Hong Life CalculatorsUnderstanding the wheel offset calculator — concept and principle
This calculator compares a current and a new wheel by rim width and offset (ET). It shows how far the inner rim edge moves toward the suspension, how far the outer edge moves toward the fender, and how much the track width changes. Use it before buying aftermarket wheels or wheel spacers.
How it is calculated
Half rim width = rim width (in) × 25.4 ÷ 2 = W/2 in mm
Inner edge from the hub face = W/2 + ET
Outer edge from the hub face = W/2 − ET
Inner change = new inner − old inner; outer change = new outer − old outer
Centreline shift = old ET − new ET; track change = 2 × centreline shift
- Positive results mean the edge moves further from the hub face: inward for the inner edge, outward for the outer edge.
The principle
Offset is the distance from the wheel's centreline to the face that bolts onto the hub. A positive offset puts the mounting face toward the outside of the wheel, so more of the rim sits inboard. Measured from the hub face, the inner edge is half the width plus the offset, and the outer edge is half the width minus the offset. Lowering ET by 10 mm moves the whole wheel 10 mm outward, and since both sides do this, the track widens by 20 mm. A wider rim adds half its extra width to each side.
Worked example
Default inputs: current 7 in ET45, new 8 in ET35.
- Half widths: 7 × 12.7 = 88.9 mm and 8 × 12.7 = 101.6 mm.
- Old inner = 88.9 + 45 = 133.9 mm; new inner = 101.6 + 35 = 136.6 mm, so the inner edge moves 2.7 mm toward the suspension.
- Old outer = 88.9 − 45 = 43.9 mm; new outer = 101.6 − 35 = 66.6 mm, so the outer edge moves out 22.7 mm.
- Centreline moves out 45 − 35 = 10 mm; track widens 20 mm.
| New rim width (in) | Outer edge moves outward by (mm) |
|---|---|
| 2 | -53.5 |
| 4 | -28.1 |
| 6 | -2.7 |
| 8 | 22.7 |
| 10 | 48.1 |
| 12 | 73.5 |
| 16 | 124.3 |
How to read the result
Compare the inner change with your current gap to the strut, spring seat and brake caliper, and the outer change with the room under the fender. A few millimetres inward is often fine; 20 mm outward may rub on the fender lip when the suspension compresses. Remember the tyre: its sidewall bulges past the rim, and a wider tyre adds more.
Common mistakes
- Mixing up the sign of ET: a lower ET pushes the wheel out.
- Forgetting that a wider rim moves both edges.
- Ignoring the tyre width, which usually stands out beyond the rim.
- Checking only straight ahead and not at full steering lock.
Frequently asked questions
What does ET45 mean?
The hub mounting face is 45 mm outboard of the wheel's centreline.
How does a spacer change offset?
A 10 mm spacer acts like reducing ET by 10 mm.
Is a lower offset better?
Not in itself; it widens the stance but can increase rubbing and load on wheel bearings.
Sources: Standard wheel geometry: offset (ET) = distance from rim centreline to hub mounting face, positive toward the outside · 1 in = 25.4 mm exactly (NIST SP 811)