Wheel Offset and ET Calculator with transparent formulas, clear units, and instant results. Effective ET: Eₑ = E₂ − S; I = ΔW ÷ 2 + (Eₑ−E₁); O = ΔW ÷ 2 − (Eₑ−E₁).
Stable formula
This calculator uses a stable mathematical formula. Always verify the values you enter.
Accuracy level
High when inputs and units are correct.
Last reviewed
July 9, 2026
Formula or source
Stable mathematical formula explained on the page.
Guide reading time
4 min
Confidence
High for the stated calculation.
Result type
Formula result, not an official certification.
Do not use for: Cases with missing data, unclear units, or a required professional certification.
How Wheel Offset and ET Calculator works
The Wheel Offset and ET Calculator uses these inputs: Old rim width (in), Old ET (mm), New rim width (in), New ET (mm), Spacer (mm). Its primary output is Effective ET. The calculation uses the entered tire and wheel geometry.
The engine implements Eₑ = E₂ − S; I = ΔW ÷ 2 + (Eₑ−E₁); O = ΔW ÷ 2 − (Eₑ−E₁). Validation runs first to reject zero divisors and non-finite values.
Numeric example using the starting values: Wheel Offset and ET Calculator: Old rim width (in): 7.5 · Old ET (mm): 45 · New rim width (in): 8 · New ET (mm): 40. The resulting output is Effective ET: 40.0 mm.
Wheel Offset and ET Calculator: This result is a calculation estimate only. It does not approve installation, safety, legality, or vehicle fitment. Verify it with the vehicle manufacturer, a qualified professional, and local law. Suspension, fender, brake, load, speed-rating, and legal clearance cannot be approved from geometry alone.
💡 Useful Tips
Do not mix units between Old rim width (in) and Old ET (mm); make sure both describe the same scenario. Verify fitment against manufacturer specifications, approved sizes, and a qualified installer.
Do not treat Wheel Offset and ET Calculator — Effective ET as mechanical, safety, legal, or financial approval.
It is the direct output of the formula and entered values, and applies only to the defined scenario.
Which inputs change the result?
The active inputs are Old rim width (in), Old ET (mm), New rim width (in), New ET (mm), Spacer (mm). Changing any one runs the same formula again. A small diameter difference does not guarantee mechanical or legal fitment.
What to check next
The result is a starting point. For a clearer picture, continue to a related calculator or read a short guide that explains the assumptions.