Wheel Offset Calculator

backspacing = rim width ÷ 2 + offset; front spacing = rim width − backspacing

currentWidth

Existing rim width, in inches

currentOffset

Existing offset (ET), in millimetres

newWidth

New rim width, in inches

newOffset

New offset (ET), in millimetres

Use it when you are choosing a wheel and want to know how far it will move relative to the one on the car, or when a wheel is quoted in the notation you do not think in — backspacing from an American seller, ET from a European one. The change figures are the useful half: run your current wheel against itself first, then against the candidate, and compare the two inner and outer numbers. If you only want the backspacing of a given width and offset, the chart below has eleven offsets across four rim widths already worked out.

1. A 7.5 inch ET 42 wheel replaced by a 9 inch ET 30

   1. Current backspacing: 7.5 ÷ 2 = 3.75 in, plus 42 mm of offset, which is 1.65 in — 5.40 in
   2. New backspacing: 9 ÷ 2 = 4.50 in, plus 30 mm, which is 1.18 in — 5.68 in
   3. Inner edge: the mounting face is now 5.68 − 5.40 = 0.276 in further from the inner edge, so 0.276 × 25.4 = 7.0 mm less inner clearance
   4. Outer edge: front spacing was 7.50 − 5.40 = 2.10 in and is now 9.00 − 5.68 = 3.32 in
   5. That is 3.32 − 2.10 = 1.22 in further out, or 31.1 mm

   The outer edge moved 31.1 mm while the offset changed only 12 mm, and the missing 19.05 mm is exactly half of the 1.5 inch the rim grew. Offset decides where the wheel sits relative to its own centreline; width decides how far the edges are from that centreline. A change of wheel moves each edge by the sum of the two, and quoting only the offset change understates a wider wheel by roughly half.

2. An 8 inch ET 35 wheel replaced by a 9 inch ET 45

   1. Current backspacing: 4.00 + 1.38 = 5.38 in
   2. New backspacing: 4.50 + 1.77 = 6.27 in
   3. Inner edge: 0.892 in closer to the suspension, which is 22.7 mm less clearance
   4. Outer edge: front spacing goes from 2.62 in to 2.73 in, so the outer edge moves out by only 2.7 mm

   The mirror image of the first example, and the reason both outputs are given. Here the wheel is an inch wider and the offset is 10 mm larger, so the extra width goes inboard: the outer edge barely moves while the inner edge loses 22.7 mm. Fitting this wheel is the opposite risk — it is the inside that will touch a strut or a brake caliper, not the arch.

3. The same wheel in and out: 8 inch ET 0 to 8 inch ET 0

   1. Offset 0 puts the mounting face exactly on the centreline, so backspacing is half the width: 8 ÷ 2 = 4.00 in
   2. Nothing changed, so both edges move by 0.0 mm

   Worth running once as a check on yourself: enter the same wheel twice and both change figures must come out at zero. If they do not, one of the four fields has been swapped or the offset sign has been flipped — and a sign error in the offset field is easy to make, because a positive ET moves the wheel inboard while a negative one moves it out.

This is geometry, and nothing else. Fitment — whether the wheel actually fits the car — depends on the shape of the arch, the position of the strut, the size of the brake caliper and the steering angle at full lock, and none of them is an input here. Two cars that take the same bolt pattern and the same nominal wheel can have completely different clearances. The changing inner clearance figure is a comparison against the wheel currently fitted and assumes that wheel fits with the amount of room you believe it has; if the baseline is already rubbing, everything here is relative to a bad starting point. Rim width is the width between the beads, not the overall width of the wheel, and the offset is stamped on the wheel as ET followed by millimetres — check both rather than trusting a listing, since the same model of wheel comes in several offsets. Tyre width is not accounted for: a wider tire on the same rim adds to the outer edge by roughly half the width increase, and that is often what actually rubs.

What is the difference between offset and backspacing?

Offset is measured to the centreline of the rim and is quoted in millimetres with a sign; backspacing is measured to the inner edge and is quoted in inches. One is a property of the wheel, the other is what a tape measure finds when the wheel is off the car. The conversion is backspacing = width ÷ 2 + offset, with the offset converted to inches first — a 9 inch ET 30 wheel is 5.68 inches of backspacing.

Why is offset in millimetres and backspacing in inches?

Because they come from two different markets. Offset is a European convention and every wheel stamped ET is in millimetres; backspacing is an American one, and American wheel sellers quote inches. Rim width is in inches on both sides, which is the trap: the two terms in the formula are in different units, and adding 42 to 3.75 gives 45.75 instead of the 5.40 inches the wheel actually has.

How much further out does a wider wheel sit?

Half the width increase, plus the change in offset. Going from a 7.5 inch to a 9 inch rim moves each edge out by 0.75 inch, or 19.05 mm, before the offset is considered at all; reducing the offset from ET 42 to ET 30 adds another 12 mm outward. The total is 31.1 mm, and quoting the 12 mm alone understates it by nearly two thirds.

What does a negative offset mean?

That the mounting face is inboard of the centreline, so more of the wheel sits outside the hub face and the wheel sticks out. Deep-dish and wide-body wheels are negative; most factory front-wheel-drive cars are between ET 35 and ET 50. Going from ET -12 to ET -25 moves the wheel 13 mm further out with the same rim width, and both edges move by that same 13 mm because the width did not change.

Will a 9 inch wheel with ET 30 fit my car?

This page can tell you that its inner edge sits 7 mm further out and its outer edge 31.1 mm further out than the 7.5 inch ET 42 wheel you have now, but not whether that clears. Fitment is decided by the car rather than the wheel — arch shape, strut position, caliper size and steering angle all have a say — so use the change figures to judge against the clearance you can see, and measure before buying. The tool removes the arithmetic, not the tape measure.

• NIST Guide to the SI, Special Publication 811 — US National Institute of Standards and Technology
• The International System of Units (SI Brochure) — Bureau International des Poids et Mesures