Wheel offset calculator

Compare a reference wheel with another width and offset to see the theoretical change at the inner and outer rim edges.

Formulaouter position = wheel width ÷ 2 − offset

Width is split around the centerline

Offset moves the mounting face away from the wheel centerline. The calculator converts wheel width to millimeters, then compares the inner and outer half-widths after offset.

Rim position is not a fitment verdict

Tire section width, spoke and brake clearance, suspension travel, alignment, bodywork and manufacturer tolerances are not represented by two wheel dimensions.

A worked example

Take an 8-inch wheel at ET45 and compare it with a 9-inch wheel at ET35. The half-widths are 101.6 mm and 114.3 mm. Adding the offsets puts the inner edge at 146.6 mm and 149.3 mm, so the new wheel sits 2.7 mm closer to the suspension. Subtracting them puts the outer edge at 56.6 mm and 79.3 mm, so it stands 22.7 mm further out.

That is the pattern to watch: an extra inch of width with ten millimetres less offset barely moves inward but sticks out by nearly an inch. Whether that clears the arch is a question for a tape measure and full suspension travel, not for the arithmetic.

Frequently asked questions

What does ET mean on a wheel?

It is the German Einpresstiefe, insertion depth, and it is the offset in millimetres. ET45 means the mounting face sits 45 mm outboard of the wheel’s centreline. ET0 puts it exactly on the centreline, and a negative number, common on deep-dish and off-road wheels, puts it inboard.

Which way does a lower offset move the wheel?

Outwards. Reducing the offset moves the mounting face towards the middle of the car, which pushes the rim further into the arch opening. Going from ET45 to ET35 on the same width moves the whole wheel out by 10 mm, gaining that much clearance to the strut and losing that much to the bodywork.

How is offset different from backspacing?

They describe the same geometry from opposite ends. Offset is measured in millimetres from the centreline; backspacing is measured in inches from the inner flange to the mounting face. Backspacing also uses the overall rim width, which is about an inch more than the quoted width because it includes both flanges, which is why converting between the two catches people out.

Do spacers change the offset?

Effectively, yes. A 15 mm spacer on an ET45 wheel positions it like an ET30 wheel, so the arithmetic here works if you subtract the spacer thickness from the offset. What a spacer does not change is the load path: hub-centric spacers with correctly rated longer studs or bolts are a different proposition from cheap slip-on plates, and some jurisdictions regulate them.

Why does the tire stick out further than the calculation says?

Because this compares rim edges, and the tire is wider than the rim. Section width is measured at the widest point of the bulge, and mounting a wide tire on a narrow rim makes that bulge more pronounced. For arch clearance, add the tire’s section width and check at full lock and full compression, where the wheel is closest to everything.