Quick Answer
- An 8" wide wheel with 5.5" backspacing is ET+25 (+25.4 mm). Going the other way, ET+25 on 8" is 5.48" backspacing.
- Backspacing (in) = (wheel width + 1) ÷ 2 + offset (mm) ÷ 25.4. Offset (mm) = (backspacing − (wheel width + 1) ÷ 2) × 25.4.
- Each 10 mm less offset moves the whole wheel 10 mm (0.39") outward and the inner lip the same distance away from the suspension.
Enter the wheel width and the backspacing from the wheel specs to get the offset (ET) in millimetres.
Width is measured between tire bead seats. We add ~1" internally to approximate total lip-to-lip width when converting to backspacing.
Measure from the inner wheel lip (rear edge) to the mounting face.
Offset
+25.4mm
Wheel Width
8.0"
Backspace
5.50in
Offset
+25.4mm
Positive offset: the mounting surface is toward the outside face, so the wheel sits further in.
Formula: Offset = (Backspacing − (Wheel Width + 1) ÷ 2) × 25.4 = (5.50 − 4.50) × 25.4 = 25.4mm
Backspacing assumes the overall wheel width is the listed width plus 1". The flange depth differs by wheel, so check the backspacing in the wheel maker's specifications.
Get a plain-English explanation of your results and what they mean for your vehicle.
Backspacing to Offset Chart
Offset in millimetres for each backspacing and wheel width, rounded to the nearest millimetre. A plus sign is a positive offset. Calculated as (backspacing − (width + 1) ÷ 2) × 25.4.
| Backspacing | 7.0" wide | 7.5" wide | 8.0" wide | 8.5" wide | 9.0" wide | 9.5" wide | 10.0" wide |
|---|---|---|---|---|---|---|---|
| 4.00" | 0 | -6 | -13 | -19 | -25 | -32 | -38 |
| 4.25" | +6 | 0 | -6 | -13 | -19 | -25 | -32 |
| 4.50" | +13 | +6 | 0 | -6 | -13 | -19 | -25 |
| 4.75" | +19 | +13 | +6 | 0 | -6 | -13 | -19 |
| 5.00" | +25 | +19 | +13 | +6 | 0 | -6 | -13 |
| 5.25" | +32 | +25 | +19 | +13 | +6 | 0 | -6 |
| 5.50" | +38 | +32 | +25 | +19 | +13 | +6 | 0 |
| 5.75" | +44 | +38 | +32 | +25 | +19 | +13 | +6 |
| 6.00" | +51 | +44 | +38 | +32 | +25 | +19 | +13 |
| 6.50" | +64 | +57 | +51 | +44 | +38 | +32 | +25 |
| 7.00" | +76 | +70 | +64 | +57 | +51 | +44 | +38 |
Offset to Backspacing Chart
Backspacing in inches for each offset and wheel width. Calculated as (width + 1) ÷ 2 + offset ÷ 25.4. At ET0 and ET±25 the values for 7 to 10 inch wide wheels match the table published by Metal Cloak.
| Offset (ET) | 7.0" wide | 7.5" wide | 8.0" wide | 8.5" wide | 9.0" wide | 9.5" wide | 10.0" wide |
|---|---|---|---|---|---|---|---|
| ET-44 | 2.27" | 2.52" | 2.77" | 3.02" | 3.27" | 3.52" | 3.77" |
| ET-25 | 3.02" | 3.27" | 3.52" | 3.77" | 4.02" | 4.27" | 4.52" |
| ET-19 | 3.25" | 3.50" | 3.75" | 4.00" | 4.25" | 4.50" | 4.75" |
| ET-12 | 3.53" | 3.78" | 4.03" | 4.28" | 4.53" | 4.78" | 5.03" |
| ET0 | 4.00" | 4.25" | 4.50" | 4.75" | 5.00" | 5.25" | 5.50" |
| ET+12 | 4.47" | 4.72" | 4.97" | 5.22" | 5.47" | 5.72" | 5.97" |
| ET+18 | 4.71" | 4.96" | 5.21" | 5.46" | 5.71" | 5.96" | 6.21" |
| ET+25 | 4.98" | 5.23" | 5.48" | 5.73" | 5.98" | 6.23" | 6.48" |
| ET+35 | 5.38" | 5.63" | 5.88" | 6.13" | 6.38" | 6.63" | 6.88" |
| ET+45 | 5.77" | 6.02" | 6.27" | 6.52" | 6.77" | 7.02" | 7.27" |
| ET+55 | 6.17" | 6.42" | 6.67" | 6.92" | 7.17" | 7.42" | 7.67" |
Offset in Inches and the Effect of Wheel Spacers
Offset is in millimetres, so divide by 25.4 for inches. A negative offset means the mounting surface is toward the inside of the wheel and the wheel sits further out.
Offset in millimetres and inches
| Offset (ET) | Inches | Mounting surface |
|---|---|---|
| ET-44 | -1.73" | Toward the inside of the wheel |
| ET-25 | -0.98" | Toward the inside of the wheel |
| ET-19 | -0.75" | Toward the inside of the wheel |
| ET-12 | -0.47" | Toward the inside of the wheel |
| ET0 | 0.00" | At the centerline |
| ET+12 | +0.47" | Toward the outside face |
| ET+18 | +0.71" | Toward the outside face |
| ET+25 | +0.98" | Toward the outside face |
| ET+35 | +1.38" | Toward the outside face |
| ET+45 | +1.77" | Toward the outside face |
| ET+55 | +2.17" | Toward the outside face |
Wheel spacers on an ET+45 wheel
A spacer moves the wheel outward by its thickness, which is the same as lowering the offset by that amount. The scrub radius calculator shows how that change affects steering geometry.
| Spacer | Effective offset | Wheel moves outward |
|---|---|---|
| 5 mm | ET+40 | 0.20" |
| 10 mm | ET+35 | 0.39" |
| 15 mm | ET+30 | 0.59" |
| 20 mm | ET+25 | 0.79" |
| 25 mm | ET+20 | 0.98" |
How Width and Offset Changes Move the Inner and Outer Lip
Calculated with the formulas below. A positive inner lip change moves the lip toward the suspension, and a positive outer lip change moves the lip outward.
| Current to new wheel | New backspacing | Inner lip | Outer lip |
|---|---|---|---|
| 8.0" ET+35 to 8.0" ET+25 | 5.48" | -0.39" (-10.0 mm) | +0.39" (+10.0 mm) |
| 8.0" ET+35 to 9.0" ET+35 | 6.38" | +0.50" (+12.7 mm) | +0.50" (+12.7 mm) |
| 8.0" ET+35 to 9.0" ET+25 | 5.98" | +0.11" (+2.7 mm) | +0.89" (+22.7 mm) |
| 8.0" ET+45 to 8.5" ET+35 | 6.13" | -0.14" (-3.7 mm) | +0.64" (+16.4 mm) |
| 8.5" ET+35 to 9.5" ET+20 | 6.04" | -0.09" (-2.3 mm) | +1.09" (+27.7 mm) |
Static ride height geometry for the wheel only. Tire size, camber and suspension travel are not included.
Offset & Backspacing Formulas
How to Use This Calculator
- Find the wheel width (inches) and either the backspacing (inches) or the offset (ET, millimetres) in the wheel specifications or on the back of the wheel.
- Pick Backspacing to Offset or Offset to Backspacing, enter the width and the value you have, and read the converted value and the frontspace.
- To compare wheels, open Compare Two Wheels and enter the width and offset of your current wheel and the new one.
- Read how far the inner lip moves toward or away from the suspension, and how far the outer lip moves in or out.
- Treat the result as geometry only and test-fit the wheel, because tire size, camber and suspension travel are not included.
About Wheel Offset, Backspacing & Fitment
What Is Wheel Offset and Why Does It Matter?
Wheel offset is the distance in millimetres from the wheel centerline to the mounting surface. It sets how far in or out the wheel sits in the arch: a higher positive offset moves the wheel inward, and a lower or negative offset moves it outward. Together with the wheel width it fixes where the inner and outer lips sit.
Backspacing vs Frontspacing (Outer Lip)
Backspacing measures from the mounting surface to the inner edge of the wheel. Frontspace is the same distance to the outer edge. Converting between offset and backspacing lets you compare wheels that are listed in different formats. Once you know the offset, the wheel and tire clearance estimator turns it into an inner and outer gap for a chosen tire, and the staggered front and rear tire check helps when the two axles use different wheels.
Checking a Wheel Before You Buy
Use the Compare Two Wheels tab to see how a new wheel sits against your current one. Wheel and vehicle makers say wheels must suit the vehicle, so ask the vehicle manufacturer or the wheel maker whether a wheel is approved. Offset also changes the geometry at the steering axis, which you can see on the scrub radius calculator, and the camber angle calculator explains positive, negative and zero camber.
Frequently Asked Questions
Disclaimer
This tool provides geometric estimates only and cannot account for every vehicle or modification. Actual fitment depends on your specific suspension, brakes, alignment, bodywork, tire model, and how the car is loaded or driven. Always test-fit wheels and tires, check for clearance at full steering lock and through suspension travel, and follow your vehicle manufacturer’s and wheel maker’s recommendations.