Wheel offset – the deceptively simple mathematical calculation that perplexes both beginning and experienced car enthusiasts alike.

In the simplest of terms, wheel offset is a measure, typically in millimeters, of how far in a set of wheels will sit within a car’s fenders. All wheels have an offset value, but what does that number actually mean in practicality?

Wheel offset is a value given in ET, or einpress tiefe, which translates from German as “insertion depth.” If you have any experience with aftermarket wheels, you’ve likely seen that they have an ET value stamped on them already. Being able to calculate this value on your own is important, however, because not all wheels are stamped with their ET values. Additionally, building a set of 2- or 3-piece modular wheels will require this calculation before assembly to ensure they fit as expected.

When measuring offset, you first need to measure the wheel’s overall width. Take that value, divide it by two, and you have the center point of the wheel’s width.

The next step is to measure from the wheel’s mounting flange – where the wheel mates up to the car’s hub, to the rear edge of the wheel. The resulting value is called “backspacing.”

From there, simply subtract that backspacing value from the center point figure to get your offset value.

• The more positive the offset number, the deeper inside the fenders the wheels will sit. Too far positive and the wheel may foul on brake or suspension components, depending on the overall width of the wheel.
• The more negative the offset number, the further out the wheels will sit. Wheels with offset that is too low may sit outside of the fenders, causing the tires to rub and potentially cause damage to the car’s body work.

All cars are different in terms of suspension components, fender clearance, and overall room inside the wheel wells. Using the measurements from your factory wheels is a great place to start when considering the size of your next set, but it is always a smart ides to check with people who own the same car as you to get a feel for what works and what doesn’t.