Offset and backspacing describe the same thing from opposite ends, which is why wheel shopping involves converting between them. Offset is measured from the centreline; backspacing from the inner edge. Both say where the mounting face sits.
The two measurements
Offset is the distance in millimetres from the wheel's mounting face to its centreline. Positive offset puts the mounting face outboard of centre, which tucks the wheel in. Negative offset puts it inboard, which pushes the wheel out.
Backspacing is the distance in inches from the mounting face to the inner rim edge. More backspacing pulls the wheel inward.
They convert directly: backspacing equals half the overall wheel width plus the offset in inches. Higher offset and more backspacing both mean the same thing — the wheel sits further in.
The extra inch
Wheel width is quoted bead seat to bead seat, not edge to edge. The flanges that hold the tire bead add roughly half an inch on each side.
So a 9 inch wheel is about 10 inches overall, and its centreline is 5 inches from either flange. Ignoring that inch is the most common mistake in offset arithmetic and it puts the answer out by half an inch.
This is also why a 9 inch wheel with zero offset has 5 inches of backspacing, not 4.5.
Wider wheels move both ways
Going from an 8 inch wheel to a 9 inch adds an inch of width, and where that inch goes depends entirely on the offset.
At the same offset, the extra inch is split evenly — half an inch further in and half an inch further out. Drop the offset and more goes outward; raise it and more goes inward.
The calculator reports both movements because they have different consequences. Outboard movement runs into the fender; inboard movement runs into the strut, the spring and the brake caliper.
What goes wrong in each direction
Too far in and the tire rubs the strut or the inner fender liner, often only under compression or at steering lock — so it passes a static check and fails on the road.
Too far out and the tire catches the fender lip, usually over bumps with passengers aboard. It also sticks out past the bodywork, which is illegal in some jurisdictions.
Offset also changes scrub radius, which alters steering feel and loads the wheel bearing differently. A large offset change on a modern car is felt through the wheel, not just seen.
What to measure instead of calculate
Clearance to the strut at full compression and full lock. Clearance to the caliper, which depends on the spoke shape as much as the offset. Clearance to the fender with the suspension loaded.
None of that is in the offset number, and no calculator can produce it. The reliable method is a straightedge across the hub face and a tape measure, or a test fit.
Use this page to translate between the numbers in a listing. Use the car to decide whether the wheel fits.