Tire Size Calculator
Decode a P-metric tire size.
Results update as you type.
How it works
The code stamped on a tyre's sidewall hides its real dimensions, and this tire size calculator translates it into the numbers that actually matter when fitting new rubber. Enter the section width in millimetres, the aspect ratio as a percentage, and the rim diameter in inches, and it returns the overall diameter, the sidewall height, and how many times the tyre turns in a mile. Those figures decide whether a tyre will fit and whether your speedometer stays honest. Overall diameter affects ride height, clearance, and gearing, while revolutions per mile drive the speedometer and odometer readings. Fit a tyre with a different diameter and every speed and distance reading shifts, which is why comparing a prospective size against your current one before buying is so important. The math follows the standard P-metric convention. Aspect ratio expresses sidewall height as a percentage of section width, so multiplying the two gives the sidewall in millimetres. The wheel contributes its rim diameter, converted from inches, and the tyre adds a sidewall at the top and bottom, producing the overall diameter. From there the circumference sets the revolutions per mile. Whether you are plus-sizing wheels, chasing a specific look, or restoring factory gearing, seeing these numbers side by side lets you choose a replacement that keeps handling and instrument accuracy intact.
diameter = rim × 25.4 + 2 × (width × aspect ÷ 100)
A P-metric size like 225/45R17 packs three numbers. Sidewall height in millimetres is the section width times the aspect-ratio percentage, since aspect ratio is the sidewall expressed as a share of width. Overall diameter is the rim diameter converted to millimetres plus two sidewalls, one at the top and one at the bottom. Dividing 63,360 inches per mile by the tyre's circumference gives revolutions per mile.
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 rim 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 a tire size like 225/45R17?
The first number, 225, is the section width in millimetres. The second, 45, is the aspect ratio, meaning the sidewall height is 45 percent of the width. The letter R indicates radial construction, and the final number, 17, is the wheel rim diameter in inches. Enter the width, aspect ratio, and rim diameter into this calculator to see the tyre's real overall dimensions.
Why does overall tire diameter matter?
Overall diameter determines ride height, ground clearance, and effective gearing, and it directly affects speedometer accuracy. A larger diameter raises the vehicle and makes the speedometer read slightly slow, while a smaller one does the opposite. When replacing tyres, keeping the overall diameter close to the original preserves handling, instrument accuracy, and the electronic systems that rely on wheel speed, such as ABS and traction control.
What are revolutions per mile used for?
Revolutions per mile counts how many full turns a tyre makes to cover one mile, derived from its circumference. Speedometers and odometers are calibrated to a specific value, so a tyre with a different count will make those readings inaccurate. It also matters for matching tyres across an axle and for setups that read wheel speed, since mismatched revolutions can confuse the vehicle's electronics.
Can I safely change my tire size?
Modest changes are common, especially plus-sizing where a wider tyre on a larger rim keeps the overall diameter similar. The key is to hold overall diameter close to the original, ideally within about three percent, so speedometer accuracy and clearance are preserved. Always confirm the new size fits the wheel well without rubbing and matches the load and speed ratings your vehicle requires before fitting.
Does aspect ratio affect ride quality?
Yes. A lower aspect ratio means a shorter, stiffer sidewall, which sharpens steering response and cornering but transmits more road bumps into the cabin. A higher aspect ratio gives a taller, more cushioned sidewall that smooths the ride and better resists pothole damage. Choosing an aspect ratio is a trade-off between handling precision and comfort, alongside its effect on the tyre's overall diameter.