Tire Size Calculator — Free Online Tire Dimension Tool

How to Use the Tire Size Calculator

1. Enter tire width: Type the tire width in millimeters into the first field. This is the first number in your tire's sidewall marking (for example, 215 in 215/65R15). You can find this number on the tire sidewall or in your vehicle's owner's manual. Common passenger tire widths range from 185 mm to 275 mm, while truck and SUV tires can range from 235 mm to 315 mm.
2. Enter aspect ratio: Type the aspect ratio into the second field. This is the two-digit number after the slash in your tire's designation (for example, 65 in 215/65R15). The aspect ratio represents the sidewall height as a percentage of the tire width. Typical values range from 30 (low-profile performance tires) to 80 (tall-sidewall truck tires).
3. Enter rim diameter: Type the wheel rim diameter in inches into the third field. This is the last number in your tire's designation (for example, 15 in 215/65R15). Common rim sizes for passenger vehicles range from 14 to 22 inches. You can find this on your wheel or in your vehicle specifications.
4. Review tire specifications: The results panel instantly calculates and displays the overall tire diameter in inches, circumference in inches, sidewall height in both millimeters and inches, revolutions per mile, and estimated speed at 2,000 and 3,000 RPM. A tire notation display also shows your input in standard tire format (e.g., 215/65R15).

All results update in real time. Adjust any value to instantly see how changes in width, aspect ratio, or rim size affect the overall tire dimensions and performance characteristics.

Tire Size Formulas

Variables Explained

• Tire Width (mm): The section width of the tire measured in millimeters across the widest point of the tread from outer sidewall to outer sidewall when mounted and inflated to recommended pressure. This is the first number in the tire designation. Wider tires generally provide more grip but increase rolling resistance and may affect fuel economy.
• Aspect Ratio (%): The ratio of the sidewall height to the tire width, expressed as a percentage. An aspect ratio of 65 means the sidewall height is 65% of the tire width. Lower aspect ratios provide better cornering response and steering feel, while higher aspect ratios offer a more comfortable ride by absorbing road imperfections.
• Rim Diameter (in): The diameter of the wheel rim in inches, measured bead seat to bead seat. This must match the tire's designated rim size for safe mounting. Larger rims allow for wider, lower-profile tires (plus-sizing) while maintaining similar overall diameter.
• Overall Diameter (in): The total height of the tire from the ground to the top when mounted and inflated. This is the rim diameter plus two sidewall heights (one for each side). Overall diameter directly affects speedometer accuracy, ground clearance, and gearing.
• Revolutions Per Mile: The number of complete rotations the tire makes to cover one mile. This is calculated by dividing the number of inches in a mile (63,360) by the tire circumference. This metric is important for speedometer calibration and understanding drivetrain performance.

Step-by-Step Example

Calculate the specifications for a 225/55R17 tire:

1. Calculate sidewall height: 225 x 55 / 100 = 123.75 mm
2. Convert sidewall to inches: 123.75 / 25.4 = 4.87 inches
3. Calculate overall diameter: 17 + (2 x 4.87) = 26.74 inches
4. Calculate circumference: 26.74 x 3.14159 = 84.01 inches
5. Calculate revolutions per mile: 63,360 / 84.01 = 754 rev/mile

A 225/55R17 tire has an overall diameter of about 26.74 inches, a circumference of 84.01 inches, and completes approximately 754 revolutions per mile. This information helps when comparing this tire to alternatives and understanding how a size change would affect your vehicle.

Practical Examples

Example 1: Alex Upgrades Wheels with Plus-Sizing

Alex drives a sedan with factory 205/65R15 tires and wants to upgrade to 17-inch wheels for a sportier look. He needs to find a tire size that maintains approximately the same overall diameter to keep his speedometer accurate. Using the tire size calculator:

• Original 205/65R15: sidewall = 133.25 mm, overall diameter = 25.49 in
• Option 1: 225/45R17: sidewall = 101.25 mm, overall diameter = 24.97 in (2.0% smaller)
• Option 2: 225/50R17: sidewall = 112.5 mm, overall diameter = 25.86 in (1.4% larger)
• Option 3: 215/50R17: sidewall = 107.5 mm, overall diameter = 25.46 in (0.1% smaller)

Alex chooses the 215/50R17 because it has the closest overall diameter to his original tires (only 0.1% difference), which means virtually no speedometer error. The wider tread and lower profile give him the sportier look he wanted without compromising measurement accuracy or requiring speedometer recalibration.

Example 2: Maria Considers All-Terrain Tires for Her SUV

Maria drives a midsize SUV with factory 235/60R18 tires and wants to install larger all-terrain tires for occasional off-roading. She is considering 265/65R18 tires and uses the calculator to check the diameter difference:

• Original 235/60R18: sidewall = 141 mm, overall diameter = 29.10 in
• Proposed 265/65R18: sidewall = 172.25 mm, overall diameter = 31.56 in
• Diameter increase: 31.56 - 29.10 = 2.46 in (8.5% larger)
• Speedometer error: at 60 mph indicated, actual speed would be about 65.1 mph

The 8.5% diameter increase exceeds the recommended 3% maximum. At 60 mph on the speedometer, Maria would actually be traveling at about 65 mph, risking speeding tickets. She also needs to verify fender clearance and may need a lift kit. She instead considers 255/60R18 (overall diameter 30.05 in, just 3.3% larger) as a more practical compromise.

Example 3: Kevin Compares Winter Tire Options

Kevin needs winter tires for his compact car that currently uses 205/55R16 all-season tires. His tire shop offers two winter tire options: 195/65R15 (on cheaper 15-inch steel wheels) and 205/55R16 (same size, winter compound). He compares the sizes:

• Current 205/55R16: overall diameter = 24.88 in, revs/mile = 811
• Option 1: 195/65R15: overall diameter = 24.98 in, revs/mile = 808
• Option 2: 205/55R16: overall diameter = 24.88 in, revs/mile = 811
• Diameter difference (option 1): 0.10 in (0.4% larger, negligible)

Both options maintain virtually identical overall diameters. Kevin chooses the 195/65R15 option because the narrower tire actually performs better in snow (cuts through to pavement), the higher aspect ratio absorbs pothole impacts better in winter, and the 15-inch steel wheels are less expensive to replace if damaged by road salt or curbs.

Example 4: Daniel Calculates Speedometer Correction

Daniel already installed 285/70R17 tires on his truck, replacing the factory 265/65R17. He wants to understand exactly how far off his speedometer reads and calculate the true speed:

• Factory 265/65R17: overall diameter = 30.52 in, circumference = 95.88 in
• Installed 285/70R17: overall diameter = 32.71 in, circumference = 102.76 in
• Size increase: (32.71 - 30.52) / 30.52 = 7.2%
• When speedometer reads 60 mph: actual speed = 60 x 1.072 = 64.3 mph
• When speedometer reads 70 mph: actual speed = 70 x 1.072 = 75.0 mph

Daniel discovers that his speedometer reads about 7.2% low, meaning he is traveling 4.3 mph faster than displayed at highway speeds. He takes his truck to the dealer for a speedometer recalibration to avoid unknowingly speeding. He also notes that his odometer will undercount mileage by the same 7.2%, affecting maintenance interval tracking and resale mileage.

Common Tire Size Reference Table

All values calculated using standard tire size formulas. Actual dimensions may vary by manufacturer and inflation pressure.

Tips and Complete Guide

Understanding Plus-Sizing and Minus-Sizing

Plus-sizing means increasing the wheel diameter while maintaining the same overall tire diameter by using a lower aspect ratio tire. For example, moving from 205/65R15 to 225/50R17 keeps the overall diameter similar while adding wheel size and width. This approach improves handling response and gives a sportier appearance, but the shorter sidewall provides less cushioning over bumps. Minus-sizing works in reverse: moving to a smaller wheel with a higher aspect ratio tire. This is often done for winter tire setups, where the taller sidewall protects the wheel from potholes and the narrower tire cuts through snow better. Our calculator makes it easy to compare different size combinations and find options that maintain your target overall diameter.

Tire Size and Fuel Economy

Tire size affects fuel economy in several ways. Wider tires have a larger contact patch, which increases rolling resistance and typically decreases fuel economy by 1% to 3% per 10 mm increase in width. Larger overall diameter tires can improve highway fuel economy slightly because the engine turns at lower RPM at a given speed, but they may hurt city fuel economy due to increased rotational mass. Low rolling resistance tires in the correct size for your vehicle can improve fuel economy by 3% to 5% compared to standard tires. When using our calculator to compare tire options, remember that the tire closest to your factory size will likely maintain the fuel economy your vehicle was engineered for.

Reading Tire Sidewall Information

Beyond the basic size designation, tire sidewalls contain important additional information. The load index number (e.g., 95) indicates the maximum weight the tire can support when properly inflated: index 95 means 1,521 pounds per tire. The speed rating letter (e.g., H) indicates the maximum sustained speed: H means 130 mph, V means 149 mph, W means 168 mph, and Y means 186 mph. The DOT code includes the manufacturing date in the last four digits (e.g., 2325 means the 23rd week of 2025). The UTQG rating shows Treadwear (higher is longer lasting), Traction (AA is best), and Temperature (A is best). Always match or exceed the load index and speed rating specified by your vehicle manufacturer.

Common Mistakes to Avoid

• Changing overall diameter by more than 3%: Going beyond 3% diameter change from factory specification causes significant speedometer error and can affect ABS, traction control, and stability control systems that rely on accurate wheel speed data. Some modern vehicles may trigger warning lights or limit functionality with incorrect tire sizes.
• Ignoring load rating when upsizing: A larger tire is not necessarily rated for more weight. When changing tire sizes, verify that the new tire's load index meets or exceeds your vehicle's requirements. Running tires with insufficient load rating is dangerous and can lead to blowouts, especially on loaded trucks and SUVs.
• Mixing tire sizes on the same axle: All tires on the same axle must be the same size, brand, model, and wear level. Mixing different sizes on the same axle causes handling instability, uneven braking, and accelerated wear. All-wheel-drive vehicles typically require all four tires to be identical to protect the drivetrain.
• Not checking fender clearance before upsizing: Larger tires may rub against fender wells, suspension components, or body panels during turns or over bumps. This can damage both the tire and the vehicle. Always verify clearance at full steering lock and under compression before committing to a larger tire size.
• Confusing metric and imperial tire sizing: Some older trucks and light trailers use a different sizing format like 31x10.50R15, which describes the tire in inches (31-inch diameter, 10.5-inch width). This is not the same as metric sizing and cannot be directly compared without conversion. Our calculator uses the more common metric format.

Frequently Asked Questions