How overall wheel size is calculated
The sidewall height is the section width multiplied by the aspect ratio, divided by 100. Overall diameter is the rim diameter in millimetres plus two sidewalls, because the tyre sits on both sides of the rim.
Revolutions per mile = 1,609,344 mm divided by the circumference, so a larger wheel turns fewer times over the same distance.
Worked example
For 225/45 R17 the sidewall is 101.3 mm, so the diameter is 431.8 + 202.5 = 634.3 mm. That is 24.97 inches. The rolling circumference is the diameter multiplied by pi, which gives 1,993 mm, and the wheel turns 808 times per mile.
These figures come from the nominal size. A worn or under-inflated tyre will be slightly smaller in practice.
Why the inch figure matters
Tyre and wheel sizes are often discussed in inches, especially for off-road, classic and American vehicles where a 33 inch tyre or a 20 inch wheel is the normal shorthand. Converting the metric sidewall code into an overall inch diameter makes those comparisons possible.
The inch figure is the overall diameter, not the rim size. A 17 inch wheel with a 225/45 tyre has an overall diameter of about 25 inches, which is the number that matters for gearing, clearance and the speedometer.
Nominal size versus real size
Size codes are nominal. Real diameters vary with manufacturing tolerance, tread depth and inflation, and a worn tyre is measurably smaller than a new one of the same size.
If you need an accurate figure for a speedometer correction, measure the wheel across its centre with the car on level ground and the tyres at the specified pressure.
Limits of this calculator
It reports dimensions from the code you enter. It does not check rim width suitability, load index, speed rating, brake clearance or whether the size is approved for your vehicle.
To compare two sizes and see the speedometer error between them, use the tyre size calculator. For the dial reading itself, use the speedometer calculator.
To compare two sizes and their speedometer error, use the UK tyre size calculator.
For US spelling and tire terminology, use the US wheel size calculator.
Inputs for the wheel size calculator
Rim diameter
The figure after R on the tyre sidewall, in inches. Half-inch sizes such as 16.5 are accepted.
Tyre section width
The first number on the sidewall in millimetres, such as 225 in 225/45 R17.
Aspect ratio
The second number, the sidewall height as a percentage of the section width.
CALCULATOR QUESTIONS
Wheel size calculator UK questions
Is the wheel size the same as the rim size?
No. The rim size is the metal wheel, such as 17 inches. The wheel size in common use usually means the overall diameter including the tyre.
Why is my tyre 25 inches when the wheel is 17?
The tyre adds a sidewall on each side of the rim. Two 101 mm sidewalls add about 203 mm to a 432 mm rim.
Does circumference affect the speedometer?
Yes. The speedometer counts revolutions, so any change in circumference changes the speed shown for a given road speed.
Can I use this to choose a tyre?
It gives the dimensions, but not fitment. Check rim width, load index, speed rating and vehicle approval separately.