Use the free tire diameter calculator below to find the overall height of any P-metric tire in seconds. Enter a size like 225/55R17 (or fill in the three fields manually) and instantly get the overall diameter, sidewall height, circumference, and revolutions per mile and per kilometer in both inches and millimeters.
Knowing your tire's overall diameter is important when changing wheel sizes, installing a lift kit, recalibrating your speedometer, or comparing replacement tires. Even a small change in diameter can affect vehicle clearance, gearing, fuel economy, and the accuracy of your speedometer. Use the calculator below to instantly determine the diameter, sidewall height, circumference, and revolutions per mile for almost any P-metric tire size.
Tire Diameter Calculator
Enter a tire size in standard P-metric format, or use the individual fields.
Results for
| Measurement | Inches | Millimeters |
|---|---|---|
| Overall Diameter | — | — |
| Sidewall Height | — | — |
| Circumference | — | — |
| Revs per Mile | — | |
| Revs per Km | — | |
How to Read a P-Metric Tire Size
The number molded into the sidewall of every passenger tire follows a standard three part format. Take 225/55R17 as an example:
- 225 - Section width in millimeters. This is the widest point of the tire from sidewall to sidewall when mounted on the specified rim.
- 55 - Aspect ratio, expressed as a percentage. The sidewall height equals 55% of the section width - in this case 123.75 mm (roughly 4.87 inches).
- R17 - Radial construction; the rim diameter the tire is designed to fit, in inches.
Some sizes carry a prefix letter - P (passenger), LT (light truck), or ST (special trailer). The calculator strips any prefix automatically, so you can paste the full size string as it appears on the sidewall.
Tire Size Calculator Formula
How to calculate tire diameter? The overall tire diameter is calculated from first principles using the three values above:
- Sidewall height (mm) = Section width × (Aspect ratio ÷ 100)
- Overall diameter (mm) = (Rim diameter × 25.4) + (Sidewall height × 2)
- Overall diameter (in) = Overall diameter (mm) ÷ 25.4
For a 225/55R17: sidewall = 225 × 0.55 = 123.75 mm (4.87″). Overall diameter = (17 × 25.4) + (123.75 × 2) = 431.8 + 247.5 = 679.3 mm (26.74″).
What Each Result Means
Overall Diameter
The total height of the mounted, inflated tire from the ground to the top of the tread. This is the number that matters most for fitment - clearance in the wheel well, lift kit calculations, and speedometer accuracy all depend on it.
Sidewall Height
The distance from the rim edge to the outer tread face. A taller sidewall absorbs more road shock; a shorter sidewall improves steering response and handling feel. When you "plus size" to a larger rim, the aspect ratio typically drops to keep the overall diameter - and therefore the speedometer reading - consistent.
Circumference
The distance the tire travels in one complete revolution. Circumference = π × overall diameter. This figure feeds directly into the revolutions-per-mile and revolutions-per-kilometer calculations.
Revolutions per Mile / per Kilometer
How many full rotations the tire completes per mile (or per km) of distance traveled. These numbers are used when programming aftermarket speedometers, differential gear ratios, and ABS or traction-control systems after a tire size change. A larger tire has a longer circumference, so it completes fewer revolutions per mile - which causes a stock speedometer to read low if no recalibration is done.
How Much Diameter Change Is Safe?
The general guideline is to stay within ±3% of the factory overall diameter. Go beyond that and you risk:
- Speedometer error - a speedometer that consistently reads high or low, which can affect cruise-control accuracy and traffic-law compliance.
- ABS and traction-control calibration issues - the wheel-speed sensors report revolutions, not actual speed; a significant diameter change shifts those readings.
- Transmission and differential wear - especially relevant on AWD vehicles, where all four tires must remain within a very tight diameter tolerance to avoid binding in the center differential.
- Clearance problems - a taller tire can contact the inner fender or suspension components under full steering lock or suspension compression.
To check your percentage difference: ((new diameter − original diameter) ÷ original diameter) × 100. If that number is between −3 and +3, you're in a safe range for most vehicles. Always cross-check with a fitment guide specific to your year, make, and model.
Note: I've pushed closer to 4% on full sized trucks and haven't personally encountered any issues other than the speedometer reading a few mph off at higher speeds.
Common Tire Sizes - Quick Reference
The table below lists calculated overall diameters for frequently encountered P-metric sizes:
| Tire Size | Sidewall (in) | Overall Diameter (in) | Overall Diameter (mm) | Revs / Mile |
|---|---|---|---|---|
| 195/65R15 | 4.99″ | 24.99″ | 634.7 mm | 806 |
| 205/55R16 | 4.45″ | 24.89″ | 632.0 mm | 809 |
| 215/55R17 | 4.66″ | 26.32″ | 668.5 mm | 765 |
| 225/45R17 | 3.98″ | 24.97″ | 634.1 mm | 808 |
| 225/50R17 | 4.43″ | 25.86″ | 656.8 mm | 780 |
| 225/55R17 | 4.87″ | 26.74″ | 679.3 mm | 754 |
| 235/45R18 | 4.16″ | 26.33″ | 668.5 mm | 765 |
| 245/40R18 | 3.86″ | 25.71″ | 652.9 mm | 785 |
| 265/70R17 | 7.31″ | 31.61″ | 803.0 mm | 639 |
| 275/55R20 | 5.95″ | 31.91″ | 810.5 mm | 633 |
| 285/75R16 | 8.43″ | 32.85″ | 834.3 mm | 614 |
| 305/70R18 | 8.39″ | 35.79″ | 909.0 mm | 563 |
Plus Sizing and Keeping Diameter Consistent
Plus sizing means increasing wheel diameter (going from 17" to 18", for example) while reducing the tire aspect ratio so the overall diameter stays close to the original. It is popular for improving handling feel and fitting larger brake calipers.
A typical progression keeping diameter near 26.7":
- 225/55R17 → 26.74"
- 225/50R18 → 26.83"
- 225/45R19 → 26.91"
Each step up in rim size is offset by a lower aspect ratio (shorter sidewall), preserving the overall diameter. Run the sizes through the calculator above to confirm before purchasing.
Frequently Asked Questions
Does tire diameter affect speedometer reading?
Yes. Your vehicle's speedometer is calibrated to a specific number of tire revolutions per mile. A taller tire covers more ground per revolution, so the wheel spins fewer times per mile, causing the speedometer to read lower than your actual speed. A shorter tire does the opposite. Staying within 3% of the factory diameter keeps the error negligible for most drivers.
Is overall diameter the same as overall height?
Yes. The terms "overall diameter" and "overall height" are used interchangeably. When a tire is standing upright on flat ground, its height from the ground to the top of the tread is exactly its overall diameter.
Why does the calculator show results in both inches and millimeters?
Tire width and sidewall height are specified in metric units (millimeters), while wheel diameters, lift heights, and many fitment measurements in North America are typically given in inches. Displaying both units makes it easier to compare specifications and avoid conversion errors.
How accurate are these calculations?
The results are based on the standard P-metric geometry formulas used throughout the tire industry and represent nominal dimensions. Actual mounted tire dimensions can vary slightly depending on the manufacturer, tread design, approved rim width, and inflation pressure. For critical fitment decisions, always verify dimensions against the tire manufacturer's published specifications.
Can I use this for LT or flotation tires?
This calculator works for any tire that uses the standard metric sizing format, including LT-metric tires such as LT265/75R16. However, flotation sizes such as 35×12.50R17 use a different sizing system and are not currently supported.
How do I calculate tire diameter manually?
First, calculate the sidewall height by multiplying the tire width by the aspect ratio. Then multiply the sidewall height by two and add the wheel diameter converted to millimeters. For example, a 225/55R17 tire has a sidewall height of 123.75 mm and an overall diameter of approximately 679.3 mm or 26.74 inches.
What happens if I increase tire diameter?
Increasing tire diameter raises ground clearance and changes your vehicle's effective gearing. A larger tire covers more distance per revolution, causing the speedometer to read lower than your actual speed. Larger tires can also affect acceleration, fuel economy, and wheel well clearance.
Find the Right Tire Size with Confidence
Whether you're checking speedometer accuracy, planning a plus-size wheel upgrade, comparing replacement tires, or verifying clearance before installing larger tires, knowing your tire's overall diameter is essential.
Use the tire diameter calculator above to quickly determine overall height, sidewall size, circumference, and revolutions per mile for virtually any P-metric or LT-metric tire size.
If you're considering changing tire sizes, always verify fitment with your vehicle manufacturer and try to stay within ±3% of your original tire diameter to avoid unwanted speedometer and drivability issues.