Wheel Offset and Backspacing Explained
Updated 8 November 2026. Every figure on this page comes from the same formulas as our calculators.
Offset and backspacing both describe where a wheel sits relative to the hub. Getting them right is what decides whether a new wheel clears the suspension on the inside and stays under the fender on the outside.
Offset (ET)
Offset is the distance in millimetres from the centre line of the wheel to the face that bolts against the hub. European wheels write it as ET, so a wheel marked 7Jx17 ET45 is 7 inches wide, 17 inches in diameter, with a +45 mm offset (the J describes the shape of the rim flange).
- Positive offset: the mounting face is towards the outside of the wheel, so the wheel sits further in, under the car. Most cars use positive offsets.
- Zero offset: the mounting face is exactly in the middle.
- Negative offset: the mounting face is towards the inside, so the wheel sticks out further. Common on lifted trucks for a wider stance.
Lower offset means further out: going from ET45 to ET40 moves the whole wheel 5 mm towards the fender.
Backspacing
Backspacing is the distance from the mounting face to the inner edge of the wheel, usually given in inches. It tells you directly how much room the wheel needs on the inside. It depends on both the width and the offset:
Backspacing = (wheel width + 1 in) ÷ 2 + offset in inches
The extra inch is because the nominal width is measured inside the rim flanges; the outside of the wheel is roughly half an inch wider on each side. For a 7x17 ET45: (7 + 1) ÷ 2 + 45 ÷ 25.4 = 4 + 1.77 = 5.77 in.
Example: a wider wheel with a lower offset
Changing from a 7x17 ET45 to an 8x17 ET40:
- The extra inch of width (25.4 mm) is split between the two sides: 12.7 mm each.
- The 5 mm lower offset moves the whole wheel 5 mm out.
- Inner edge: 12.7 − 5 = 7.7 mm (0.30 in) closer to the suspension.
- Outer edge: 12.7 + 5 = 17.7 mm (0.70 in) further out towards the fender.
- Backspacing goes from 5.77 in to 6.07 in.
Open this example in the offset calculator.
Example: two truck wheels
A common question on truck forums is whether to choose a 17x8.5 with 0 offset or a 17x9 with −12 offset. Compared with the 8.5 +0:
- The 9 −12 sits 18.4 mm (0.72 in) further out at the outer lip.
- Its inner edge is 5.6 mm (0.22 in) further from the suspension, because the 12 mm lower offset more than cancels the extra quarter inch of width on the inside.
- Backspacing is 4.75 in for the 8.5 +0 and 4.53 in for the 9 −12.
Open this example in the offset calculator.
Spacers change the effective offset
A wheel spacer pushes the wheel out by its own thickness, which is the same as lowering the offset by that amount. A +13 mm wheel on 1.25-inch (31.75 mm) spacers behaves like a wheel with about −19 mm offset. Spacers add a joint between hub and wheel, so they need hub-centric fitment and correct torque; that is a job to check with the spacer maker's instructions.
What the numbers cannot tell you
- Brake clearance: whether the spokes clear the caliper depends on the wheel's spoke design, not just the offset.
- Tire size: a wider or taller tire on the same wheel also grows inwards and outwards. The tire comparison tool shows how much per side.
- Steering and bearings: moving a wheel far out changes how the steering feels and adds leverage on the wheel bearings.
- Local rules: some places have inspection rules on tires sticking out past the bodywork.
Bolt pattern and centre bore must also match the vehicle. The offset only describes position; check everything with the wheels fitted, at full steering lock and with the suspension compressed.