Tire size calculator

Turn width, aspect ratio and rim diameter into theoretical unloaded dimensions.

Formuladiameter = rim inches × 25.4 + 2 × width × aspect ÷ 100

What the label describes

For 225/45R17, the sidewall is 45% of 225 mm. Two sidewalls plus the 17-inch rim produce the theoretical overall diameter.

Label geometry is not a fitment check

Actual mounted dimensions, approved rim widths, load capacity, clearance and compatibility come from the tire, wheel and vehicle manufacturers.

Reading 225/45R17 end to end

The 225 is the section width in millimetres. The 45 is the aspect ratio, meaning the sidewall stands 45% of that width, so 101.3 mm. R marks radial construction, and 17 is the rim diameter in inches, the one measurement on the label that is not metric.

Two sidewalls plus the rim give an overall diameter of 634.3 mm, which is 24.97 inches, a circumference of 78.45 inches and about 808 revolutions per mile. Anything printed after the size, such as 91V, is the load index and speed rating, and those come from the manufacturer rather than from geometry.

Frequently asked questions

What do the numbers on a tire actually mean?

In 225/45R17: width 225 mm across the section, sidewall height 45% of that width, radial construction, and a 17-inch rim. Because the sidewall is a percentage rather than a fixed figure, widening a tire while keeping the same aspect ratio also makes it taller, which is why the two numbers cannot be changed independently.

Why is my measured diameter smaller than the calculated one?

Because the calculation describes an unloaded tire and a mounted one is squashed by the weight of the car. The loaded radius is typically a couple of percent smaller, and it moves with pressure, load and wear. Rolling circumference, the distance actually covered per revolution, sits between the two and is what manufacturers publish for accurate work.

What is a P, LT or ZR prefix on the label?

P marks a US passenger-car tire and LT a light-truck tire built for higher loads and pressures. ZR indicates a speed rating above 240 km/h. None of them change this arithmetic, but they do change what the tire is rated to carry, which is a manufacturer question rather than a geometric one.

How do I convert an old-style size like 31x10.50R15?

That format states the dimensions directly: about 31 inches overall diameter, 10.5 inches wide, on a 15-inch rim. There is no aspect ratio to apply, so this calculator, which expects the metric form, is not the right tool for it. To compare the two systems, calculate the metric size here and set the result against the inches printed on the flotation size.

Can I use this to check whether a size will fit my car?

No, and that is worth stating plainly. Geometry tells you how tall and how far around a tire is. Fitment depends on approved rim widths, arch and suspension clearance at full lock and full droop, load and speed ratings, and how the vehicle’s ABS and stability systems were calibrated. Use the placard on the door frame and the manufacturer’s list.