Wheel Offset Calculator
Enter the width and offset of your current wheels and the ones you want. See how far the new wheel moves in toward the suspension and out toward the fender, the new backspacing and the change in track width, with an optional clearance check from your own measurements.
Last updated:
The 8 in a 17x8 wheel.
ET35 = +35. Use a minus sign for negative offset.
Rim flanges add about 1″ to the overall width (½″ per side).
Measure with the current wheel. Leave blank to skip.
Plumb line from the fender lip to the wheel face. Blank to skip.
How to use the Wheel Offset Calculator
- Enter the current wheel: width in inches and offset in millimeters (ET).
- Enter the new wheel you are considering.
- Optionally measure the gap from the inner lip to the strut or caliper and from the wheel face to the fender lip, and enter them.
- Press Calculate offset change to see poke, inner clearance change and new backspacing.
- Check the offset table to see how other offsets in the same width would sit.
Wheel offset and backspacing formulas
All positions are measured from the hub mounting face, the flat surface that bolts to the car. Overall wheel width is the nominal width plus about 1 inch for the two rim flanges.
- Backspacing = (width + 1″) ÷ 2 + offset ÷ 25.4 (distance from mounting face to inner lip)
- Front spacing = (width + 1″) ÷ 2 − offset ÷ 25.4 (distance from mounting face to outer lip)
- Poke change = new front spacing − old front spacing
- Inner change = new backspacing − old backspacing (positive means closer to the suspension)
- Track width change = 2 × (old offset − new offset) in mm
Offset to backspacing reference
| Offset | 7″ wide | 8″ wide | 9″ wide | 10″ wide |
|---|---|---|---|---|
| −25 mm | 3.02″ | 3.52″ | 4.02″ | 4.52″ |
| −12 mm | 3.53″ | 4.03″ | 4.53″ | 5.03″ |
| 0 mm | 4.00″ | 4.50″ | 5.00″ | 5.50″ |
| +20 mm | 4.79″ | 5.29″ | 5.79″ | 6.29″ |
| +35 mm | 5.38″ | 5.88″ | 6.38″ | 6.88″ |
| +45 mm | 5.77″ | 6.27″ | 6.77″ | 7.27″ |
Fitment tips
- Measure before you buy: with the current wheel on, measure from the inner lip to the closest suspension part and from the fender lip straight down to the wheel face, then enter both for a clearance check.
- Width goes both ways: an extra inch of width adds half an inch inside and half an inch outside at the same offset.
- Spacers act like lower offset: a 10 mm spacer moves the wheel out exactly like reducing the offset by 10 mm.
- Check the brakes: large calipers need clearance at the wheel spokes, not only at the barrel, which this calculator cannot measure.
Frequently asked questions
What is wheel offset?
Offset is the distance in millimeters from the wheel's centerline to its mounting face. Positive offset means the mounting face is toward the outside (street side) of the wheel, pulling the wheel in under the car; negative offset pushes the wheel out. ET45 means +45 mm.
How do I convert offset to backspacing?
Backspacing = (wheel width + 1 inch) ÷ 2 + offset ÷ 25.4. The extra inch accounts for the rim flanges. A 17x8 wheel with +35 mm offset has (8 + 1) ÷ 2 + 35 ÷ 25.4 = 4.5 + 1.38 = 5.88 inches of backspacing.
How much will my new wheels poke out?
The outer face moves by half the width increase plus the offset decrease. Going from 8 inches +35 to 9 inches +20 moves the face out by 12.7 mm (half an inch of width) plus 15 mm (offset) = 27.7 mm, about 1.09 inches.
Does lower offset mean the wheel sticks out more?
Yes. At the same width, every millimeter less offset moves the whole wheel, inner and outer edge, 1 mm further out. Going from +40 to +30 moves the wheel 10 mm out on each side and widens the track by 20 mm.
How much inner clearance do I need?
Many fitment guides suggest keeping at least about 5 mm (0.2 inch) between the inner barrel and the strut, spring seat or caliper so the wheel does not rub under load or when the suspension compresses. Check at full steering lock too.
Does this include the tire?
No. The calculator measures the wheel itself. Tires often bulge past the rim, and a wider tire on the same wheel adds width on both sides, so test-fit or check the tire's section width before buying.
Can changing offset affect handling?
Moving the wheel out changes the scrub radius and increases leverage on wheel bearings and suspension parts, which can affect steering feel and wear. Small changes are common; large changes are best checked with a fitment specialist.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.