Wheel Offset Calculator: ET and Backspacing
What does a different wheel offset do to fitment? Enter your current wheel width and ET offset and the wheel you want to fit, and the calculator gives the backspacing for both and how many millimetres of inner and outer clearance change. Positive changes mean more room than your current setup. To compare tire sizes for the new wheel, use the tire size calculator.
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Formula
- Width in mm =
inches × 25.4; centerline =width ÷ 2 - Backspacing =
width ÷ 2 + ET; outside depth =width − backspacing - Inner clearance change = old backspacing − new backspacing
- Outer clearance change = old outside depth − new outside depth
Positive means more room than the current wheel.
Worked example
8 in ET35: width 203.2 mm, backspacing 136.6 mm (5.38 in), outside 66.6 mm. New 9 in ET20: width 228.6 mm, backspacing 134.3 mm, outside 94.3 mm.
Inner clearance +2.3 mm (+0.09 in); outer clearance −27.7 mm (−1.09 in).
Second example: 8 in ET45 to 9 in ET35 gives −2.7 mm inside and −22.7 mm outside.
Use this when
You are choosing aftermarket wheels and want to know whether they will rub the suspension or fender. ET is stamped on the wheel, and the numbers matter most when you change width and offset together. Typical passenger-car offsets are roughly ET30 to ET50, but the right value depends on the car.
Wheel offset vs tire size
This page covers where the wheel sits. The tire size calculator covers how tall and wide the tire is. A wider wheel with a lower offset moves the whole assembly outward; a taller tire changes speedometer reading and ride height without moving it sideways.
Common mistakes
- Confusing offset with backspacing. Offset is measured from the centerline, backspacing from the back of the wheel.
- Treating ET as the same sign convention everywhere: a larger positive ET pulls the wheel inward.
- Mixing millimetres and inches.
- Skipping the other fitment checks, such as brake clearance, bolt pattern and tire width.
Frequently asked questions
What is wheel offset (ET)?
Offset is the distance in millimetres from the wheel's centerline to the surface that bolts to the hub. Positive ET tucks the wheel inward, negative ET pushes it outward.
How do I convert offset to backspacing?
Backspacing = half the width + ET, in the same unit. An 8 inch wheel is 203.2 mm wide, so with ET35: 101.6 + 35 = 136.6 mm, or 5.38 inches.
How do I compare two wheels?
Subtract backspacings for the inside and outside depths for the outside. Going from 8 in ET35 to 9 in ET20 gives 2.3 mm more room inside and 27.7 mm less room outside.
Is this enough to know a wheel will fit?
No. Brake calipers, suspension parts, tyre size, fender shape and bolt pattern also matter. Use the numbers as a first check and confirm with the wheel maker or a fitter.
How do I convert ET offset to backspacing?
Add half the wheel width to the offset, in the same unit. An 8 inch wheel at ET45 has a backspacing of 101.6 + 45 = 146.6 mm, or 5.77 inches. Positive ET moves the mounting face outward, so backspacing grows.
What happens when I change from 8 in ET45 to 9 in ET35?
The wheel gets 25.4 mm wider and the offset drops 10 mm. You lose 2.7 mm of clearance on the inside and 22.7 mm on the outside, so the wheel sits further out by about 0.89 inches.
How much does one extra inch of width change fitment at the same ET?
The extra 25.4 mm of width is split evenly, about 12.7 mm each way. Going from 7 in ET40 to 8 in ET40 reduces clearance by 12.7 mm (0.5 in) on both the inside and the outside.