Wheel Offset & Fitment Calculator

How offset & backspacing work

Offset (ET) is the distance from the wheel's centreline to its mounting face. A higher ET tucks the wheel inward; a lower or negative ET pushes it out (poke). Backspacing is the distance from the mounting face to the inner lip:

backspacing = (width × 25.4 ÷ 2) + ET

Going wider or dropping the offset moves the outer lip out (more poke / fender risk) and the inner lip in (closer to struts, brakes and arms). This tool shows both deltas so you can judge clearance before you buy.

ET offset chart

The chart people mean when they search for one. Both tables are generated by running the calculator above, so a row here cannot disagree with the tool.

ET to backspacing chart

Backspacing in millimetres for each rim width. Offset and backspacing describe the same thing from opposite edges, which is why a single ET number means a different backspacing on every width.

What each wheel does, against a stock 8J ET45

Positive inner means the wheel moves toward the strut and coilover; positive poke means it moves out toward the arch lip. Both in millimetres.

ET40 versus ET−40: the 80 mm question

A minus sign in front of an offset is not a small change. Offset is measured from the wheel's centreline to its mounting face, so ET40 and ET−40 are 80 mm apart — the same wheel would sit four centimetres further out of the arch on the negative one. That is deep-dish, stanced, tyre-past-the-fender territory, not a fitment nudge.

Positive offset means the mounting face sits outboard of the centreline, pulling the wheel in under the car. That is what nearly every modern front-drive and transverse-engined car uses, typically ET35 to ET50. Negative offset puts the face inboard and throws the wheel out; you mostly see it on trucks, off-roaders and deliberately aggressive fitments. Zero means the face is exactly on the centreline.

Width moves things too, and only half as much per side. Going from an 8J ET45 to a 9J ET35 gains 22.7 mm of poke: 10 mm from the offset change plus half of the extra inch of width. That is why you cannot judge a wheel from its ET alone — a 9J ET35 and an 8J ET45 have completely different outer edges despite being "only 10 mm apart" on paper.

The two clearances that actually bite

Every fitment problem is one of two edges, and they move in opposite directions.

• The inner edge is the one that costs money. It moves toward the strut body, the spring perch, the brake caliper and the suspension arms — and unlike the outer edge, you find out at full lock or full compression, not in the car park. Going wider or raising the offset both bring it in.
• The outer edge is the one you can see. It moves toward the arch lip, and it is what people mean by poke. Too much and the tyre catches the lip over bumps, throws water down the side of the car, and in some countries fails an inspection for sitting proud of the bodywork.

The usual mistake is to solve an outer-edge problem by raising offset, and quietly create an inner-edge problem that only appears on a bumpy corner with the steering wound on. Check both numbers in the tool before you buy, and remember it cannot see your brakes — measure the caliper clearance yourself.

Spacers, and what they really change

A spacer subtracts from offset, one millimetre for one millimetre. A 15 mm spacer on an ET45 wheel behaves exactly like an ET30 wheel: same outer edge, same inner clearance, same everything. That makes spacers a legitimate fine-tuning tool — you can correct a wheel that tucks 10 mm too far without buying different wheels.

Two things they do not change. They do not alter the bolt pattern, so a spacer will never make a 5×112 wheel fit a 5×114.3 hub — that needs an adapter, which is a different and more serious component. And they push the wheel away from the hub face, so anything over about 5 mm needs longer studs or bolts to keep full thread engagement. Check the pattern first with the bolt pattern calculator.

The things the offset number cannot tell you

A wheel can have perfect offset and still be wrong. Before you buy, check the centre bore — the wheel's centre hole must be equal to or larger than your hub's spigot, with a hub-centric ring to take up the difference, or the wheel is carried on its bolts alone and will vibrate. Check the load rating, especially on a heavy car or an SUV. Check the face and spoke profile, because a concave spoke can foul a big brake caliper that a flat one clears at the same offset. And check the tyre you plan to fit, since a stretched or a bulging sidewall moves the widest point of the assembly well away from the rim edge this tool measures.

FAQ