Wheel Offset & Backspacing Calculator

Convert offset (mm) to backspacing (in) and back — plus hub-to-inner-edge distance and a rough fender vs frame clearance flag

Enter listed wheel width, a bolt-pattern reference, and either target offset or target backspacing. The other value, hub-to-inner-edge distance, and a clearance flag come back immediately. Math walkthrough: guide. Rubbing, scrub, and bearing wear: troubleshooting. Spacers vs buying the right wheel: comparison. Related tire geometry: tire size calculator. Automotive index: automotive calculators hub.

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How to Use This Wheel Offset & Backspacing Calculator

Enter the listed (bead-seat) wheel width. Pick a bolt-pattern reference so the cheat sheet matches the wheel you are pricing. Choose offset mode if the catalog shows ET in millimeters, or backspacing mode if you taped an existing wheel. The results panel returns the other value, hub-to-inner-edge (same as backspacing), hub-to-outer-edge, and a rough clearance flag.

Backspacing (in) = (W + lip) / 2 + ET / 25.4
Offset (mm) = (BS − (W + lip) / 2) × 25.4

Worked 8 in / +35 mm example and how to tape a used wheel: guide. Fender rub vs strut/caliper rub: troubleshooting. Whether a spacer is the cheaper fix: comparison.

What These Numbers Are (and Are Not)

Offset (ET) is the distance from the wheel’s centerline to the mounting pad, in millimeters. Positive ET puts the pad outboard of centerline, so more of the barrel hangs inboard of the hub — inner lip closer to the frame. Negative ET puts the pad inboard of centerline, so more of the barrel hangs outboard — outer lip closer to the fender.

Backspacing is the distance from that same mounting pad to the inner lip, usually in inches. It is the hub-to-inner-edge number. The two values describe the same geometry; catalogs print ET, and a tape on a used wheel prints backspacing.

Assumptions baked into every result

Listed width is bead-seat width. Overall width = listed width + lip allowance (default 1.00 in). The conversion does not know your stock ET, fender roll, lift, or caliper package. Bolt pattern is a label, not a check.

A clearance flag is not a mock-fit. Positive offset raises backspacing and tucks the wheel toward the frame and strut. Negative offset lowers backspacing and pokes the wheel toward the fender lip. Confirm both ends at full lock and full bump. Symptom walkthrough: troubleshooting.

When this tool is the wrong tool

Certifying a lift kit, reconstructing a crash, or picking load rating / hub bore. Pair offset math with rolling-diameter checks on the tire size calculator when the new wheel also changes tire size.

Need the measurement steps? How to tape backspacing on a used wheel and convert before you order.

Open Offset Guide →

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Wheel Fitment Shop Gear

The calculator converts ET and backspacing. A caliper confirms lip stack on a used wheel; a 1/2 in socket set and a click torque wrench install the wheel to spec after you mock-fit. None of this replaces a test-fit at lock and bump.

Starrett 0-6 inch electronic slide caliper

Starrett Stainless Steel Electronic Slide Caliper 0-6in EC799A

Measure lip thickness and hub-pad stack on a used wheel when the ET stamp is gone. Compare the tape backspace to the calculator before you order a replacement.

View on Amazon →

DEWALT Impact Socket Set Metric/SAE 1/2in Drive 23-Piece

Lug nuts and hub-centric rings come off and go back on during a mock-fit. Use impact sockets on an impact, then finish to spec with a torque wrench — not the gun.

View on Amazon →

CDI 2503MFRPH 1/2-Inch Drive Torque Wrench 30-250 ft-lb

After a spacer or a new-offset wheel, lug torque is not optional. Click to the vehicle spec in a star pattern on the ground, not hanging on a jack.

View on Amazon →

Check Rolling Diameter Too

A new wheel often arrives with a new tire size. Diameter percent, true vs indicated speed, and the common ±3% band.

Open Tire Size Calculator →

Tow Vehicle Wheel Setup

Custom wheels on a tow vehicle still need tongue-weight discipline. 10–15% band from loaded trailer weight.

Open Tongue Weight Calculator →

GCWR Headroom Before You Tow

Wheel offset does not raise hitch class. Leftover trailer budget after curb, passengers, and cargo.

Open Towing Capacity Calculator →

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Wheel Offset & Backspacing Calculator

How to Use This Wheel Offset & Backspacing Calculator