How to use the wheel offset calculator
- Enter the width and offset of the wheel currently fitted. Width is in inches and is usually stamped as a number followed by J, such as 7.5J. Offset is in millimetres, often stamped as ET45 or “45”.
- Enter the width and offset of the wheel you are considering, plus any spacer thickness.
- Press Calculate to see how far each edge moves and in which direction.
Sign convention used here
Positions are measured in millimetres along the axle from the hub mounting face. Positive numbers are toward the outside of the vehicle (the fender or wheel-arch lip); negative numbers are toward the inside (suspension and vehicle centre).
effective offset = offset − spacer thickness
wheel centreline = −effective offset
inner edge = centreline − width (in) × 25.4 ÷ 2
outer edge = centreline + width (in) × 25.4 ÷ 2
change = proposed position − existing position A positive change means the edge moves outward. For the inner edge that means more room to the suspension; for the outer edge it means the wheel sits further out toward the fender. This matches the convention described by Tire Rack’s offset guide: higher positive offset tucks the wheel in, negative offset pushes it out.
Worked example: 7.5J ET45 to 8.5J ET35
The proposed wheel is one inch (25.4 mm) wider, which on its own adds 12.7 mm on each side. Its offset is 10 mm lower, which moves the whole wheel 10 mm outward. Combined:
- Inner edge: −12.7 + 10 = −2.7 mm, so it moves 2.7 mm closer to the suspension.
- Outer edge: +12.7 + 10 = +22.7 mm, so it sits 22.7 mm further out.
How spacers are modelled
A spacer sits between the hub and the wheel, so it moves the wheel’s mounting face outward by its thickness. Mathematically this is the same as reducing the offset by the spacer thickness. An 8J ET45 wheel with a 15 mm spacer sits like an 8J ET30 wheel, with both edges 15.0 mm further out.
No recommendation is made. The calculator shows geometry only. It does not recommend spacers, check stud or bolt thread engagement, hub-centric fit, or whether a spacer is permitted where you drive.
What this calculator doesn’t check
- Suspension, brake, fender or body clearance. Edge positions are not the same as clearance.
- Tyre width and bulge, which usually extend past the rim flange.
- Bolt pattern (PCD), centre bore, seat type, load rating or brake caliper clearance behind the spokes.
- Changes in scrub radius, steering feel or wheel-bearing loads.
- Vehicle compatibility or legal requirements.
Frequently asked questions
What is positive and negative wheel offset?
Offset is the distance from the wheel’s centreline to the face that bolts against the hub. Positive offset means the mounting face sits outboard of the centreline (toward the street side), which tucks the wheel inward. Negative offset puts the mounting face inboard of the centreline, so the wheel sits further out. Offset is often marked on the wheel as “ET” followed by millimetres, for example ET45.
Why does a wider wheel move both edges?
Width is added around the centreline. With the same offset, one extra inch (25.4 mm) of width moves each edge out by about 12.7 mm: the inner edge toward the suspension and the outer edge toward the fender.
How is a spacer modelled?
A spacer of thickness s moves the wheel’s mounting face outward by s. The wheel then sits exactly as if it had an offset of ET − s. For example, an ET45 wheel on a 15 mm spacer sits like an ET30 wheel. The calculator shows this effective offset. It does not assess whether a spacer is suitable, legal or safe.
Is wheel width the same as the overall width of the wheel?
No. The marked width (for example the 8 in 8J) is measured between the bead seats where the tyre sits. The rim flanges extend roughly half an inch further on each side, so the overall width is larger. This calculator measures edge positions at the bead seats; for wheels with similar flange profiles the change in edge position is the same either way.
Will the new wheels clear my brakes, suspension and fenders?
This tool cannot tell you. It only shows how the wheel edges move relative to the hub face. Clearance also depends on tyre size and shape, spoke design over the brake caliper, suspension travel, steering lock and body shape. Bolt pattern, centre bore and load rating are separate checks.
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