Wheel Offset and Backspacing Calculator: Poke, Inner Clearance and Spacers
Convert wheel offset (ET) to backspacing and front spacing or back again, and compare a new wheel with your current one: how far the outer lip pokes out, how much inner clearance you lose, track width change, wheel spacers and tire width.
How it works
Overall width = stamped width + 2 × lip (≈ 1 in) · Backspacing = overall width ÷ 2 + offset ÷ 25.4 · Front spacing = overall width − backspacing · Offset = (backspacing − overall width ÷ 2) × 25.4 · Effective offset = offset − spacer · Outer lip change = (new overall ÷ 2 − new effective offset) − (current overall ÷ 2 − current effective offset) · Inner lip change = (new overall ÷ 2 + new effective offset) − (current overall ÷ 2 + current effective offset)
Offset (ET) is the distance in millimeters from the wheel’s centerline to its mounting face. Positive offset puts the mounting face toward the street side, which tucks the wheel in toward the suspension; zero or negative offset pushes it out for a wider stance. Backspacing describes the same geometry from the inner edge of the rim lip, in inches, and decides clearance to the strut and brakes; front spacing is the distance from the mounting face to the outer lip, which decides poke at the fender. The stamped width (the 8 in an 18×8 wheel) is measured between the bead seats, and the lips add about half an inch per side, so the calculator uses width + 1 in as the overall width; you can change that allowance under More options if you have measured your wheel. To compare wheels, enter the current width and offset: the outer and inner lip changes are worked out separately, because a wider wheel at the same offset grows both ways. A spacer acts like a lower offset by its thickness. Add tire section widths to see how the sidewalls move too.
Worked example
An 8-inch wheel with a +35 mm offset is 9 in wide overall, so its backspacing is 9 ÷ 2 + 35 ÷ 25.4 = 4.5 + 1.38 = 5.88 in and its front spacing 9 − 5.88 = 3.12 in. Compared with an 8-inch +45 mm wheel, the outer lip sits 10 mm (0.39 in) further out (more poke), the inner lip gains 10 mm of clearance from the suspension, and the track is 20 mm (0.79 in) wider across the axle.
Sources: Wheel offset, backspace and frontspace (Powerhouse Racing) · Wheel offset comparison: inner clearance and outer position (Inch Calculator) · Offset, backspacing and spacers explained (Hurrey’s Performance)
Frequently asked questions
What is the difference between offset and backspacing?
Offset is measured in millimeters from the wheel’s centerline to its mounting face; backspacing is measured in inches from the mounting face to the inner rim edge. They describe the same thing: for a given width, more offset means more backspacing.
Will a lower offset make my wheels stick out?
Yes. A numerically lower (or negative) offset moves the wheel outward, adding poke and a wider track. Go too far and the tire can rub the fender; too much positive offset pulls the wheel inward toward the suspension or brakes.
What happens if I go wider but keep the same offset?
The extra width is split between both sides. Going from 8 to 9 inches at the same offset moves the outer lip 12.7 mm out and the inner lip 12.7 mm closer to the suspension. To keep the inner clearance the same, the offset would have to drop by 12.7 mm, which pushes all the extra width outward.
How much offset change is safe?
Changes of a few millimeters are usually fine. Bigger swings alter scrub radius and steering feel, can speed up wheel-bearing wear, and need a check of inner (suspension, brakes) and outer (fender) clearance. Stay close to the factory offset unless the fitment is verified.
How do wheel spacers change offset?
A spacer moves the wheel out by its thickness, so a +35 mm wheel on a 10 mm spacer behaves like a +25 mm wheel. Use hub-centric spacers and keep enough lug-nut thread engagement; thick spacers usually need longer studs or a bolt-on design.
What does ET mean on a wheel?
ET is short for the German Einpresstiefe, the offset in millimeters. An ET35 wheel has its mounting face 35 mm outboard of its centerline.
Related calculators
Educational estimate. Real fitment depends on the exact wheel, tire, suspension and alignment; test-fit or ask a wheel shop before buying.