et rim calculator - Aaron Graves, PhDude Replica

Wheel ET (Offset) Calculator

Compare your current and new wheel setup to estimate poke and inner clearance changes.

Current wheel width (inches)

Example: 8, 8.5, 9.5

Current offset ET (mm)

Positive ET is common on modern cars

New wheel width (inches)

New wheel offset ET (mm)

Spacer thickness (mm)

Spacer lowers effective ET

What is an ET rim calculator?

An ET rim calculator helps you estimate how a wheel change affects fitment. In wheel terminology, ET means offset (from the German word Einpresstiefe). Offset is the distance in millimeters between the wheel centerline and the mounting face that sits against your hub.

Changing wheel width, offset, or adding spacers can push the wheel outward toward the fender, or inward toward the suspension. This calculator gives you a fast comparison before you buy wheels.

How ET, width, and spacers work together

Offset basics

Higher ET (for example ET45) usually tucks the wheel further inward.
Lower ET (for example ET30) moves the wheel outward.
Spacers reduce effective offset: ET35 with a 10 mm spacer behaves like ET25.

Width also changes fitment

A wider wheel grows equally to both sides from the wheel centerline. That means even if offset stays the same, you still get:

More outer poke
Less inner clearance

Formula used by this calculator

Outer position = (wheel_width_mm / 2) - effective_offset_mm Inner position = (wheel_width_mm / 2) + effective_offset_mm effective_offset_mm = wheel_offset_mm - spacer_mm Outer change = new_outer - old_outer Inner change = new_inner - old_inner

Interpretation:

Outer change + = wheel sticks out more.
Outer change - = wheel tucks in more.
Inner change + = wheel moves closer to strut/suspension (less clearance).
Inner change - = more inner clearance.

Practical fitment tips

1) Don’t check wheels alone

Tire width and sidewall shape can add several millimeters beyond the wheel lip. A tire with a square shoulder may rub earlier than a rounded shoulder tire on the same wheel.

2) Verify both sides of clearance

Good fitment means avoiding contact at both ends:

Inside: strut, spring perch, control arm, brake lines, inner liner
Outside: fender lip, fender liner, bumper tab

3) Account for suspension movement

Static fitment can still rub when turning, braking, or hitting bumps. Lowered vehicles and aggressive alignment (especially camber changes) alter real-world clearance significantly.

4) Hub-centric and hardware safety

If you use spacers, make sure they are high quality and compatible with your wheel/hub bore. Confirm stud or bolt engagement is sufficient. Safety and torque specs always come first.

Example scenario

Suppose your current setup is 8J ET35 and you want 9J ET30 with a 5 mm spacer.

Effective new ET becomes ET25.
The wheel sits noticeably farther out.
Inner barrel may move closer or farther depending on the width/offset combination.

That combination often looks more flush, but you must still verify suspension and fender clearance with your exact tire size.

Final notes

This ET rim calculator is meant for planning and comparison. Manufacturing tolerances, tire shape, alignment, brake package, and suspension geometry all influence final fitment. Use this as a baseline, then confirm with real measurements before purchase.