Wheel Offset Calculator
Compare your current wheels against a new set. See how much further the wheel pokes out, how much clearance you lose inboard, and what the backspacing works out to.
Current and new wheels
- Backspacing assumes a 1 inch flange allowance, which is the industry convention.
- This compares wheels only. Wider tires add their own clearance problems on top.
- Numbers are per side. Total track width change is roughly double the poke figure.
Offset, backspacing and why both exist
Offset is the distance from the wheel's centerline to its hub mounting face, in millimeters, stamped on the wheel as ET. Positive offset puts the mounting face outboard of center and tucks the wheel in. Negative offset pushes the wheel out. Zero means the mounting face is dead center.
Backspacing measures the same thing differently: from the mounting face to the inboard edge of the rim, in inches. Truck and off-road shops talk in backspacing. Import and European shops talk in offset. They describe the same geometry.
Which direction actually causes problems
Inboard is where you run out of room first. Struts, control arms, tie rod ends and brake calipers all live in there. Move a wheel 20mm inboard and you can find the barrel touching a strut at full lock.
Outboard is more forgiving until it is not. Too much poke and the tire catches the fender lip over bumps, throws road spray down the side of the car, and in some places fails inspection for sticking out past the bodywork.
Wider wheels move both edges
Going from an 8 inch wheel to a 9 inch wheel at the same offset adds half an inch on each side. People forget the inboard half.
You can protect one edge or the other, not both. Raise the offset by about 13mm per inch of added width and the outer edge stays exactly where it was, but the whole extra inch goes inboard toward the strut. Drop the offset by 13mm instead and the inner edge holds station while the wheel pokes out a full inch further. Leave the offset alone and you split the difference, half an inch each way.
Frequently asked questions
How much offset change is safe?
Within about 5mm of stock you will not notice. Up to 12mm is usually fine on a stock vehicle. Past 25mm you are into checking clearance yourself, because published fitment guides stop being reliable.
Does changing offset hurt anything mechanically?
It changes the scrub radius, which changes how the steering loads up and how the car behaves when you brake on a split surface. Large offset changes also add leverage on the wheel bearing, which shortens its life. Small changes are not worth worrying about.
Can I use spacers instead of buying different wheels?
A spacer reduces effective offset by its thickness, so a 15mm spacer on a +45 wheel behaves like a +30 wheel. Use hub-centric spacers and make sure the studs still have full thread engagement. Thin spacers without extended studs are how wheels come off.
What is the formula for backspacing?
Backspacing in inches equals half the overall wheel width plus the offset converted to inches. Overall width is the stated width plus one inch for the flanges. So a 9 inch wheel with +20 offset has 5 inches of backspacing.
Everything on this page is an estimate worked out from the numbers you entered and typical figures for the conditions you picked. Your vehicle, your parts and your conditions will differ. Check anything you intend to act on against the manufacturer's specification, the service manual, or a qualified technician. Verifying the result before relying on it is up to you.