Find Tire Diameter Calculator

Easily calculate the overall diameter, sidewall height, and circumference of your car or truck tires using our free Tire Diameter Calculator. Understand how tire size affects your vehicle.

Calculate Tire Diameter

Common Tire Sizes and Diameters

Tire Size	Width (mm)	Aspect Ratio (%)	Wheel (in)	Overall Diameter (in)
205/55R16	205	55	16	24.88
225/45R17	225	45	17	24.97
235/40R18	235	40	18	25.40
245/40R18	245	40	18	25.72
275/35R19	275	35	19	26.58
265/70R17	265	70	17	31.61
315/70R17	315	70	17	34.36

Examples of common tire sizes and their calculated overall diameters.

What is a Tire Diameter Calculator?

A Tire Diameter Calculator is a tool used to determine the overall outer diameter of a vehicle's tire based on the tire size specifications typically found on the sidewall. These specifications follow a standard format like "P225/45R17" or "225/45R17", where '225' is the width in millimeters, '45' is the aspect ratio (sidewall height as a percentage of width), and '17' is the wheel diameter in inches.

This calculator is essential for anyone looking to change their tire size, as it helps understand how a different tire will affect speedometer accuracy, gearing, and the physical fitment within the wheel well. Vehicle owners, mechanics, tire shop professionals, and car enthusiasts looking to upgrade or change their wheels and tires should use a Tire Diameter Calculator.

A common misconception is that all tires with the same wheel diameter (e.g., 17 inches) are the same overall height. However, the width and aspect ratio significantly impact the sidewall height, and thus the total diameter. Our Tire Diameter Calculator accurately computes this.

Tire Diameter Calculator Formula and Mathematical Explanation

The calculation for the overall tire diameter involves a few steps:

1. Calculate Sidewall Height in Millimeters: Multiply the tire width (in mm) by the aspect ratio (as a decimal).
   Sidewall Height (mm) = Tire Width * (Aspect Ratio / 100)
2. Convert Sidewall Height to Inches: Divide the sidewall height in millimeters by 25.4 (since there are 25.4 mm in an inch).
   Sidewall Height (inches) = Sidewall Height (mm) / 25.4
3. Calculate Overall Diameter: Add twice the sidewall height (in inches) to the wheel diameter (in inches). We multiply by two because the sidewall is on both the top and bottom of the wheel relative to the diameter line.
   Overall Diameter (inches) = (2 * Sidewall Height (inches)) + Wheel Diameter (inches)
4. Calculate Circumference: Multiply the Overall Diameter by Pi (π ≈ 3.14159).
   Circumference (inches) = Overall Diameter (inches) * π

The Tire Diameter Calculator uses these exact formulas.

Variables Used:

Variable	Meaning	Unit	Typical Range
Tire Width (W)	The nominal width of the tire from sidewall to sidewall.	mm	155 – 355+
Aspect Ratio (AR)	The height of the sidewall as a percentage of the tire width.	%	25 – 85
Wheel Diameter (D)	The diameter of the wheel rim.	inches	13 – 24+
Sidewall Height (SH)	The height of one sidewall.	mm or inches	Varies
Overall Diameter (OD)	The total height of the tire.	inches	Varies
Circumference (C)	The distance around the outside of the tire.	inches	Varies

Variables involved in the tire diameter calculation.

Practical Examples (Real-World Use Cases)

Example 1: Upgrading to Larger Wheels

Someone has a car with stock tires sized 205/55R16 and wants to upgrade to 17-inch wheels while keeping the overall diameter similar to maintain speedometer accuracy. They are considering 225/45R17 tires.

• Stock Tire (205/55R16): Using the Tire Diameter Calculator, Width=205, Aspect Ratio=55, Wheel=16 gives an overall diameter of approximately 24.88 inches.
• New Tire (225/45R17): Width=225, Aspect Ratio=45, Wheel=17 gives an overall diameter of approximately 24.97 inches.

The difference is very small (0.09 inches), meaning the speedometer reading will remain very accurate, and the new tires are a good "plus-one" fitment. Our Tire Diameter Calculator confirms this.

Example 2: Off-Road Truck Tire Change

A truck owner has 265/70R17 tires and wants to fit larger 315/70R17 tires for better off-road capability.

• Stock Tire (265/70R17): Using the Tire Diameter Calculator, Width=265, Aspect Ratio=70, Wheel=17 gives an overall diameter of about 31.61 inches.
• New Tire (315/70R17): Width=315, Aspect Ratio=70, Wheel=17 gives an overall diameter of about 34.36 inches.

The new tires are significantly larger (2.75 inches taller). This will affect the speedometer (it will read lower than actual speed), effective gearing (lower RPM at the same speed), and might require a lift kit for clearance. The Tire Diameter Calculator highlights this substantial change.

How to Use This Tire Diameter Calculator

1. Enter Tire Width: Input the first number from your tire size (e.g., '225' from 225/45R17) into the "Tire Width (mm)" field.
2. Enter Aspect Ratio: Input the second number (e.g., '45') into the "Aspect Ratio (%)" field.
3. Enter Wheel Diameter: Input the third number (e.g., '17') into the "Wheel Diameter (inches)" field.
4. View Results: The calculator will automatically display the Overall Diameter, Sidewall Height, and Circumference as you type or after you click "Calculate". The chart will also update.
5. Interpret Results: The "Overall Diameter" is the key figure for comparing tire sizes and understanding potential impacts on your vehicle.
6. Reset: Click "Reset" to clear the fields to their default values.
7. Copy: Click "Copy Results" to copy the inputs and results to your clipboard.

When changing tire sizes, aim for an overall diameter close to your original equipment (OE) size to minimize effects on speedometer, odometer, and vehicle systems like ABS and traction control. The Tire Diameter Calculator is crucial for this.

Key Factors That Affect Tire Diameter Results

• Tire Width: A wider tire, at the same aspect ratio and wheel size, will have taller sidewalls and thus a larger overall diameter.
• Aspect Ratio: This is a direct multiplier for the sidewall height relative to the width. A higher aspect ratio means taller sidewalls and a larger diameter for the same width and wheel size.
• Wheel Diameter: The base diameter on which the tire is mounted. Larger wheels directly increase the overall diameter if sidewall height remains constant (which it doesn't when plus-sizing correctly).
• Tire Pressure: While not part of the standard size calculation, under or over-inflation can slightly alter the rolling diameter and contact patch. The Tire Diameter Calculator assumes correct inflation and standard sizing.
• Tire Wear: New tires have a larger diameter than worn-out tires. The calculation is based on the specified size of a new tire.
• Manufacturing Tolerances: Different tire brands, even with the same nominal size, can have slight variations in actual measured diameter. The Tire Diameter Calculator provides a theoretical value based on the numbers.
• Load on the Tire: The weight on the tire will cause it to deflect slightly, reducing the effective rolling diameter compared to the unladen diameter. The calculator gives the unladen diameter. Explore our tire size calculator for more details.

Frequently Asked Questions (FAQ)

What is the 'P' or 'LT' before some tire sizes?

'P' stands for P-metric (passenger car) and 'LT' for Light Truck. These designations indicate the load range and construction type but don't directly enter the primary diameter calculation made by our Tire Diameter Calculator, though they relate to the typical ranges of width and aspect ratio.

How much can I change my tire diameter without issues?

It's generally recommended to stay within +/- 3% of the original equipment (OE) tire diameter to avoid significant speedometer errors or issues with vehicle systems like ABS and traction control. Our Tire Diameter Calculator helps you find sizes within this range.

Will a larger diameter tire affect my fuel economy?

Yes, larger diameter tires (and often wider ones) can increase rolling resistance and weight, potentially reducing fuel economy slightly. They also change the effective gear ratio.

Will changing tire diameter affect my speedometer?

Yes. If you install tires with a larger overall diameter, your speedometer will read lower than your actual speed. Smaller diameter tires will cause it to read higher. Learn more about vehicle speedometer accuracy.

What is "plus sizing" tires?

"Plus sizing" (e.g., +1, +2) involves increasing the wheel diameter while decreasing the tire's aspect ratio to maintain a similar overall tire diameter. For instance, moving from 205/55R16 to 225/45R17 is a +1 size. Use our Tire Diameter Calculator to check diameter when plus sizing tires.

Does the Tire Diameter Calculator work for all tire types?

Yes, it works for any tire that uses the standard width/aspect ratio/wheel diameter sizing convention (e.g., P-metric, Euro-metric, LT).

What is tire circumference, and why is it important?

Circumference is the distance the tire travels in one revolution. It's directly related to the diameter and affects speedometer calibration and the distance traveled per engine revolution. Our tire circumference calculator can give more focused results.

Can I mix tire sizes on my vehicle?

Generally, no, unless your vehicle is designed for staggered fitments (different sizes front and rear), and even then, the overall diameters should be very close, especially on AWD vehicles. Use the Tire Diameter Calculator to compare.