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Enter your values, then click Calculate result.How this calculator helps
The rim offset calculator compares two wheel specifications using their nominal widths and signed offsets. It reports how the outer edge moves relative to the current wheel and how the inner edge moves toward the suspension. Positive outward movement and positive inward encroachment are labeled separately so the signs remain useful. The tool does not compare tyres, inspect a vehicle or certify that a wheel fits. Use the geometry as a starting point for actual clearance measurements and verified wheel specifications, keeping mounting-face offset distinct from rim width and from tyre-section dimensions.
How to use it
- 1
Read current and proposed wheel widths in inches and their signed manufacturer offsets in millimetres.
- 2
Enter the values in the displayed units and verify the source record or measurement basis.
- 3
Click Calculate result to submit the current inputs.
- 4
Read the labeled outputs with the methodology and checks. Input edits retain the previous result until Calculate is clicked again.
Formula and methodology
Half-width change mm = (new width in − old width in) × 25.4/2. Inner movement toward suspension = half-width change + new offset − old offset. Outer outward movement = half-width change − new offset + old offset.
The calculator applies the displayed arithmetic to the values entered on this device. It does not silently load a local tax rate, currency conversion or commercial assumption.
Worked calculation example
Changing from an 8 in wheel at +40 mm offset to a 9 in wheel at +35 mm gives a half-width increase of 12.7 mm. The nominal outer edge moves outward 17.7 mm, while the inner edge moves 7.7 mm toward the suspension.
How to interpret your result
Changing from an 8 in wheel at +40 mm offset to a 9 in wheel at +35 mm gives a half-width increase of 12.7 mm. The nominal outer edge moves outward 17.7 mm, while the inner edge moves 7.7 mm toward the suspension. Nominal wheel-specification geometry only. No tyre-profile modeling, exterior flange correction, spacer handling, brake clearance, load rating or vehicle fitment approval. Compare the main quantity with the separately labeled intermediate values before using it in another worksheet. Retain units, input basis and any selected convention with the result so it can be reproduced.
For different inputs or formulas, use Tire Size Calculator; Gear Ratio Speed Calculator; Torque Converter.
Related questions this calculator covers
- rim offset calculator
Scenario comparison
| Scenario | What it shows |
|---|---|
| 8 in +40 to 9 in +35 | outer +17.7 mm, inner +7.7 mm. |
| Identical wheel specifications | both changes zero. |
| Same width with offset reduced 5 mm | outer +5 mm and inner −5 mm. |
Common mistakes to avoid
- Entering backspacing as signed offset.
- Reading positive inner movement as increased clearance.
- Treating nominal rim movement as full tyre fitment approval.
Convert the width difference to millimetres and split it equally. Apply the offset difference with opposite signs to the two sides. Identical inputs must yield zero changes. Confirm offsets from the manufacturer and compare nominal arithmetic with actual vehicle clearances.
Authoritative reference. NIST verifies the inch-to-millimetre unit factor. The page derives nominal edge movement from its disclosed centerline geometry; the source does not certify wheel fitment. Reference reviewed 4 October 2026. Mohammad Qasim reviews the arithmetic and editorial scope; individual professional assessment is not performed.What can affect the result?
Width and offset describe different dimensions
Wheel width is entered in inches and converted to millimetres before the edge comparison. Offset is already in millimetres and is signed relative to the wheel centerline. A positive offset places the mounting face on the outward side of that centerline under the stated convention. Do not enter backspacing as offset without a verified conversion and rim-profile basis. The calculation compares nominal specification widths rather than measuring the full exterior flange dimensions of two different wheel designs.
Read inner and outer signs separately
A positive outer movement means the proposed nominal outer edge extends farther outward. A negative outer movement means it retracts. A positive inner movement means the proposed inner edge approaches the suspension, reducing geometric space on that side. These are two different directions, so the same positive sign does not mean the same physical movement. The interface includes the convention in the labels; keep those labels when recording values instead of copying only unsigned magnitudes.
A wider wheel changes both sides
With offset held constant, an increased width adds half its converted width change to both edge distances. With width held constant, a lower offset moves the outer edge outward and the inner edge away from the suspension by equal amounts. When width and offset both change, the two effects combine. These limiting cases provide quick checks for the formula and make it easier to identify a sign mistake before interpreting a proposed setup.
Nominal rim geometry is not tyre clearance
The tyre can extend beyond the wheel edges, and its mounted shape depends on the tyre specification and wheel combination. Suspension motion, steering lock, brake hardware, wheel spokes, hub dimensions and body clearances also matter. A nominal edge comparison cannot validate those conditions. Obtain actual measurements and vehicle or component requirements before selecting equipment. The related tyre-size calculator answers a different dimensional question and should not be treated as a clearance certificate either.
Use one mounting-reference basis for both wheels
The comparison assumes the same hub mounting plane and no unmodeled spacer, adapter or mounting change. If those components are present, the effective mounting basis must be established separately rather than silently added to the wheel offset. A simple numerical difference also does not assess load rating, fastener engagement, bolt pattern or legal suitability. Retain the complete original and proposed specifications and compare the result with an appropriate physical inspection.
Privacy and browser processing
Values entered on this page are processed in the current browser session. SolvePilot does not require an account and does not receive the values entered into the calculator. Refreshing or closing the page clears the working values unless the browser itself restores a previous session. Avoid entering identifying or account information because the calculation needs summary values only.
Accuracy and verification
Accuracy depends first on input quality. Confirm definitions, scales, dates and source information before entering a value. Keep an independent record of any result used for planning because this page does not create an official statement or retain a calculation history.
Limits of this estimate
Nominal wheel-specification geometry only. No tyre-profile modeling, exterior flange correction, spacer handling, brake clearance, load rating or vehicle fitment approval. Results use the entered assumptions and do not verify their real-world applicability. Calculator inputs are processed locally by the browser interface. Avoid entering identifying records; save only the quantities and context necessary for your own review.
Sources and review information
Frequently asked questions
Does positive outer movement mean more outward extension?＋
Yes, under the labeled convention. Negative values mean the nominal outer edge retracts.
Does positive inner movement increase clearance?＋
No. It means movement toward the suspension, which reduces geometric space on that side.
Are tyre widths included?＋
No. The inputs are wheel width and offset. Actual tyre shape and clearances need separate verification.
Can backspacing be entered as offset?＋
Not directly. Establish the correct signed offset from a verified specification first.
Does this prove a wheel fits?＋
No. Brake, hub, tyre, steering, suspension, load and mounting requirements are not validated.