What is the difference between wheel offset and backspacing?

They measure the same thing from opposite ends. Offset is the distance from the hub mounting face to the wheel centreline in millimetres; backspacing is the distance from that face to the inboard lip in inches. Given the width, either converts to the other exactly.

How do I convert offset to backspacing?

Backspacing equals half the overall width plus the offset in inches: (width + 1) ÷ 2 + ET ÷ 25.4. The extra inch accounts for the bead flanges on both sides of the rim.

Does a lower offset make wheels stick out?

Yes. Lower offset moves the mounting face inboard relative to the centreline, so the wheel sits further toward the fender. Every 10 mm of offset reduction is 10 mm of extra poke.

How much offset change is safe?

There is no universal figure — it depends on your fender clearance, suspension travel and brake package. As a guide, changes beyond about 25 mm in either direction are worth a test fit at full lock and full compression before buying tyres.

Do wheel spacers change offset?

Effectively, yes. A spacer of X millimetres reduces the effective offset by X, moving the whole wheel outboard by that amount while leaving backspacing measured from the new mounting surface unchanged.

Wheel Offset Calculator

Convert offset to backspacing, add spacers, and see exactly how far a new wheel pokes out or tucks in.

The Wheel Offset Calculator runs entirely in your browser. Wheel dimensions you enter are converted on your device and never uploaded.

Open the Tire Size Calculator

About Wheel Offset Calculator

Wheels are specified two different ways depending on where you shop. Metric wheels quote offset in millimetres — the distance from the hub mounting face to the wheel's centreline — while American wheels quote backspacing in inches, measured from the mounting face to the inboard lip. The bridge between them is overall width, which is the quoted width plus one inch of bead flange, and forgetting that inch is how most fitment maths goes wrong. This calculator converts either way, applies a spacer if you are running one, and then answers the question that actually matters: compared with the wheels already on the car, how much further out does the new set sit, and how much closer to the strut?

Features

Offset to backspacing and backspacing to offset, in one place
Adds the bead flange automatically, so overall width is right
Spacer thickness folded into the effective offset
Side-by-side comparison of an old and a new wheel
Reports outer poke and inner clearance change in millimetres and inches
Warns when a change is large enough to deserve a test fit
Plain-language explanation of positive, zero and negative offset
Copyable summary of the whole fitment

How to use the Wheel Offset Calculator

Enter the wheel width in inches and the offset in millimetres
Read the backspacing, frontspacing and overall width
Add a spacer thickness if you plan to run one
Switch to compare mode and enter the new wheel to see how the fitment moves

Example

Input

Old: 8.5J ET35   New: 9.5J ET20

Output

Outer face: +27.7 mm (1.09 in)
Inner lip: -2.3 mm (-0.09 in)

A wider wheel with less offset gains almost all of its width on the outside — check the fender before ordering tyres.

Common errors & troubleshooting

Backspacing works out half an inch off the catalogue figure. — Overall width is the quoted width plus one inch, because there is half an inch of flange at each bead seat. An 8.5J wheel measures 9.5 inches across the lips.
The new wheels rub the strut instead of the fender. — You gained inner reach, not poke. Compare backspacing rather than offset: a wider wheel with the same offset grows equally in both directions.
A spacer seems to make the offset larger. — It makes it smaller. Spacers push the wheel outboard, and moving outboard is arithmetically a reduction in offset — a 15 mm spacer turns ET35 into an effective ET20.

Frequently asked questions

What is the difference between wheel offset and backspacing?: They measure the same thing from opposite ends. Offset is the distance from the hub mounting face to the wheel centreline in millimetres; backspacing is the distance from that face to the inboard lip in inches. Given the width, either converts to the other exactly.
How do I convert offset to backspacing?: Backspacing equals half the overall width plus the offset in inches: (width + 1) ÷ 2 + ET ÷ 25.4. The extra inch accounts for the bead flanges on both sides of the rim.
Does a lower offset make wheels stick out?: Yes. Lower offset moves the mounting face inboard relative to the centreline, so the wheel sits further toward the fender. Every 10 mm of offset reduction is 10 mm of extra poke.
How much offset change is safe?: There is no universal figure — it depends on your fender clearance, suspension travel and brake package. As a guide, changes beyond about 25 mm in either direction are worth a test fit at full lock and full compression before buying tyres.
Do wheel spacers change offset?: Effectively, yes. A spacer of X millimetres reduces the effective offset by X, moving the whole wheel outboard by that amount while leaving backspacing measured from the new mounting surface unchanged.

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