Wheel Backspacing & Offset Calculator
Instantly convert wheel backspacing (inches) to offset (ET in mm) or vice-versa. Accurately calculate hub mounting pad position, strut clearance, and outer fender poke with our engineering-grade visual cross-section diagram and benchmark conversion matrix.
Quick Navigation
Convert Backspacing to Offset (ET)
Enter wheel width and backspacing to calculate offset in millimeters.
Interactive Wheel & Stance Cross-Section Diagram
Real-time graphic of wheel barrel, tire profile, hub mounting surface, centerline, and backspacing depth.
Wheel Backspacing & Offset Conversion Matrix
Use this reference chart to quickly cross-reference standard rim widths with measured backspacing (inches) to determine the exact wheel offset (ET in millimeters). Positive numbers denote mounting face closer to the street side; negative numbers denote a deep-dish concave look.
| Wheel Width | 3.50" BS | 4.00" BS | 4.50" BS | 5.00" BS | 5.50" BS | 6.00" BS | 6.50" BS | 7.00" BS |
|---|---|---|---|---|---|---|---|---|
| 7.0" (8.0" Total) | -13 mm | 0 mm | +13 mm | +25 mm | +38 mm | +51 mm | - | - |
| 8.0" (9.0" Total) | -25 mm | -13 mm | 0 mm | +13 mm | +25 mm | +38 mm | +51 mm | - |
| 8.5" (9.5" Total) | -32 mm | -19 mm | -6 mm | +6 mm | +19 mm | +32 mm | +45 mm | +57 mm |
| 9.0" (10.0" Total) | -38 mm | -25 mm | -13 mm | 0 mm | +13 mm | +25 mm | +38 mm | +51 mm |
| 9.5" (10.5" Total) | -44 mm | -32 mm | -19 mm | -6 mm | +6 mm | +19 mm | +32 mm | +45 mm |
| 10.0" (11.0" Total) | -51 mm | -38 mm | -25 mm | -13 mm | 0 mm | +13 mm | +25 mm | +38 mm |
Understanding Wheel Backspacing vs. Wheel Offset
When fitting new wheels or custom tire setups on cars, trucks, or 4x4s, two primary measurements dictate whether the wheel will fit inside the wheel well without rubbing the suspension struts or protruding past the fenders:
Wheel Backspacing (Inches)
Backspacing is the straight-line physical distance from the wheel's hub mounting pad (where it bolts to the rotor) to the extreme inboard lip edge of the rim. It is predominantly measured in inches and directly tells you how much space the wheel occupies toward the vehicle's frame, struts, brake calipers, and inner fender liners.
Wheel Offset / ET (Millimeters)
Offset (often designated as ET from the German Einpresstiefe) is the distance between the wheel's true centerline and the hub mounting surface. It is measured in millimeters and can be positive (hub is pushed toward the street side), zero (hub is exactly in the center), or negative (hub is sunken inwards toward the suspension, giving a deep lip).
The Mathematical Conversion Equations
Where 1.0" represents the standard 0.50" bead flange lip on both outer and inner sides of the wheel.
To find frontspacing (outer poke depth): Frontspacing = (Nominal Width + 1.0) - Backspacing.
How to Measure Backspacing on an Unmounted Wheel
- Place Wheel Face Down: Lay your bare wheel or tire face down on a flat, clean surface or piece of cardboard so the front cosmetic face doesn't scratch.
- Lay a Straightedge Across the Back: Place a rigid steel ruler or straight carpenter's level across the back edge of the rim from lip to lip.
- Measure Down to the Hub Face: Use a tape measure or ruler to measure the perpendicular distance from the underside of the straightedge straight down to the flat mounting pad of the wheel center hub.
- Read the Measurement: That distance is your exact Wheel Backspacing in inches (e.g. 4.5", 5.25", 6.0").
Frequently Asked Questions (FAQ)
Why is total wheel width 1 inch wider than advertised?
Wheel sizes (e.g., 18x9) are measured between the tire bead mounting surfaces inside the rim. The outer structural metal lips that retain the tire bead typically add approximately 0.5 inches on each side, making a 9-inch wheel physically 10 inches wide from lip edge to lip edge.
What happens if my backspacing is too large?
Excessive backspacing pulls the wheel deeper into the vehicle. This can cause the inner rim lip, tire sidewall, or tire tread to rub against suspension struts, control arms, sway bars, or inner wheel-well splash guards. You can resolve this with bolt-on wheel spacers or lower offset wheels.
What does negative offset mean for truck and 4x4 stance?
Negative offset (e.g., -12mm or -24mm) pushes the hub mounting pad deep into the wheel, forcing the tire and rim to "poke" outward beyond the vehicle fenders. While this gives a wide, aggressive stance, it increases leverage on wheel bearings and may require lift kits or fender flares to prevent fender rubbing.