Wheel Offset Calculator — Clearance and Poke

Background.

Wheel offset is the signed distance from a wheel's imaginary centerline to its hub-mounting pad, normally marked in millimeters as ET. Wheel Pros explains that a higher positive offset tucks a wheel farther under the vehicle, while lowering the offset moves it toward the fender. Width matters at the same time: a wider wheel extends on both sides of its centerline, so comparing offset alone cannot tell you whether the inner barrel or outer lip will move closer to surrounding parts.

This calculator compares two nominal wheel widths and offsets. It converts each width from inches to millimeters using the exact 25.4 mm per inch relationship, divides the width in half, and places those half-widths on each side of the mounting pad. With the usual positive-offset convention, inner reach from the hub is half-width plus offset. Outer reach is half-width minus offset. Subtracting the two wheel positions gives changes that are easier to interpret during an initial fitment comparison.

A positive inner-clearance result means the new wheel edge sits farther away from the suspension side of the hub than the current wheel edge. A negative result means less mathematical clearance. A positive outer-poke result means the new wheel lip extends farther toward the fender. The centerline shift depends only on the offset difference, and doubling that shift estimates the axle track-width change if identical replacements are installed on both sides.

For the worked example, changing from an 8-inch ET45 wheel to a 9-inch ET35 wheel adds 12.7 mm of half-width on both edges and also shifts the centerline outward by 10 mm. The new inner reach is therefore 2.7 mm farther inward, giving 2.7 mm less barrel clearance. The outer edge extends 22.7 mm farther outward. Installing that offset change on both sides moves the theoretical wheel centerlines 20 mm farther apart.

These numbers compare nominal wheel edges, not the complete mounted tire. A wheel's published width is generally the bead-seat width, while its physical flanges extend beyond that nominal dimension. Tire section width, sidewall shape, rim protector, load, pressure, alignment, steering angle, suspension travel, brake-caliper envelope, hub face, spacers, studs or bolts, and manufacturing tolerance all affect actual fitment. A tire can be the limiting part even when the wheel-barrel comparison appears comfortable.

Do not treat zero or positive clearance as approval for a fitment. BBS application documents identify vehicle, wheel, tire, offset, hardware, and approval conditions together; width and ET alone do not establish legal or mechanical compatibility. Verify the vehicle and wheel manufacturer's application data, load rating, bolt pattern, center bore, fastener seating, brake clearance, and tire approval. Then perform a physical clearance check through full steering and suspension movement with qualified fitment help. The calculator is a geometry comparison, not a certification or modification instruction.

What is wheel offset calculator?

Offset locates a wheel centerline relative to the hub face. Positive ET places the mounting face toward the wheel's street side and moves the wheel inward; lower or negative ET moves its centerline outward. Combining width and offset lets the two barrel edges be compared before a physical trial fit.

How to use this calculator.

1. 01Read the current width and ET marking from the wheel specification.
2. 02Enter the proposed wheel's nominal width and signed offset in millimeters.
3. 03Treat negative inner-clearance change as movement toward suspension or brake components.
4. 04Treat positive outer poke as movement toward the fender.
5. 05Verify the full wheel-and-tire assembly and approved vehicle application physically.

The formula.

Current inner reach minus new inner reach gives the change in inner clearance. New outer reach minus current outer reach gives the change in outer extension. Current ET minus new ET gives outward centerline shift, and twice that value gives the theoretical same-axle track change. Decimal arithmetic preserves the exact inch conversion until output rounding.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Wheel Pros Help Center. What backspace or offset do I need? Defines offset, backspace, tuck and poke and explains the positive-to-negative direction.
2. [2]Wheel Pros Help Center. How do I identify my wheel? Explains wheel width and ET markings, including negative-offset notation.
3. [3]BBS Service. Vehicle- and wheel-specific application certificates and technical fitment data.