2 \times SH

Circumference

C = OD \times \pi

Revolutions per Mile

N = \frac{63360}{C}

Speedometer Correction

V_{actual} = V_{indicated} \times \frac{OD_2}{OD_1}

Engine RPM at Speed

RPM = \frac{Speed \times GearRatio \times 336}{OD}

Frequently asked questions

How do I convert a metric tire size to inches?

Enter your metric tire size in the calculator using the Metric input format. The tool will show the inch equivalent in the Equivalent Size section. For example, a 265/70R17 is close to a 31.5X10.5R17 in inch format. The calculator does all the unit conversion math for you.

How do I convert an inch tire size to metric?

Switch the input format to Inch, then type in the overall height, section width, and rim diameter. The calculator will display the closest metric equivalent. For example, a 33X12.5R17 is close to a 315/70R17 in metric format.

How is overall tire diameter calculated?

For a metric tire, the formula is: Overall Diameter = (Rim Diameter × 25.4) + (2 × Sidewall Height). The sidewall height equals the section width times the aspect ratio divided by 100. Everything is converted to the same unit before adding. The calculator shows this math in the Step-by-Step Solution section.

What does revolutions per mile mean?

Revolutions per mile tells you how many times a tire rotates to travel one mile. A smaller tire spins more times per mile, and a larger tire spins fewer times. This number affects your speedometer accuracy, odometer readings, and fuel economy. The calculator finds it by dividing 63,360 (inches in a mile) by the tire's circumference in inches.

How close in size should my new tires be to my original tires?

A common fitment guideline is to stay within 3% of your original tire's overall diameter. Going beyond 3% can cause noticeable speedometer error, affect your braking distance, and put extra stress on your drivetrain. The comparison tab shows the exact percentage difference between two tire sizes so you can check this easily.

How does a bigger tire affect my speedometer?

A bigger tire covers more ground per rotation. Your speedometer thinks each rotation is the same distance as the original tire, so it reads lower than your actual speed. For example, if your speedometer says 60 mph, you might really be going 62 mph. The comparison tab includes a speedometer correction chart that shows the exact difference at various speeds.

How does a smaller tire affect my speedometer?

A smaller tire covers less ground per rotation, so your speedometer reads higher than your actual speed. If your speedometer says 60 mph, you might really be going 58 mph. Your odometer will also add miles faster than you actually drive.

What is the Convert to a Different Wheel Size feature?

This feature finds tire sizes that fit a new wheel diameter while keeping an overall diameter close to your current tire. Enter a new rim size (like 20 inches), and the calculator lists up to 8 compatible tire sizes within 1.5 inches of your original tire's diameter. This is useful when upgrading to larger wheels or downsizing for winter.

What is aspect ratio on a tire?

Aspect ratio is the height of the tire's sidewall expressed as a percentage of its width. A tire with a 70 aspect ratio has a sidewall that is 70% as tall as the tire is wide. A lower aspect ratio means a shorter, stiffer sidewall. A higher aspect ratio means a taller, more cushioned sidewall.

What is section width on a tire?

Section width is the measurement of the tire from one sidewall to the other at its widest point, not counting any raised lettering or trim. In metric tire sizes, it is given in millimeters. For example, in 265/70R17, the section width is 265 mm (about 10.4 inches).

What is sidewall height and why does it matter?

Sidewall height is the distance from the wheel rim to the outer edge of the tread. It is calculated by multiplying the section width by the aspect ratio. A taller sidewall gives a softer ride and better protection against potholes. A shorter sidewall gives sharper handling and a sportier look but a firmer ride.

Will changing tire size void my warranty?

It depends on your vehicle manufacturer and how far you deviate from the factory tire size. Small changes (within 3% of the original diameter) usually do not cause issues. Large changes can affect safety systems like ABS and traction control, which could give a dealer reason to deny a warranty claim related to the drivetrain. Always check your owner's manual or ask your dealer before making big changes.

Do I need to recalibrate my speedometer after changing tire size?

If your new tire's overall diameter is more than 3% different from the original, you should recalibrate your speedometer. Many modern vehicles allow a dealer to reprogram the ECU for the new tire size. Aftermarket speedometer calibrators are also available. The comparison tab's speedometer chart shows exactly how far off your readings will be, so you can decide if a recalibration is needed.

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