Free wheel offset calculator

Wheel offset calculator backspacing and poke

Convert offset in millimeters to backspacing in inches (or back), then compare two wheels to see how far the inner edge moves toward the suspension and the outer edge toward the fender.

Free, no sign-upRuns in your browserNothing is uploaded
I know each wheel's
Rim width (in)Offset (mm)
Current wheel
New wheel

Rim width is the number in the wheel size (17x8), measured between the flanges. Overall width is about an inch more. For a close fit, measure the real backspacing with a straightedge across the back of the wheel.

Both wheels
CurrentNew
What changes on the vehicle-
Inner edge (toward the suspension)-
Outer edge (toward the fender)-

In short

Backspacing = half the wheel's overall width + offset ÷ 25.4. Overall width is about an inch more than the rim width in the size, because of the flanges, so a 17x8 +35 mm wheel has about 4.5 + 1.38 = 5.88 in of backspacing. A 17x9 +12 sits 10.3 mm further from the strut and pokes out 35.7 mm more. Measure the real wheel to be sure.

How it works

How to use the Wheel Offset Calculator

1

Pick what you know

Each wheel's offset in mm, or its backspacing in inches.

2

Enter both wheels

Rim width and offset (or backspacing) for the current and the new wheel.

3

Read the change

Offset, backspacing and front spacing for each, and how the inner and outer edges move.

What it does

What the wheel offset calculator works out

Offset ↔ backspacing

Converts either way, using overall width = rim width + a flange allowance.

Adjustable flange allowance

1 inch by default; change it to match a measured wheel.

Inner clearance

How much closer the inner edge sits to the strut, spring and brake parts.

Outer poke

How much further the outer edge sticks out toward the fender.

Front spacing

Mounting face to the outer edge, for both wheels.

Backspacing
overall width ÷ 2 + offset (mm) ÷ 25.4
Offset
(backspacing − overall width ÷ 2) × 25.4
Overall width
rim width + about 1 in for the flanges (an approximation; measure)
Inner edge change
new backspacing − old backspacing
Outer edge change
new front spacing − old front spacing
FAQ

Questions about the wheel offset calculator

How do you convert wheel offset to backspacing?
Add 1 inch to the rim width for the overall width, halve it, and add the offset in inches (mm ÷ 25.4). A 17x8 with +35 mm offset: 9 ÷ 2 + 35 ÷ 25.4 = 5.88 in of backspacing.
What is positive and negative offset?
Offset is the distance from the wheel's centerline to its mounting face. Positive means the mounting face is toward the outside of the wheel, tucking it in; negative pushes the wheel out.
Why add an inch to the wheel width?
The width in a wheel size is measured between the flanges, where the tire bead sits. The flanges add about half an inch each side, so overall width is about an inch more. It varies, so measure if it matters.
Will a lower offset rub?
Lower offset moves the wheel out: more room at the strut, more poke at the fender. A wider wheel adds width on both sides. Tire size, steering lock and suspension travel decide whether it rubs, so test fit.
How do I measure backspacing?
Lay the wheel face down, put a straightedge across the back flange, and measure from the mounting surface up to the straightedge. Measure the overall width flange to flange too; then offset = backspacing − half the overall width.
Keep going

Guides and features for this

Tire size calculatorBayRobinAlignment pricing for shopsGuideTire pricing for shopsGuideBrake and tire shopsBayRobinAll guides on running an auto repair shopTopic guides
Sources

Where the formulas and rules come from

Checked October 2026. Overall width is approximated as rim width + the flange allowance. Measure the actual wheel for a close fit.

Link to this tool

Useful for your team or your readers? Paste this into a blog post, forum or resource page.

More free shop tools

All free tools
Tire Date Code CheckerFree tire age checkerTire Load Index and Speed Rating ChartFree load index lookupEffective Labor Rate CalculatorFree labor rate calculatorParts Markup CalculatorFree parts pricing calculatorRepair Order Gross Profit CalculatorFree RO profit calculatorTechnician Efficiency CalculatorFree tech efficiency calculator

Never miss a service call

BayRobin's AI receptionist answers every call with your shop's name, gets the concern and the year, make and model, and books the appointment into your schedule. Free until your first 10 booked jobs, no card up front.

Start freeSee the AI receptionist