What is wheel offset (ET)?

ET stands for Einpresstiefe, German for 'insertion depth'. It is the distance in millimetres from the wheel's mounting flange (the face that bolts to the hub) to the exact centreline of the rim. A positive ET means the flange is on the street side of centre, pushing the wheel inward; a negative ET means the flange is on the brake side of centre, pushing the wheel outward for a 'poke' or 'dish' stance. Zero ET means the flange is exactly at the centreline.

Backspacing is the distance from the wheel's mounting flange to the inner lip (the side facing the brakes and suspension). It is measured in inches in North America and in mm in Europe. The formula is: Backspacing (mm) = (rim width in inches x 25.4 / 2) + ET. A higher backspacing value means more of the wheel sits inside the arch, reducing the risk of outer-arch rub but increasing the risk of hitting inner suspension or calliper components.

How do I find the ET stamped on my wheels?

Most alloy wheels have the specification cast or stamped on the rear barrel in the format 'JxD ET offset', for example '8J x 18 ET35'. Here 8J is the 8-inch bead-seat width, 18 is the diameter in inches, and ET35 means a +35 mm offset. Some older or aftermarket wheels use 'offset' or 'OS' instead of 'ET', but the value is the same measurement.

What happens to handling when I change the offset?

Changing ET alters the scrub radius — the distance between the steering axis and the tyre contact patch on the ground plane. A lower (more negative) ET increases scrub radius, which can make steering feel heavier and increases stress on wheel bearings and steering rack. A higher (more positive) ET decreases scrub radius. Most manufacturers allow a tolerance of plus or minus 5-8 mm from OEM before noticeable handling effects appear, but this varies by car — always check your vehicle's service manual or a fitment database.

Can I fit wheels with a different offset on my car?

Often yes, within limits. Common safe ranges are plus or minus 10-15 mm from the OEM ET for most passenger cars, but you must physically verify clearance at four points: the outer tyre wall versus the arch liner at full lock and on full compression; the inner barrel versus the strut or shock body; the inner lip versus the brake calliper; and the inner barrel versus any suspension wishbones or subframe. Spacers can be used to push a high-ET wheel outward, but they also move the load further from the bearing, increasing stress.

What is the difference between positive and negative ET?

Positive ET (e.g. ET+35, common on modern front-wheel-drive hatchbacks and saloons) positions the hub face towards the street side of the wheel, pulling the wheel inward for a flush or tucked look. Negative ET (e.g. ET-10, common on classic muscle cars, lifted trucks and widebody builds) positions the hub face towards the brake side, pushing the wheel outward for the 'dish' or 'poke' look. Higher positive ET = more inward position; more negative ET = more outward position.

What is the Wheel Offset Calculator?

Free wheel offset calculator. Enter rim width and ET offset to get backspacing and lip clearance instantly, or compute the track-width change when swapping to a different offset. Convert between ET and backspacing in both directions. 100% client-side — nothing is uploaded. It runs free in your browser on Gera Tools, with nothing uploaded.

Wheel Offset Calculator

Name: Wheel Offset Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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Wheel offset — measured in millimetres and stamped on the back of almost every alloy wheel as ET (from the German Einpresstiefe, meaning “insertion depth”) — determines exactly where a wheel sits in relation to the hub, the brake calliper, the suspension and the arch. Get it wrong by even 10 mm and you risk rubbing on the inner arch liner at full steering lock, hitting the strut body, or stressing wheel bearings beyond their design tolerance. Get it right and the wheel fits flush, the tyre clears everything cleanly, and handling stays predictable. This free calculator handles all three jobs: computing backspacing and lip clearance from a given rim width and ET, calculating the exact track width change when swapping from one offset to another, and converting between ET (the European mm system) and backspacing (the North American inch system) in either direction.

How the formulas work

The geometry is straightforward. A wheel of internal bead-seat width W inches has its centreline at W/2 inches = W x 12.7 mm from each lip. The mounting flange is placed at distance ET mm from that centreline towards the street side (positive) or brake side (negative). Two distances follow directly:

Backspacing (inner clearance) = (W x 25.4 / 2) + ET, in mm. Divide by 25.4 for inches. This is how far the inner lip stands from the hub face.
Outer lip to hub centreline = (W x 25.4 / 2) - ET, in mm.

When you swap wheels, the track width change per side equals newET - oldET, and the total track change across both sides is 2 x (newET - oldET). A negative result means the track widens; a positive result means it narrows.

Worked example

A common aftermarket wheel is 8J ET25 on a car whose factory wheels are 7.5J ET38.

New backspacing = (8 x 25.4 / 2) + 25 = 101.6 + 25 = 126.6 mm (4.98 in)
Factory backspacing = (7.5 x 25.4 / 2) + 38 = 95.25 + 38 = 133.25 mm (5.25 in)
ET change = 25 - 38 = -13 mm per side, so the total track widens by 26 mm (13 mm outward on each side).

That 13 mm outward shift per side is significant: on many hatchbacks the arch clearance at full lock is only 15-20 mm, so physical verification on the car — not just arithmetic — is essential before driving.

Rim	ET	Backspacing	Outer lip offset
7J	+38	127.0 mm	51.0 mm
8J	+35	146.6 mm	66.6 mm
8J	+25	126.6 mm	76.6 mm
9J	-10	104.2 mm	124.2 mm

Formula note

All calculations use the identity:

Backspacing (mm) = (rim_width_inches × 25.4 ÷ 2) + ET

Rearranged for the reverse converter:

ET (mm) = (backspacing_inches × 25.4) − (rim_width_inches × 25.4 ÷ 2)

The only inputs needed are the rim’s internal bead-seat width (in inches, as stamped on the wheel) and the ET offset (in mm). No tyre data is required because offset is a wheel-only measurement independent of tyre profile.