Wheel Offset Calculator Compare backspacing, poke, and fitment between wheel setups
Original Setup
→
New Setup
Fitment Analysis
✗ Likely Won't Fit
Inner Clearance Change
-2.7 mm
Closer to suspension
Outer Poke Change
+22.7 mm
Sticks out more
Track Width Change
+45.4 mm
Total (both sides)
Backspacing
5.52" → 5.63"
140.3mm → 143mm
Original Backspacing140.3 mm (5.52")
New Backspacing143 mm (5.63")
Original Outer Distance50.3 mm from fender
New Outer Distance72.9 mm from fender
How to Use This Calculator
Enter your original wheel width and offset on the left, and your new wheel specifications on the right. The calculator instantly shows:
- Inner clearance change: How much closer (or further) the wheel's inner lip moves relative to the suspension and brake components.
- Outer poke change: How much further the wheel sticks out from (or tucks under) the fender compared to stock.
- Track width change: The total widening or narrowing of the vehicle's track (both sides combined).
- Backspacing: The distance from the wheel's inner edge to the mounting surface, shown in both mm and inches.
If you're using wheel spacers, enter the spacer thickness to see the effective offset with spacers installed.
Offset & Backspacing Formulas
Backspacing (mm) = (Wheel Width mm / 2) + Offset - Spacer
Outer Poke (mm) = (Wheel Width mm / 2) - Offset + Spacer
Track Width Change = (New Outer Poke - Original Outer Poke) × 2
Outer Poke (mm) = (Wheel Width mm / 2) - Offset + Spacer
Track Width Change = (New Outer Poke - Original Outer Poke) × 2
Example
Jake — fitting wider wheels on his WRX
Jake's 2023 WRX has stock 18×8.5" wheels with ET+55 offset. He wants to install 18×9.5" wheels with ET+38 offset for a wider, more aggressive stance. The calculator shows: inner clearance increases by 13.4mm (more room from suspension), outer poke increases by 30mm per side (wheels stick out more), and total track width increases by 60mm. The "Check Clearance" warning tells Jake he should verify fender clearance — with this much poke change, he may need to roll his fenders or add small fender flares.
Frequently Asked Questions
Wheel offset (ET, from the German "Einpresstiefe") is the distance in millimeters from the wheel's mounting surface to the centerline of the wheel. Positive offset means the mounting surface is toward the outside (street side) of the wheel. Negative offset means it's toward the inside (suspension side). A higher positive offset tucks the wheel further under the fender, while a lower or negative offset pushes it outward.
Backspacing is the distance from the inner edge of the wheel to the mounting surface, measured in inches or millimeters. It determines how close the inner lip of the wheel sits to the suspension and brake components. Backspacing = (wheel width / 2) + offset. For example, an 8-inch wide wheel with +40mm offset has a backspacing of about 5.6 inches (142mm).
Generally, a change of +/- 5mm from the factory offset is safe and unlikely to cause issues. A change of +/- 10mm is usually acceptable but should be verified for clearance. Beyond 15mm from stock offset, you risk rubbing on fenders (with less offset) or hitting suspension components (with more offset). Always test-fit wheels before mounting tires.
If the offset is too low (or too negative), the wheel will stick out further from the fender. This can cause the tire to rub on the fender when turning or going over bumps, increased wear on wheel bearings and suspension components due to the wider lever arm, and potential legality issues in some regions where tires must be within the fender line.