What Does the Wheel Offset Calculator Do?
Wheel width and offset together determine where the nominal inner and outer wheel edges sit relative to the mounting face. This calculator compares an existing wheel with a proposed wheel and reports the change in inner clearance and outer projection. It helps separate the effect of a wider wheel from the effect of moving the mounting face within that width.
How to Use This Calculator
- Enter the current wheel width in inches and offset in millimetres, using the manufacturer's specification. The nominal width refers to the rim's bead-seat width, not necessarily the full external width across its lips.
- Enter the replacement width and offset on the same basis. Positive and negative offset have different effects, so retain the sign from the specification rather than entering only the magnitude.
- Calculate and read inner-position change and outer projection separately. A wheel can move closer to suspension components while also projecting farther out if it is wider.
- Use the differences as a starting point for a complete fitment assessment. Check the tyre, spokes, brakes, steering and suspension movement independently; width and offset alone do not describe the complete assembly.
The Maths & Assumptions Behind the Tool
Convert width from inches to millimetres by multiplying by 25.4. Half-width plus offset describes the nominal inner extent from the hub mounting plane; half-width minus offset describes the nominal outer extent. Subtract old from new for each side. A positive inner increase means less inboard clearance under this convention. Increasing offset alone moves the wheel inward, while increasing width alone adds half the extra width to each side.
Worked example
Changing from 8J ET35 to 9J ET35 adds 25.4 mm of wheel width. With unchanged offset, half is added to each side: 12.7 mm more outer projection and 12.7 mm less inner clearance.
Changing the new wheel’s offset to ET45 moves it 10 mm further inboard, leaving 2.7 mm more outer projection and 22.7 mm less inner clearance than the original. These are wheel-edge geometry changes; tyre shape, brake clearance and suspension movement still require a fitment check.
Why This Matters for UK Drivers
Enter wheel width in inches and offset, commonly marked ET, in millimetres. The tool converts width before comparing the two edge positions. A larger positive offset moves the wheel further inboard; reducing offset moves it outward. If width also changes, the two effects can reinforce or partly cancel each other. Looking at offset alone is therefore insufficient when the replacement has a different width.
The calculation describes nominal wheel-edge position, not the complete fitted tyre envelope. Rim-flange shape, tyre width, brake clearance, suspension travel and steering movement can all matter. A result showing more inner clearance does not establish that the wheel clears the brake caliper, and more outer projection does not establish acceptable body clearance. Use this page to understand the geometry and prepare a fitment discussion, then verify the actual combination against manufacturer information and physical clearances.
Interpreting the result in practice
Consider changes separately before combining them. Adding one inch of width contributes 12.7 mm to each nominal side. Adding 10 mm of positive offset moves the whole wheel 10 mm inward. Together, those changes add 22.7 mm inboard and 2.7 mm outboard. This decomposition is a useful check on the signs and explains why simply comparing offset numbers can mislead when widths also differ.
The calculation is relative to the hub plane and assumes the quoted widths are comparable. It cannot know spoke curvature, barrel shape or the tyre's actual section width. A wheel with apparently favourable edge positions may still contact a brake caliper. Static clearance also does not establish clearance throughout steering travel and suspension compression.
If spacers or other mounting changes are involved, obtain suitable technical advice and vehicle-specific information. This page does not assess fastener engagement, hub location, loads or approval. It also does not recommend a spacer size. Use the nominal position differences to frame the fitment question accurately, not as a substitute for checking the complete installation.
An unchanged width makes the offset effect particularly clear. Moving from ET35 to ET45 shifts the nominal wheel 10 mm inward: the inner edge is 10 mm closer to inboard components and outer projection decreases by 10 mm. Moving from ET35 to ET25 reverses those changes. If a result seems to move both edges outward at unchanged width, recheck the sign convention or the inputs.
Actual clearance is a separate measurement. Knowing that a replacement moves an inner edge 8 mm inward is only useful when you also know the available clearance and how it changes in use. The tool reports the change, not the remaining gap. It has no access to the vehicle's suspension geometry or the wheel manufacturer's detailed profile.
This is a wheel-position calculation, not a fitment guarantee
Brake caliper shape, spoke design, tyre sidewall width, suspension movement, steering angle and body clearance are not captured by width and offset alone.
Check the complete package
Use the tyre-size comparison alongside this calculator, then verify clearances physically or against reliable vehicle-specific fitment data before purchasing wheels.