Wheel Offset & Backspacing Calculator
Convert wheel offset (ET) to backspacing and back for any wheel width, or compare a new wheel with the stock one: how far the tyre and rim move toward the strut and the fender, clearance left, the offset range that fits, spacer effect and track width change.
How it works
overall width = (stamped width + 1″) × 25.4 · backspacing = overall ÷ 2 + offset · front spacing = overall ÷ 2 − offset · offset = backspacing − overall ÷ 2 · effective offset = offset − spacer · centre-line shift = stock offset − new offset · tyre sidewall move = centre-line shift ± tyre width change ÷ 2
Offset (ET) is the distance from a wheel’s mounting face to its centre-line. It is positive when the face sits outboard of the centre, which tucks the wheel in. Backspacing measures from the same face to the inner lip. The two are linked through the overall width, which is about an inch more than the stamped size because of the flanges (adjustable under More options). Choose Offset → backspacing or Backspacing → offset for a straight conversion, with a table for every common width. Compare with stock answers the fitment question in two parts: how far the inside moves toward the strut and how far the outside moves toward the fender. The rim lips move with the backspacing and front spacing, so a wider wheel grows both ways. A tyre’s sidewalls follow the wheel’s centre-line instead, plus half of any change in tyre width. Pick what you measured from. Enter today’s clearances and the calculator shows what is left, flags rubbing or poke, and gives the range of offsets that fit, with the spacer you would need for a flush fit. Spacers simply subtract from the offset. Moving the centre-line also changes the scrub radius and the track width, so staying within about 15 mm of stock keeps the steering close to stock.
Worked example
Replacing a 7.5″ ET45 wheel with an 8.5″ ET35: the overall widths are 215.9 and 241.3 mm, so backspacing goes from 107.95 + 45 = 153 mm (6.02″) to 120.65 + 35 = 155.7 mm (6.13″) and front spacing from 2.48″ to 3.37″. The rim lips move 2.7 mm in and 22.7 mm out, and the centre-line moves 10 mm out, which widens the track by 20 mm. With a 245 tyre replacing a 225 (+10 mm per side), the inner sidewall stays where it was (−10 + 10 = 0), leaving all 15 mm to the strut. The outer sidewall moves 10 + 10 = 20 mm, so a fender that had 20 mm is now exactly flush. The offsets that fit this tyre run from ET+35 to ET+45.
Frequently asked questions
What does 8.5J ET35 mean?
8.5 inches between the bead seats, J-profile flange, 35 mm positive offset (mounting face 35 mm outboard of the wheel centre-line). Negative ET (common on trucks) pushes the wheel out.
How do I convert offset to backspacing?
Add 1 inch to the stamped width to get the overall width, halve it, and add the offset in inches (mm ÷ 25.4). An 8″ ET+25 wheel: 9 ÷ 2 + 0.98 = 5.48″ of backspacing. For the reverse, subtract half the overall width from the measured backspacing and multiply by 25.4.
Why does the tyre move less than the rim lip on a wider wheel?
A wider rim moves both lips outward from the centre, but the tyre’s sidewalls sit where the tyre’s own width puts them, around the wheel’s centre-line. The same tyre on a wheel 1″ wider only spreads about 10 mm in total. That is why the calculator moves the tyre by the centre-line shift plus half the tyre width change, and offers a rim-edge mode for when the lip is the tight spot.
How do I measure backspacing?
Lay the wheel face down, put a straight edge across the inner lip, and measure from the straight edge down to the mounting face. Measuring to the lip edge, not the bead seat, is what this calculator expects.
Are spacers safe?
Hub-centric spacers from a reputable maker, with enough stud engagement (≥ 6–8 turns) or bolt-on adapters, are widely used. Slip-on spacers over 5 mm without longer studs are not. Re-torque after the first drive.
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