Tire Diameter Calculator
Find a tire's overall diameter in inches.
Results update as you type.
How it works
The overall diameter of a tyre decides whether it fits your car and whether your speedometer stays accurate, and this tire diameter calculator computes it straight from the numbers on the sidewall. Enter the section width in millimetres, the aspect ratio as a percentage, and the wheel diameter in inches, and it returns the tyre's overall diameter in inches, the single dimension that matters most when comparing sizes. That figure carries a lot of weight. Overall diameter sets how tall the tyre stands, which affects ground clearance, fender gap, and the effective gearing that determines how hard your engine works at a given speed. It also drives speedometer and odometer accuracy, because those instruments assume a specific rolling distance per revolution. Fit a tyre of a different diameter and every reported speed and distance shifts, subtly but consistently. The calculation follows the standard P-metric convention that every tyre size encodes. The first number is the width, the middle number is the aspect ratio describing sidewall height as a percentage of that width, and the last number is the wheel diameter. Multiplying width by aspect ratio gives the sidewall, and adding one at the top and one at the bottom of the wheel yields the overall diameter. Whether you are plus-sizing wheels, fitting a taller all-terrain tyre, or simply checking that a replacement matches your originals, seeing the diameter lets you keep handling and instrument accuracy intact rather than discovering a mismatch after the tyres are mounted.
diameter = wheel + 2 × (width × aspect ÷ 100) ÷ 25.4
A tyre's overall diameter is the wheel it wraps plus a sidewall on the top and bottom. The sidewall height in millimetres is the section width multiplied by the aspect ratio percentage, because aspect ratio is defined as the sidewall expressed as a share of width. Dividing that height by 25.4 converts millimetres to inches, and doubling it accounts for both sidewalls before adding the wheel's own diameter, which is already given in inches.
Source:Standard P-metric tire sizing
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter section width, aspect ratio and wheel diameter.
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
Overall diameter updates instantly as you type — no button to press.
Frequently asked questions
How do I read the numbers in a tire size?
In a size like 225/45R17, the 225 is the section width in millimetres, the 45 is the aspect ratio meaning the sidewall is 45 percent of the width, the R marks radial construction, and the 17 is the wheel diameter in inches. Enter the width, aspect ratio, and wheel diameter into this calculator and it returns the tyre's overall diameter, the figure you actually need for fitment.
Why does overall diameter matter for my speedometer?
Your speedometer and odometer are calibrated to the rolling distance of the original tyre, which depends on its diameter. A larger diameter travels farther per revolution, making the speedometer read low and the odometer undercount, while a smaller diameter does the opposite. Keeping the new diameter within a few percent of the original preserves accuracy and avoids confusing the electronic systems that rely on wheel speed data.
How much can overall diameter safely change?
A common guideline is to keep the overall diameter within about three percent of the original so speedometer error stays small and clearance is preserved. Larger jumps can cause rubbing against the fender or suspension, alter gearing noticeably, and throw off speed readings. When plus-sizing, people deliberately pair a wider tyre with a lower aspect ratio and larger wheel to keep the overall diameter close to stock.
Does a lower aspect ratio always mean a smaller diameter?
Not necessarily, because diameter depends on width, aspect ratio, and wheel size together. A lower aspect ratio shortens the sidewall, but if it is paired with a larger wheel or wider tyre, the overall diameter can stay the same or even grow. That is exactly how plus-sizing works: a bigger wheel with a shorter sidewall keeps the overall diameter constant while changing the look and handling feel.
Why convert the sidewall from millimetres to inches?
Tyre widths and sidewall heights are specified in millimetres, but wheel diameters and overall tyre diameters are conventionally quoted in inches in many markets. To add the sidewall to the wheel, both must share a unit, so the sidewall height is divided by 25.4, the number of millimetres in an inch. This mixed convention is why the calculation looks slightly awkward, but it produces a diameter in the inches that fitment charts use.