Et50 Calculator helps compare wheel offset changes before fitting new wheels. Enter current width, current ET, new width, and new ET to see outer poke, inner clearance, backspacing, and track width shift.
When a “Wider and Deeper” Wheel Doesn’t Fit Wider or Deeper
Someone runs the numbers on a wheel upgrade, sees the width go up an inch and the offset go up 10mm, and assumes the fitment basically stayed put. Then the new wheel goes on and the inner lip is suddenly clipping a strut bolt that had clearance to spare before. Width and offset don’t cancel each other out — they push the wheel’s lips in opposite directions, and this calculator is built to isolate exactly how far each one moves.
Calculator Used Formula
Overall Wheel Width: Overall Width (mm) = (Wheel Width (in) + 1.0) × 25.4
Inner Depth: Inner Depth (mm) = Overall Width (mm) ÷ 2 + Offset (mm)
Outer Depth: Outer Depth (mm) = Overall Width (mm) ÷ 2 − Offset (mm)
Inner Clearance Shift: Inner Shift (mm) = New Inner Depth − Old Inner Depth
Outer Extension Shift (Poke): Outer Shift (mm) = New Outer Depth − Old Outer Depth
New Wheel Backspacing: Backspacing (in) = (New Width (in) + 1.0) ÷ 2 + New Offset (mm) ÷ 25.4
Offset in Inches: Offset (in) = New Offset (mm) ÷ 25.4
Track Width Shift, Per Side: Per-Side Shift (mm) = Old Offset (mm) − New Offset (mm)
Track Width Shift, Total: Total Shift (mm) = Per-Side Shift (mm) × 2
Offset Difference: Offset Difference (mm) = New Offset (mm) − Old Offset (mm)
What “Overall Width” Actually Means Here
Every depth figure in this tool starts from an overall width, not the bare rim width you type in. The calculator adds 1.0 inch to whatever width you enter before converting to millimeters, which assumes a 0.5-inch flange on each side of the rim. That’s a built-in approximation, not a measurement — actual flange thickness varies between wheel manufacturers and styles. If a wheel’s real flange profile is noticeably thicker or thinner than that, every inner depth, outer depth, and backspacing figure below will be off by the same small margin.
Inner Depth and Outer Depth Move in Opposite Directions
Inner Depth is the distance from the hub mounting pad to the inboard lip; Outer Depth is the same measurement to the outboard lip. Because offset is added to one and subtracted from the other, raising the offset number pushes the hub toward the outside of the wheel — the inner lip retreats toward the suspension while the outer lip pulls in toward the fender. That’s why a higher ET reads as a clearance risk on the strut side and a reduction in poke on the fender side at the same time, even with no width change at all.
Reading the Backspacing Number
The backspacing figure is the same physical measurement as the new wheel’s Inner Depth — it’s just converted to inches because that’s the unit most US wheel listings quote backspacing in. If a wheel’s spec sheet lists backspacing in inches, this number is what you compare it against directly, without doing your own unit math.
Why Track Width Can Shrink on a Wider Wheel
Wheel width by itself has no effect on track width — only the offset difference moves the wheel’s centerline relative to the hub. The Per-Side Change figure and the New ET − Old ET figure will always carry opposite signs, because raising the offset pulls the wheel inward (negative track shift) while the raw offset number itself goes up (positive difference). Going to a wider wheel does not guarantee a wider track; if the offset increase is large enough, the car can end up narrower at that axle than before, even though the tire itself is physically wider.
Worked Example
Starting from an 8-inch wheel at ET40 and moving to a 9-inch wheel at ET50: Inner Depth goes from 154.3mm to 177.0mm, a 22.7mm shift toward the strut. Outer Depth goes from 74.3mm to 77.0mm, only a 2.7mm increase in poke. Backspacing on the new wheel works out to 6.97 inches. Despite the wider tire, the per-side offset change of −10mm doubles into a −20mm total track width shift — the axle actually gets narrower, while the real fitment concern shifts almost entirely to inner clearance rather than outer poke.
A Few Things Worth Knowing Before You Trust the Numbers
- Width must be entered as a positive number; offset accepts negative values, since plenty of wheels — especially deep-dish and truck applications — run negative ET.
- Every depth and backspacing figure depends on the 0.5-inch-per-side flange assumption, so treat the results as a planning estimate to confirm against the wheel manufacturer’s printed specs, not a substitute for a physical test-fit.
- The track width and clearance shifts only describe the change between the two setups you entered — they don’t account for tire size, suspension travel, or fender rolling, which still need to be checked separately.