Backspacing-to-Offset Converter
Turn backspacing into offset, and back. Use the calculator below. Results update as you type.
Backspacing = (Wheel Width / 2) + Offset + ~0.5" wheel lipHow the Backspacing-to-Offset Converter Works
The Backspacing to Offset Converter works by subtracting half the wheel width and the 0.5 in flange allowance from the backspacing, then converting inches to millimeters.
- Enter the wheel width, for example 10.0 in.
- Enter the backspacing, for example 5.0 in.
- Read the offset: −12.7 mm.
- Read the closest ET value: ET−13.
The Formula
The Backspacing to Offset Converter formula is Offset (mm) = (Backspacing − Width ÷ 2 − 0.5) × 25.4.
Backspacing = (Wheel Width / 2) + Offset + ~0.5" wheel lip Offset (mm) = (Backspacing (in) − Width (in) ÷ 2 − 0.5) × 25.4Worked example: an 8.0 in wheel with 5.5 in backspacing gives (5.5 − 4.0 − 0.5) × 25.4 = +25.4 mm, listed as ET25.
Conversion Table
The Backspacing to Offset Converter conversion table converts 12 width and backspacing pairs into offset in millimeters and the closest ET value. Offsets run from −50.8 mm to +25.4 mm.
A 4.5 in backspacing on a 9.0 in wheel is ET−13, and on an 8.0 in wheel it is ET0. The same backspacing gives different offsets on different widths.
| Wheel Width (in) | Backspacing (in) | Offset (mm) | Closest ET |
|---|---|---|---|
| 8 | 4.5 | 0.0 | ET0 |
| 8 | 5.0 | 12.7 | ET13 |
| 8 | 5.5 | 25.4 | ET25 |
| 9 | 4.5 | -12.7 | ET-13 |
| 9 | 5.0 | 0.0 | ET0 |
| 9 | 5.5 | 12.7 | ET13 |
| 10 | 4.5 | -25.4 | ET-25 |
| 10 | 5.0 | -12.7 | ET-13 |
| 10 | 5.75 | 6.3 | ET6 |
| 12 | 4.5 | -50.8 | ET-51 |
| 12 | 5.0 | -38.1 | ET-38 |
| 12 | 6.0 | -12.7 | ET-13 |
What Is the Backspacing-to-Offset Converter?
The Backspacing to Offset Converter is a tool that converts the US backspacing measurement into the metric offset (ET) used worldwide. Many off-road wheels list backspacing, while vehicle fitment guides list offset.
The conversion depends on wheel width. That is why a single "backspacing to offset" number does not exist without a width.
Common Uses
The Backspacing to Offset Converter has 4 uses: catalog matching, fitment-guide checks, import wheels and measured wheels.
- Match catalogs: Compare US and European wheel listings.
- Check fitment guides: Turn guide offsets into backspacing and back.
- Buy imported wheels: Convert ET for US vehicles.
- Convert measured wheels: Turn a tape measurement into ET.