Rim Offset Calculator
Use the tools below to calculate wheel offset from backspacing and to compare your current wheel setup against a new one.
1) Calculate Offset from Width + Backspacing
2) Compare Current vs New Wheel Setup
What Is Rim Offset?
Rim offset is the distance (in millimeters) between the wheel centerline and the wheel mounting face. It is commonly written as ET followed by a number, such as ET35.
- Positive offset: Mounting face is toward the outside of the wheel. The wheel sits further inward.
- Zero offset: Mounting face is exactly on the centerline.
- Negative offset: Mounting face is toward the inside of the wheel. The wheel sits further outward.
Why Offset Matters
Changing offset affects more than looks. It changes where your wheel and tire sit relative to suspension components and fenders. Even a 5–10 mm difference can create rubbing issues on some vehicles.
Main effects of changing offset
- Outer position (poke): How far the wheel sits toward the fender.
- Inner clearance: How close the wheel gets to struts, control arms, and brake components.
- Steering feel and scrub radius: Extreme changes can alter handling behavior.
- Bearing load: Large offset changes may increase stress on hubs and bearings.
How the Calculator Works
Offset from backspacing
If you know wheel width and backspacing, offset is calculated as:
Because many wheel widths are advertised at bead seat (not total lip-to-lip width), this page includes an optional +1.0" lip compensation checkbox.
Comparing two wheel setups
The comparison section tells you exactly how your wheel position changes:
- Outer change: Positive means more poke.
- Inner change: Positive means less suspension clearance.
- Track width change: Approximate total increase/decrease across both sides of the axle.
Example
Suppose your current setup is 18x8 ET35 and you want to run 18x9.5 ET22 with a 5 mm spacer.
Enter those values in the comparison tool and you'll see that:
- The effective new offset becomes ET17 (because spacer reduces offset).
- The outer edge moves outward significantly (more aggressive stance).
- The inner edge may also move inward or outward depending on combined width and offset changes.
This is exactly why you should evaluate both outer and inner movement, not just ET numbers alone.
Common Fitment Mistakes to Avoid
- Using offset alone without considering wheel width.
- Ignoring tire section width and sidewall bulge.
- Forgetting spacers change effective offset.
- Assuming one setup that fits a forum car will fit your suspension and brake package.
- Skipping compression and steering lock checks after installation.
Final Tips Before Buying Wheels
1. Use numbers first, looks second
Photos can be misleading. Run your specs through a calculator and validate clearances before placing an order.
2. Keep changes moderate for daily drivers
If your goal is reliability, avoid extreme offset changes unless you are prepared for alignment and suspension adjustments.
3. Confirm with test fitment when possible
Every platform is a little different. Test fit one wheel, check lock-to-lock steering and full suspension travel, then commit.