Softonic

Wheel Offset Calculator

Step 1 / 4 Current wheel
Wheel width unit

Wheel widths are commonly marked in inches; ET and spacer thickness stay in millimetres.

Current wheel

Enter the stamped nominal width and signed ET. Use 0 mm if no spacer is fitted.

Next

Changing wheel width or offset can move the rim closer to the suspension, farther toward the fender, or both. Use this calculator to compare your current and proposed wheels, including optional spacers. It estimates the change at the inner and outer wheel edges, effective offset after spacers and backspacing for each wheel. The results describe wheel geometry only: they cannot confirm tire, brake, hub or vehicle clearance.

How to compare wheel positions

  1. 1

    Enter the current wheel

    Use the width stamped on the wheel, such as 7.5J, and its signed offset in millimetres. Enter zero if the current setup has no spacer.

  2. 2

    Add the proposed wheel

    Enter the new stamped width and offset. A positive ET sits farther inward; a lower or negative ET moves the wheel outward.

  3. 3

    Include any spacers

    A spacer moves the installed wheel outward by its thickness. For example, a 5 mm spacer changes an ET+35 installation to an effective ET+30 position.

  4. 4

    Review both sides of the wheel

    Check the estimated inner-clearance change, outer-edge movement and backspacing. Then verify the complete wheel-and-tire package on the specific vehicle.

Wheel offset and position

Wheel offset, often marked ET, is the signed distance between the wheel centerline and its hub-mounting surface. Positive offset places the mounting surface toward the wheel face and generally moves the wheel inward. Negative offset places it toward the inner side of the wheel and moves the wheel outward.

The calculator first converts wheel width to millimetres. For a wheel with nominal width W, stamped offset ET and spacer thickness S:

Effective ET = ET - S

Inner position = W / 2 + Effective ET

Outer position = W / 2 - Effective ET

The positions are measured from the hub-mounting plane to the nominal bead-seat edges. A positive change in inner position means the new wheel reaches farther toward the suspension and leaves less inner clearance. A positive change in outer position means it extends farther toward the fender.

Nominal width is not overall width

A size such as 18 × 8J describes an 18-inch bead-seat diameter and an 8-inch nominal width between the bead seats. The physical wheel is usually about one inch wider across its flanges.

That distinction matters when estimating backspacing:

Backspacing in inches ≈ (Nominal width + 1) / 2 + Offset / 25.4

The one-inch allowance is an industry approximation, not a guaranteed flange dimension. A manufacturer drawing or a straightedge measurement from the inner lip to the mounting pad is more reliable. A spacer changes the installed position but does not alter the wheel’s manufactured backspacing.

Worked comparison

Compare a 7-inch ET+45 wheel with an 8-inch ET+35 wheel, with no spacers:

Measurement Current wheel New wheel Change
Nominal width 177.8 mm 203.2 mm 25.4 mm wider
Effective offset ET+45 ET+35 10 mm lower
Inner position 133.9 mm 136.6 mm 2.7 mm less clearance
Outer position 43.9 mm 66.6 mm 22.7 mm farther outward
Estimated backspacing 5.77 in 5.88 in 0.11 in more

Adding a 5 mm spacer to the new wheel makes its effective position ET+30. Compared with the original wheel, it then provides about 2.3 mm more inner clearance while extending about 27.7 mm farther outward. Its manufactured backspacing remains approximately 5.88 inches.

Why this is not a fitment guarantee

Offset and width cannot confirm whether a wheel fits. Tire section width, sidewall shape, wheel diameter, spoke design, brake-caliper clearance, bolt pattern, center bore, load rating, alignment and suspension travel all matter. Fender clearance can also change while steering, braking or compressing the suspension.

Use the result to compare geometry, then confirm the complete specification with the vehicle and wheel manufacturers or a qualified wheel installer. Tire Rack explains why vehicle-specific fitment requires more than size and offset, while Mickey Thompson documents the offset-to-backspacing convention.

Frequently Asked Questions

Positive offset places the hub-mounting surface toward the wheel face and generally positions the wheel farther inward. Negative offset places the mounting surface toward the inner lip and moves the wheel farther outward. ET0 means the mounting surface is on the wheel centerline.

Subtract the spacer thickness from the stamped offset to find the effective installed position. A 5 mm spacer on an ET+35 wheel produces the same centerline position as ET+30. The spacer does not change the offset stamped on the wheel or its manufactured backspacing.

Advertised wheel width is measured between the bead seats, while backspacing is measured from the mounting pad to the physical inner lip. The outer flanges commonly add about one inch to overall width. This is only an estimate because flange dimensions vary by wheel design.

No. It compares wheel-edge geometry but cannot check the tire, bolt pattern, center bore, brake caliper, spoke shape, load rating, steering clearance or suspension travel. Verify the complete wheel-and-tire package for the exact vehicle before buying or installing it.

Tire width does not change the wheel offset, but it can change the closest point to the suspension or fender. Actual section width also varies with rim width, tire model and inflation. Treat the calculated wheel-edge positions separately from tire clearance.

Wheel width is added on both sides of its centerline. If offset stayed unchanged, half of the extra width would move inward and half outward. Changing offset shifts the entire wheel as well, so the two effects must be calculated together.

It estimates the change between the left and right wheel centerlines if the same setup is installed on both sides of an axle. A wheel that moves 10 mm outward per side increases that centerline distance by approximately 20 mm. It does not describe tire width or prove legal clearance.

No. The calculator uses the site server to update results, and funnel links may include the entered dimensions in the URL so the next step can restore them. No wheel files are uploaded and the calculator does not contact an external vehicle-fitment service.

Related Tools

BMI Calculator

Calculate body mass index from height and weight. Shows WHO category, healthy-weight range and limitations of BMI.

Fahrenheit to Celsius Converter

Convert Fahrenheit temperatures to Celsius with the exact formula, Kelvin output and common reference points for weather, cooking and health.

Age Calculator

Calculate exact age in years, months and days from a birth date, plus total days, hours and the next birthday countdown.

Social Security Calculator

Your US Social Security retirement benefit for any start month from 62 to 70, from your date of birth and your PIA or AIME, with totals and the break-even age.

Name Compatibility Calculator

Enter two names and get a playful compatibility score plus the Pythagorean numerology number of each name. Works with the Latin alphabet (A to Z), so transliterate other scripts first.

Clock Out Time Calculator

Calculate your clock-out time from your clock-in time, required paid hours and unpaid breaks. Handles overnight shifts and shows 12- and 24-hour time.

Tool available in other languages