What offset actually measures
Picture a line running around the middle of the wheel, halfway between the inner and outer edge. Offset is the distance from that center line to the flat face that bolts against your hub, measured in millimeters.
If the mounting face sits toward the outside of the wheel, the offset is positive and the wheel tucks in toward the suspension. If it sits toward the inside, the offset is negative and the wheel pushes out toward the fender. Zero offset means the mounting face is exactly on the center line.
Offset is usually stamped on the back of a spoke or the inside of the barrel as ET followed by a number, for example ET35 or +35. ET comes from the German einpresstiefe, roughly 'insertion depth'.
Positive, negative and zero, in practice
Most passenger cars and crossovers leave the factory with positive offsets, often between +35 and +50 mm, because front-wheel-drive layouts need the wheel tucked in close to the hub.
Trucks and SUVs vary more. Factory truck wheels tend to sit in the low positives, and the aggressive aftermarket look comes from zero or negative offsets that push the wheel out to the fender line or past it.
Neither direction is right or wrong on its own. What matters is the change from what your vehicle was designed around, and whether the new position still clears everything.
Backspacing: the same idea in inches
Backspacing is the distance from the mounting face to the inner edge of the wheel, in inches. It is what many American wheel shops quote, especially for trucks, because it tells you directly how much room the wheel takes up behind the hub.
Backspacing follows from width and offset. A wheel's real width is about one inch more than its nominal width, because of the flanges, so a 9-inch wheel is about 10 inches across. Half of that, plus the offset converted to inches, is the backspacing.
More backspacing means the wheel sits deeper toward the suspension. Less backspacing means more of the wheel is outside the hub, toward the fender.
One inch is 25.4 mm. An offset change of about 25 mm moves the whole wheel one inch.
Why a wider wheel changes both sides
Width grows from the center line outward in both directions. Keep the offset the same and go one inch wider, and the wheel sits half an inch further out and half an inch further in.
That is why a wider wheel often needs a lower offset: to keep the inner edge where it was, so it doesn't reach the strut, control arm or brake caliper. The outer edge then moves further out, which is the look most people are after, and also where rubbing starts.
How much change is safe
Under about a quarter inch of change on either side, most vehicles never notice. Up to about half an inch outward is usually fine on trucks and SUVs, especially with stock-height suspension.
Beyond that, check two places. Outside: the fender line and the inner fender liner, at full steering lock and with the suspension compressed. Inside: the strut or coilover, the upper control arm and the brake caliper, which is the tightest spot on many vehicles with large brakes.
Lifted trucks, leveling kits and wider fender flares all change the answer, which is why the safest move is to compare against the wheel you already know fits.
Questions
Is a higher offset better?
Not by itself. A higher (more positive) offset moves the wheel in. That suits most cars, but on a wider wheel it can push the inner edge into the suspension. What matters is where the wheel ends up compared with the one that fits today.
Can I fix the wrong offset with spacers?
A spacer moves the wheel out by its thickness, the same as lowering the offset by that much. It can fix a wheel that sits too far in. It can't fix one that already pokes too far. Use hub-centric spacers from a known brand and re-torque after the first drive.
Where do I find my current offset?
Look on the back of a spoke or inside the barrel for ET and a number. If you can't find it, the wheel's part number, the vehicle's original spec or the retailer's fitment data will have it.