Quick answer
Offset (mm) = (backspacing − wheel width ÷ 2) × 25.4. An 8 inch wide wheel with 5.5 inches of backspacing has an offset of (5.5 − 4) × 25.4 = 38.1 mm, a positive offset.
Enter the wheel width and backspacing in inches. The calculator returns the offset and its type.
Related tools: Tire Size Calculator: Overall diameter, sidewall height and revolutions per mile from a tire size such as 225/45R17. Fuel Cost Calculator: Trip fuel cost from distance, fuel economy and price. MPG Calculator: Miles per gallon from distance and fuel used.
How to use the Wheel Offset Calculator
- Enter wheel width (in) (above 0).
- Enter backspacing (in) (above 0).
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Wheel width | A number, in in | > 0 |
| Backspacing | A number, in in | > 0 |
Formula and method
Formula
offset (mm) = (backspacing − width ÷ 2) × 25.4
IEEE-754 double precision; shown to the decimals indicated.
Positive offset puts the mounting face toward the outside of the wheel.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Wheel width = 8; Backspacing = 5.5.
| Result | Value |
|---|---|
| Offset | 38.1 mm |
| Type | Positive offset |
Example 2
Inputs: Wheel width = 9; Backspacing = 4.5.
| Result | Value |
|---|---|
| Offset | 0 mm |
| Type | Zero offset |
The first line is the headline result (offset). Below it the wheel Offset Calculator lists type, which describe the same case from other angles. IEEE-754 double precision; shown to the decimals indicated.
Reading the sign
Offset is the distance from the wheel's centreline to its mounting pad. Positive offset puts the pad toward the outside face, which tucks the wheel inward; negative offset pushes the wheel outward. Zero offset has the pad on the centreline.
Measuring backspacing
Backspacing is measured from the mounting pad to the inner rim edge. Place the wheel face-down, lay a straight edge across the back and measure to the pad. A change of 10 mm in offset moves the wheel 10 mm in or out, which can cause rubbing.
How the result changes with wheel width
Holding every other input at the first worked example, this table changes only wheel width. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Wheel width | Offset | Type |
|---|---|---|
| 4 | 88.9 mm | Positive offset |
| 8 | 38.1 mm | Positive offset |
| 16 | -63.5 mm | Negative offset |
| 32 | -266.7 mm | Negative offset |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Wheel width: entering "0" shows "Wheel width must be greater than 0." beside that field and no result.
- Backspacing: entering "0" shows "Backspacing must be greater than 0." beside that field and no result.
Choosing between the Wheel Offset Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Tire Size Calculator](/automotive/tire-size-calculator/) | section width, aspect ratio and rim diameter | Overall diameter |
| [Fuel Cost Calculator](/automotive/fuel-cost-calculator/) | distance, fuel economy and fuel price | Trip cost |
| [MPG Calculator](/automotive/mpg-calculator/) | miles driven and gallons used | Fuel economy |
| [Engine Displacement Calculator](/automotive/engine-displacement-calculator/) | bore, stroke and cylinders | Displacement |
| [Car Depreciation Calculator](/automotive/car-depreciation-calculator/) | purchase price, yearly depreciation and years | Estimated value |
Background: vehicle sizing and speed
A taller tyre travels farther per revolution, so the speedometer reads low and the effective gear ratio becomes numerically lower; a shorter tyre does the opposite. True speed is roughly the speedometer reading times the new tyre diameter divided by the old one. Verify with GPS over a known distance before changing parts.
Assumptions and limitations
- Measure backspacing from the mounting pad to the inner lip
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for wheel width and backspacing to confirm this.
Frequently asked questions
What is a typical offset?
Many cars use +35 to +50 mm.
Does a lower offset make the wheel stick out?
Yes, a lower number moves the wheel outward.
How do I convert inches to mm?
Multiply the inches by 25.4 to get millimetres.
What does the Wheel Offset Calculator calculate?
Wheel offset from width and backspacing. It returns Offset and Type.
What formula does the Wheel Offset Calculator use?
offset (mm) = (backspacing − width ÷ 2) × 25.4. Positive offset puts the mounting face toward the outside of the wheel.
What do I need to enter in the wheel Offset Calculator?
You enter Wheel width (in) and Backspacing (in).
How is the result rounded or limited?
IEEE-754 double precision; shown to the decimals indicated. Measure backspacing from the mounting pad to the inner lip.
What is an example of a wheel Offset result?
With wheel width 8 and backspacing 5.5, the offset is 38.1 mm.
Where does the method come from?
The method follows: SAE wheel geometry definitions.
Related calculators
- Tire Size Calculator — Overall diameter, sidewall height and revolutions per mile from a tire size such as 225/45R17.
- Fuel Cost Calculator — Trip fuel cost from distance, fuel economy and price.
- MPG Calculator — Miles per gallon from distance and fuel used.
- Engine Displacement Calculator — Displacement from bore, stroke and number of cylinders.
- Car Depreciation Calculator — Estimated value after years of declining-balance depreciation at a rate you enter.
- Compression Ratio Calculator — Static compression ratio from bore, stroke and clearance volume.
Sources
- SAE wheel geometry definitions
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.