Wheel Offset Calculator
Enter your current and new wheel dimensions to see exactly how much the wheel moves inward or outward and how clearance changes on both sides. The calculator works in metric (mm) or imperial (inches), converts between offset (ET) and backspacing, and shows a step-by-step breakdown of every calculation.
Formula
Worked example
Stock wheel: 8 in wide, ET35. New wheel: 9 in wide, ET20. Width difference in mm: (228.6-203.2)/2 = 12.7 mm. Offset difference: 35-20 = 15 mm. Outer change: 12.7 - (-15) = wait - outer = (nwMm/2 - effectiveNewET) - (cwMm/2 - currentET) = (114.3-20)-(101.6-35) = 94.3-66.6 = +27.7 mm outward. Inner change: (101.6+35)-(114.3+20) = 136.6-134.3 = +2.3 mm more room.
What is wheel offset and why does it matter?
Wheel offset (ET, from the German Einpresstiefe meaning press depth) is the distance in millimetres between the hub mounting face and the geometric centerline of the wheel. A positive ET means the mounting face is on the outboard side of the centerline, pulling the wheel deeper into the wheel arch. A negative ET means the mounting face is inboard of the centerline, pushing the wheel outward for a wider, more aggressive stance. Zero ET places the mounting face exactly at the centerline. Offset determines where the tyre sits relative to the suspension, fender, and bodywork. Getting it wrong causes tyre rub, scrub radius changes that affect steering feel, wheel bearing overload, and in the worst cases unsafe handling. Getting it right means the wheel fits perfectly, the tyre clears all components through the full range of suspension travel, and steering geometry is preserved.
Offset vs. backspacing - what is the difference?
Backspacing is the North American aftermarket alternative to ET. It measures the distance from the inner bead seat (the inner lip) to the hub mounting face, in inches. A larger backspacing number means the mounting face sits further inboard, which is equivalent to a higher positive ET. The relationship is: Backspacing (in) = wheel width (in) / 2 + offset (mm) / 25.4. Because backspacing depends on both width and offset, the same backspacing value means very different offsets on narrow vs. wide wheels. This is why the European ET system is more useful for direct comparison across different wheel widths. The calculator converts both ways so you can use whichever system your supplier or vehicle handbook quotes.
How inner and outer clearance changes work
When you change wheel width or offset, both the outer (fender side) and inner (suspension side) edges of the wheel shift. The outer clearance change tells you how much further the tyre protrudes toward or away from the fender. A positive value means the tyre moves outward and you must check arch liner, fender lip, and any suspension components at full lock and full compression. A negative value means the tyre tucks inward - usually safe for the fender but monitor inner clearance too. The inner clearance change tells you how much the inner edge of the wheel and tyre moves toward or away from the suspension, brake caliper, strut, and subframe. A negative inner change means reduced clearance and potential rubbing on the strut or caliper if you go too far. A positive inner change means more breathing room on the inside - often what happens when you run a lower offset on a narrower wheel.
Wheel spacers and their effect on offset
A wheel spacer is a ring bolted between the hub and the wheel. Each millimetre of spacer thickness reduces the effective ET by the same amount and shifts the wheel outward by that distance. A 15 mm spacer on a wheel with ET35 produces an effective ET of 20. The calculator includes a spacer field so you can model any spacer thickness. Note that hub-centric spacers (which have a bore that matches both the hub and the wheel) are considerably safer than lug-centric spacers because they maintain the concentric load transfer path. Spacers also change the stud engagement, so verify that sufficient thread engagement remains with the spacer installed.
Typical OEM offset ranges by vehicle type
| Vehicle type | Typical ET range (mm) | Typical backspacing (in) | Notes |
|---|---|---|---|
| Compact / economy car | +35 to +50 | 5.0-6.0 | High positive offset keeps wheel tucked |
| Sedan / mid-size car | +30 to +45 | 4.5-5.5 | Moderate positive, OEM look |
| Sports car / coupe | +20 to +45 | 4.0-5.5 | Varies widely by manufacturer |
| SUV / crossover | +20 to +50 | 4.5-6.0 | Higher backspacing for wide track |
| Full-size truck (2WD) | +18 to +25 | 4.5-5.5 | Moderate offset for level stance |
| Full-size truck (4WD) | 0 to +18 | 3.5-4.5 | Lower offset for lift kits |
| Off-road / rock crawler | -25 to +10 | 3.0-4.5 | Negative offset for wide stance |
| European luxury / performance | +35 to +55 | 5.0-6.5 | Very high positive for tucked look |
Factory offsets vary widely. Always verify against your specific vehicle's specifications before purchasing aftermarket wheels.
Frequently asked questions
What is a safe amount of offset change when fitting aftermarket wheels?
There is no universal safe number because it depends on tyre width, suspension geometry, and how much room the wheel arch provides. As a starting point, changes of up to 10 mm outward from stock are often absorbed without any rubbing. Changes of 10-25 mm outward typically require arch checking and may need rolled or pulled fender lips. Changes beyond 25 mm outward almost always require body modification or shorter tyres. Changes inward (more negative or higher ET) need checking on the inner suspension side rather than the fender.
Why does wheel width affect offset calculations?
Offset is measured from the centerline of the wheel. If you increase the wheel width, the centerline shifts outward by half the width increase even if the offset number stays the same. That is why fitting a wider wheel with the identical ET still moves the outer edge of the tyre further outward. The calculator handles this for you by accounting for both the width change and the offset change together.
How do I convert wheel offset (ET) to backspacing?
Use the formula: Backspacing (inches) = wheel width (inches) / 2 + offset (mm) / 25.4. For example, a 9-inch-wide wheel with ET25 has backspacing of 9/2 + 25/25.4 = 4.5 + 0.98 = 5.48 inches. The reverse: ET (mm) = (backspacing (in) - wheel width (in) / 2) x 25.4.
Does changing offset affect steering?
Yes. Offset affects scrub radius, which is the horizontal distance between the steering axis and the tyre contact patch centre. Moving the wheel significantly outward (lower or more-negative ET) increases scrub radius and can make steering heavier and more sensitive to road camber and braking forces. Most manufacturers engineer a small positive or zero scrub radius for predictable handling. Large offset departures from OEM, typically more than 25-30 mm, can noticeably change steering feel and increase wear on wheel bearings and steering components.
What does ET stand for on a wheel?
ET is an abbreviation of the German word Einpresstiefe, which means press-in depth or insertion depth. It describes how far the hub mounting face is pressed into the wheel relative to its centerline. The value is always in millimetres. Positive ET is by far the most common on modern passenger cars.
Is negative offset the same as zero offset?
No. Zero offset means the hub mounting face is exactly at the centerline of the wheel. Negative offset means the hub face is inboard of the centerline, so the wheel extends further outward from the vehicle. Negative offset is common on classic cars, trucks with lift kits, and off-road builds where a wide stance is desired or where the suspension design requires it.
Can I use a wheel spacer instead of a lower-offset wheel?
A spacer achieves a similar outward shift but does not change wheel width. Spacers can be a cost-effective solution for small adjustments of 5-20 mm, but they place additional stress on the wheel studs, change stud thread engagement, and must be properly hub-centric to avoid vibration. For large offset changes, a correctly-specced wheel is generally the safer and more permanent solution.