Enter your rim width and ET offset to get backspacing — or enter rim width and backspacing to get ET. The calculator also shows you how far the wheel face sits inboard and outboard of the hub centreline, which tells you whether the wheel will clear your suspension, brake caliper, and arch liner before you buy.
Wheel Offset & Backspacing Calculator
ET offset is stamped on the back of most alloy wheels. Backspacing is measured from the back edge of the rim to the hub mounting face.
What Is Wheel Offset (ET)?
Offset (ET from the German “Einpresstiefe” meaning “press depth”) is the distance in millimetres between the wheel’s hub mounting face and its geometric centreline. A positive ET means the mounting face is on the outboard side of the centreline — the wheel is tucked in. A negative ET means the mounting face is inboard — the wheel pushes out.
| Offset Type | Mounting Face Position | Effect on Stance | Common On |
|---|---|---|---|
| High Positive (ET40-ET55) | Well outboard of centreline | Wheel tucked in, narrow stance | FWD cars, economy vehicles |
| Moderate Positive (ET20-ET40) | Slightly outboard of centreline | Standard stance | Most modern cars and SUVs |
| Zero (ET0) | At centreline | Wider stance than OEM | Older RWD cars, some trucks |
| Negative (ET-5 to ET-40) | Inboard of centreline | Aggressive wide stance | Off-road builds, lifted trucks, show cars |
What Is Backspacing?
Backspacing is the distance from the back edge of the rim to the hub mounting face, measured in inches. It is the American way of expressing the same thing as ET. High backspacing = wheel pushed inward. Low backspacing = wheel pushed outward.
The conversion formula: Backspacing (inches) = (Rim Width / 2) + (ET_mm / 25.4)
On an 8-inch wide rim with ET35: Backspacing = (8/2) + (35/25.4) = 4 + 1.38 = 5.38 inches. That is considered a standard fitment for most full-size trucks and SUVs.
Common OEM Offset Specs by Vehicle Type
| Vehicle Type | Typical OEM ET | Safe Aftermarket Range |
|---|---|---|
| FWD compact/sedan (Civic, Corolla) | ET40-ET55 | ET35-ET55 |
| AWD/FWD SUV (RAV4, CRV) | ET35-ET45 | ET30-ET50 |
| RWD sports car (Mustang, Camaro) | ET25-ET40 | ET20-ET45 |
| Half-ton truck (F-150, Silverado) | ET18-ET44 | ET-12 to ET44 |
| Heavy duty truck (F-250, Ram 2500) | ET-3 to ET44 | ET-25 to ET44 |
| Jeep Wrangler (JL) | ET-18 | ET-44 to ET10 |
How Much Can You Change Offset From OEM?
The general rule is to stay within 5-8mm of OEM offset for a safe, no-issues fitment. Beyond that range, you start running into:
Suspension Clearance Issues (Too Much Positive Offset)
Running more positive offset than OEM (more ET) pushes the wheel further inboard, toward the strut, suspension arm, and brake caliper. On some vehicles there is very little clearance to play with. The back of the rim or tyre can contact the caliper under full steering lock or suspension compression.
Arch Liner and Steering Issues (Too Much Negative Offset)
Running significantly less positive or negative offset pushes the wheel outward. The tyre can contact the arch liner, particularly on full lock. On front-wheel-drive vehicles, extreme negative offset can cause steering kickback under acceleration as the scrub radius increases dramatically.
Wheel Bearing Load
Moving the wheel further from the hub centreline increases the moment arm on the wheel bearing. A significant offset change — especially toward negative — adds stress to bearings and can shorten their lifespan. Running 25mm more negative offset than OEM is not unusual for lifted trucks, but it is a long-term bearing consideration.
Spacers: Another Way to Change Effective Offset
A wheel spacer bolts between the hub and wheel, pushing the wheel outboard and effectively reducing offset by the spacer thickness. A 20mm spacer on an ET35 wheel gives an effective offset of ET15. Spacers are generally fine in moderate thicknesses (5-15mm) with quality hub-centric spacers and proper wheel bolts. Thin spacers on the wrong hub bore are where fitment problems occur.
If you are also changing tyre width, use the Tyre Size Calculator to confirm the overall diameter and width stay within safe limits alongside any offset change.
Mechanic’s Tip
Always check clearance in person before committing to an aftermarket wheel fitment. The calculator gives you the geometry, but real vehicles have casting variation, aftermarket suspension modifications, and spacer combinations that make a test fit essential. Mount one wheel and turn the steering to full lock in both directions. Then bounce the suspension through its full travel. If it clears comfortably in both extremes, you are good. If it clips anything, do not drive it.
Frequently Asked Questions
Does offset affect how the car handles?
Yes, primarily through scrub radius — the distance between the steering axis and the tyre’s contact patch centerline. OEM engineers choose offset to achieve a specific scrub radius. Changing offset by more than 15-20mm alters scrub radius enough to affect steering feel, particularly on FWD cars, where it can cause torque steer.
Where is the ET offset stamped on a wheel?
It is usually cast or machined into the back of the spoke or barrel of alloy wheels — look on the inside of the rim near the valve stem hole or center bore. It may read “ET35” or just “35.” Steel wheels have it stamped on the inner flange.
Can I use wheels from a different trim level or model year of the same car?
Often yes, but always verify the bolt pattern (PCD), center bore, and offset match. The same model year can have different offsets on different trim levels if one uses wider tyres. A 5mm offset difference is usually fine. A 20mm difference may cause clearance issues depending on the vehicle.
As an Amazon Associate we earn from qualifying purchases.
