Wheel Offset Calculator
This calculator works out wheel offset (ET), backspacing and poke, and shows exactly how far a wheel will sit inward or outward when you change wheel width or ET value. Wheel offset matters because even a small change in ET or width can push a tyre out past the guard lip or tuck it in against a brake caliper or suspension arm, so it pays to check the numbers before you buy. Enter the width and ET offset of your current wheel, plus the width and ET of the new or aftermarket wheel you are considering, using the mm inputs or the built-in ET presets from ET-10 to ET55. The calculator returns the current and new backspacing, the outward position change per wheel, and the new poke, along with a full breakdown of half width, backspacing, poke and inset for both wheels, a side-by-side position change summary table, and a fitment note flagging whether the shift is minor or significant enough to check guard clearance. Use it when comparing wheel fitments, sizing up a wider rim, or working out how a lower ET changes your stance. As a guide, most vehicles tolerate 5 to 10 mm of outward movement without rubbing, but anything more needs a proper clearance check. These are geometric estimates only; always confirm real-world fitment on your own vehicle before driving.
1. Current Wheel
2. New / Aftermarket Wheel
Current Wheel Breakdown
New Wheel Breakdown
Position Change Summary
| Current Wheel | New Wheel | Change | |
|---|---|---|---|
| Backspacing | 152.5 mm | 150.5 mm | -2.0 mm |
| Poke | 62.5 mm | 74.5 mm | +12.0 mm |
| Inset | 45.0 mm | 38.0 mm | -7.0 mm |
| Outward position | 62.5 mm outboard | 74.5 mm outboard | +12.0 mm outward |
How Wheel Offset Works
Wheel offset (ET) is the distance in millimetres from the hub mounting face to the centreline of the wheel. It controls how far inboard or outboard the wheel sits in the wheel arch.
Backspacing (mm) = (Wheel Width / 2) + ET
Poke (mm) = (Wheel Width / 2) - ET
Inset (mm) = ET (positive ET = wheel sits further inboard)
Outward position change = (Old ET - New ET) + ((New Width - Old Width) / 2)
Positive and Negative Offset Explained
| ET Value | Mounting face position | Wheel position | Typical use |
|---|---|---|---|
| Positive (e.g. ET45) | Closer to outside face | Tucked inward (more clearance) | Most modern passenger cars |
| Zero (ET0) | At centreline | Neutral | Some older and off-road vehicles |
| Negative (e.g. ET-15) | Closer to inside face | Sticks outward (poke or flush) | Modified, off-road, and wide-body builds |
Backspacing vs ET Offset
ET offset is the European standard and is stamped on most wheels sold in New Zealand. Backspacing is the US standard and is commonly used in the off-road and truck fitment market. Both describe the same physical measurement from different reference points. If you know one, you can calculate the other using the wheel width:
Backspacing (mm) = (Width / 2) + ET. To convert to inches, divide by 25.4.
ET from backspacing: ET = Backspacing - (Width / 2).
What is Poke?
Poke is the distance the outer face of the wheel sits outboard of the hub mounting face. It is calculated as: Poke = (Width / 2) - ET. A wheel with a high positive ET has less poke and sits more inboard. A lower or negative ET increases poke, making the wheel flush with or beyond the guard lip. Many car enthusiasts aim for a specific poke value to achieve a flush or aggressive stance without the wheel rubbing on the guard or suspension components.
Worked Example
Default values: current wheel is 215 mm wide with ET45; new wheel is 225 mm wide with ET38.
- Current backspacing: (215 / 2) + 45 = 107.5 + 45 = 152.5 mm
- New backspacing: (225 / 2) + 38 = 112.5 + 38 = 150.5 mm
- Current poke: (215 / 2) - 45 = 107.5 - 45 = 62.5 mm
- New poke: (225 / 2) - 38 = 112.5 - 38 = 74.5 mm
- Outward position change: (45 - 38) + ((225 - 215) / 2) = 7 + 5 = 12 mm outward per side
This matches the calculator's default output: each new wheel sits 12 mm further outward. The outer face moves from 62.5 mm to 74.5 mm outboard of the hub, which is 12 mm more poke per side.
Fitment and Guard Clearance
Moving a wheel outward increases the risk of the tyre rubbing on the guard lip or inner guard liner at full lock and over bumps. As a general guide, most vehicles can accommodate 5 to 10 mm of outward movement without modification. More than 10 mm outward usually requires checking guard clearance carefully. Moving a wheel inward (higher ET, lower poke) reduces rubbing risk but may cause the wheel to foul on brake calipers or suspension components. Always check both sides when fitting new wheels.
Converting Rim Width in Inches to Millimetres
Wheel widths are sometimes stated in inches (for example, 8J or 8.5J, where J is the flange type). To convert to millimetres, multiply by 25.4. An 8J rim has a bead-to-bead width of 8 x 25.4 = 203.2 mm. A 9J rim is 228.6 mm wide. Use the millimetre figure in this calculator.
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Sources and method: Offset formula (Backspacing = (Width / 2) + ET; Poke = (Width / 2) - ET) is the industry standard used by wheel manufacturers worldwide. ET abbreviation from German Einpresstiefe (DIN standard for wheel offset). All measurements in millimetres unless stated otherwise.
This calculator provides geometric estimates only. Always verify physical fitment clearance on your specific vehicle before driving. Guard, brake caliper, and suspension clearances vary by vehicle model and modification level. Consult a wheel fitment specialist if you are unsure.