Wheel Offset Calculator — ET Offset & Backspacing Converter
Wheel offset (ET) is the distance from the wheel's hub mounting face to its centreline. Changing it moves your wheels inward or outward in the arch, affecting stance, clearance, and handling. Enter your rim width and the current and target offsets to see backspacing and exactly how far the wheel relocates.
in
mm
mm
Wheel moves 15 mm outward with new offset
- 1
Offset change
20 − 35 = -15Positive = wheel moves inward (tucks under arch); negative = moves outward (wider stance). - 2
Wheel position change
|-15| = 15
How does this calculator work?
Backspacing (in) = rim width ÷ 2 + offset (mm) ÷ 25.4. Position change = new ET − current ET: positive = wheel moves inward, negative = moves outward. Example: ET35 → ET20 on an 8-inch rim shifts the wheel 15 mm outward and reduces backspacing from 5.88 to 5.29 inches.
Formula
How this is calculated
Wheel offset is stamped as "ET" (from German Einpresstiefe, or "insertion depth") on the wheel hub face in millimetres. Positive ET means the mounting face is offset toward the outside of the wheel — the wheel sits further inside the arch and closer to the strut/suspension. Zero ET centres the mounting face exactly at the wheel's mid-plane. Negative ET pushes the mounting face toward the inside — the wheel protrudes outward, widening stance.
Backspacing converts offset to a more practical imperial measurement: the distance from the inner edge of the rim to the mounting face. It equals half the rim width (in inches) plus the offset converted to inches. Wider rims or more positive offset increases backspacing; narrow rims or negative offset reduce it. Backspacing determines whether the inner edge of the wheel clears brake calipers, suspension arms, and inner guards.
When comparing two offsets, a positive change (e.g. ET35 → ET45) moves the wheel inward by 10 mm — more clearance from the arch but potentially rubbing on suspension components. A negative change (ET35 → ET20) moves the wheel 15 mm outward — more stance but the tyre may contact the outer arch lip under full steering lock. Always check manufacturer specifications for minimum and maximum supported offsets before fitting new wheels.
Frequently asked questions
A lower ET moves the wheel and tyre outward in the arch — increasing track width, giving a wider "stance". The trade-off is potential interference with the outer wheel arch or reduced suspension clearance. It also increases the scrub radius and can put additional load on wheel bearings and hub components.
Backspacing is the distance from the wheel's inner lip to the mounting face, measured in inches. It is the US-common alternative to the European ET offset system. The two are related by: Backspacing = (Rim width / 2) + (ET / 25.4). Both describe how centred or off-centre the mounting face is, just from different reference points.
A general rule of thumb is ±10 mm from the OEM offset as a safe starting point. Beyond that, clearance issues (outer arch rubbing or inner suspension fouling) become increasingly likely. The safest approach is to confirm physical clearance with the actual wheel/tyre combination fitted to the vehicle before driving at full suspension travel and full steering lock.
Also known as
TG we-Calculate Editorial Team. (2026). Wheel Offset Calculator — ET Offset & Backspacing Converter [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/wheel-offset-calculator
TG we-Calculate Editorial Team. "Wheel Offset Calculator — ET Offset & Backspacing Converter." TG we-Calculate. 2026. https://we-calculate.com/calculator/wheel-offset-calculator.
TG we-Calculate Editorial Team, "Wheel Offset Calculator — ET Offset & Backspacing Converter," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/wheel-offset-calculator
@misc{wecalculate_wheel_offset_calculator, title = {Wheel Offset Calculator — ET Offset & Backspacing Converter}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/wheel-offset-calculator}}, year = {2026}, note = {TG we-Calculate} }
Did this calculator help you?