What offset and backspace mean
Offset, marked ET on the wheel, is the distance in millimetres from the wheel's centre line to its mounting face. A positive offset means the mounting face is outboard of the centre, which is normal on most modern cars. A lower ET pushes the whole wheel further out of the arch.
Backspace is the distance from the inner edge of the rim to the mounting face. It equals half the rim width plus the offset, so it grows with both a wider rim and a higher offset. Backspace is what usually decides whether a wheel touches the suspension.
Worked example
A 7.5 inch wide wheel with ET45 becomes an 8 inch wide wheel with ET40. Half the width grows from 95.3 to 101.6 mm, while the offset drops by 5 mm. The inner edge ends up 11.4 mm closer to the suspension and the outer edge sits 1.4 mm further out, so the extra width goes mostly inboard. Backspace rises from 140.3 to 141.6 mm.
These figures are illustrative. Measure your own clearances before buying, because a calculation cannot see a brake caliper or a suspension strut.
Why offset matters beyond looks
Changing offset changes the scrub radius, which affects steering feel, tramlining and how the car behaves under braking. It also changes the load path through the wheel bearings and hub, and a large change can accelerate wear.
A wider track can put the tyre outside the bodywork, which raises legal and insurance questions in the UK. Check that the tyre remains within the arch and that the insurer accepts the change.
Measuring clearance
Turn the steering to full lock both ways and check the gap between the tyre and the inner arch, the suspension strut and any brake components. Compress the suspension or load the car to check the arch lip at full bump.
The inner edge figure from this calculator tells you which way the wheel moves. It does not tell you how much room you already have, so measure that gap on the car first.
Limits of this calculator
It models rim width and offset only. It ignores the tyre's section width, which can be wider than the rim and is often what actually rubs, and it ignores flange thickness, spoke profile and brake caliper shape.
For the overall diameter and speedometer effect of a tyre change, use the wheel size calculator or the tyre size calculator.
For overall wheel diameter and circumference, use the UK wheel size calculator.
For US spelling and tire terminology, use the US wheel offset calculator.
Inputs for the wheel offset calculator
Fitted wheel width
Rim width in inches, usually stamped on the wheel as 7.5J rather than measured across the outside.
Fitted offset
The ET figure in millimetres. Positive means the mounting face is outboard of the centre line.
New wheel width
Rim width of the wheel you are considering, in inches.
New offset
The ET figure of the new wheel in millimetres, taken from its stamping or specification.
CALCULATOR QUESTIONS
Wheel offset calculator UK questions
Does a lower offset push the wheel out?
Yes. Reducing ET moves the mounting face inboard relative to the rim, so the whole wheel sits further out of the arch.
Is a wider wheel always further out?
No. Half the extra width goes to each side. Keeping the same ET moves the outer edge out by half the width increase and the inner edge in by the same amount.
Can I change offset without changing width?
Yes, and the effect is simpler: the whole wheel moves by the difference in ET, in or out.
Will this tell me if the wheel fits?
No. It tells you which way and how far the edges move. You still need to measure clearance to the suspension, brakes and arches.