Wheel Offset Calculator

Wheel offset (marked ET from the German “Einpresstiefe,” meaning insertion depth) defines where the wheel’s mounting face sits relative to the centerline of the rim.
It is measured in millimeters and is critical for wheel fitment, suspension geometry, bearing load, and overall vehicle stance.

The three types of offset:

• Positive offset (ET+) - the mounting face is outboard of the wheel centerline. Most front-wheel-drive cars and modern vehicles use positive offset, so the wheel sits tucked in under the fender.
• Zero offset (ET0) - the mounting face is exactly at the wheel centerline.
• Negative offset (ET−) - the mounting face is inboard of the wheel centerline. The wheel sticks outward, the classic “deep dish” stance. Common on trucks and off-road vehicles.

The one detail that trips everyone up: marked width is not overall width.

A wheel marked 8 inches wide is 8 inches measured bead seat to bead seat, which is the part the tire actually sits on. The two rim flanges stand outside that, and they add roughly half an inch each. So the overall rim, flange edge to flange edge, is about one inch wider than the marked size, and the centerline sits half of that overall width in from either edge.

Get this wrong and every offset you calculate is out by about 12 to 13 mm, which is more than the whole tolerance most fitment guides allow.

Key formulas:

Overall Rim Width = Marked Width + 25.4 mm

Centerline (from the inboard flange edge) = Overall Rim Width ÷ 2 = (Marked Width ÷ 2) + 12.7 mm

Offset (ET) = Backspacing − Centerline

Backspacing = (Marked Width ÷ 2) + 12.7 mm + Offset

Backspacing is measured from the mounting face to the inboard flange edge, which is why it is the easy one to measure at home: lay a straightedge across the back of the wheel and measure down to the hub face.

Worked example, an 8-inch wheel with 5.5 inches of backspacing:

• Marked width: 8 in = 203.2 mm
• Overall rim width = 203.2 + 25.4 = 228.6 mm (9.0 in)
• Centerline = 228.6 ÷ 2 = 114.3 mm from the inboard edge
• Backspacing: 5.5 in = 139.7 mm
• Offset = 139.7 − 114.3 = 25.4 mm → ET+25

Sanity check it the other way: an ET+25 wheel 8 inches wide should have (203.2 ÷ 2) + 12.7 + 25.4 = 139.7 mm of backspacing, which is 5.5 inches. It closes.

Effect on fitment and handling:

• Higher positive offset → wheel sits more inboard → narrower track → may rub on suspension components
• Lower or negative offset → wheel sits more outboard → wider track → more leverage on the bearing, and may rub on the fender lip
• Changing offset by more than ±10 mm from OEM starts to matter for bearing load, steering feel and scrub radius

Note that offset alone does not change speedometer reading. That is rolling diameter, which is a tire question rather than a wheel one.

Common OEM offset ranges:

• Compact FWD cars: ET+35 to ET+50
• SUVs and pickups: ET+10 to ET+30
• Sports RWD cars: ET+20 to ET+40
• Off-road trucks (lifted): ET0 to ET−25

Metric note: offset is always given in millimeters, even on vehicles that otherwise use imperial dimensions. Backspacing is the opposite, usually quoted in inches. Mixing the two units is the second most common way to get a wrong answer here.