Wheel Offset Calculator

How wheel offset and backspacing work

Every wheel bolts to the hub through a flat mounting face, the pad around the center bore. Offset says where that face sits relative to the centerline of the rim, the plane halfway between its bead seats. A positive offset puts the mounting face outboard of the centerline, so more of the rim hangs inboard toward the strut; most front-wheel-drive and late-model cars use positive offsets. Zero offset puts the face on the centerline. A negative offset puts the face inboard, so the rim sits further out, the deep-dish look of many older trucks and drag wheels.

Backspacing describes the same wheel from a different starting point: the distance from the mounting face to the inner edge of the rim. Front spacing is the distance from the mounting face to the outer edge. Offset is quoted in millimeters and backspacing in inches, which is why a rim offset calculator that works both ways saves a lot of head-scratching when you shop across catalogs.

There is one catch. The width in a wheel size, the second number in an 18x8.5 wheel, is measured between the bead seats, where the tire sits. The lips, or flanges, add to that on each side, and backspacing is measured to the lip. The international rim standard for passenger cars (ISO 4000-2) sets each J flange at least 11 mm wide, so a typical passenger-car wheel is at least 0.87 in wider across its lips than its nominal width. Offset and backspacing conversion charts use 1 in as a convention; real wheels run about 0.5 to 1.0 in, so measure lip to lip when backspacing is critical. That allowance is an input you can change.

What to enter and where to find it

• Wheel width. The second number of the wheel size, the width in an 18x8.5 wheel. It is often cast on the back of a spoke together with the offset.
• Offset. Marked as ET followed by the offset in millimeters (ET45 is a positive offset), or as a signed number in millimeters. Some wheels marked in inches give offset as a fraction of an inch; multiply by 25.4 to get millimeters.
• Backspacing. Lay the wheel face down on a flat floor, put a straightedge across the inner lip and measure from the straightedge down to the mounting pad. That is the number to use in the Backspacing to offset tab.
• Flange allowance. Measure the wheel across its lips with a tape or calipers and subtract the nominal width. If you can't measure it, leave the 1 in that conversion charts use: it only affects the backspacing figures, not how far a new wheel moves.
• Wheel diameter. Only used to draw the wheel to scale. It doesn't change any number.

Wheel offset comparison calculator: where a new wheel ends up

Changing width and offset at the same time moves the two lips of the wheel by different amounts, which is where most fitment surprises come from. A wider wheel grows by half the extra width on each side of its centerline. A lower offset then shifts the whole wheel outward by the difference, and a higher offset shifts it inward. Add the two effects and you get how far the outer lip moves out, the poke, and how far the inner lip moves in toward the strut or spring.

The Compare wheels tab works as a wheel offset calculator visualizer: it draws both wheels in cross-section, cut through the axle and seen from behind, with the hub face as the fixed reference. The dashed wheel is the one on the car today and the solid one is the new wheel. Both moves are shown in millimeters and inches, with the direction spelled out, because a sign alone is easy to misread.

A wheel never sits alone on the car, though. The tire bulges past the lips and its width depends on the rim it is mounted on. To turn these moves into clearances, with your tire sizes and the room you measured today, use the wheel fitment calculator.

Offset backspacing calculator: converting both ways

Domestic wheels for older cars and trucks are often sold by backspacing, while import and late-model wheels are sold by offset. The Offset to backspacing tab is a backspacing calculator for the first case: enter the width and offset and it gives the backspacing and the front spacing. The Backspacing to offset tab works the other way, for a wheel listed by backspacing only, and returns the offset in millimeters and inches.

Both conversions depend on the overall width, so a wheel that is a little wider across its lips than the typical allowance will measure a touch more backspacing than the chart says. When the conversion is close to a limit, measure the wheel.

The wheel offset formula

All of it is plain geometry from the hub face, so the results are exact for the numbers entered; the only estimate is the flange allowance when you don't measure it.

1. Overall width (in) = nominal width (in) + flange allowance (in)
2. Backspacing (in) = overall width ÷ 2 + offset (mm) ÷ 25.4
3. Offset (mm) = (backspacing − overall width ÷ 2) × 25.4
4. Front spacing (in) = overall width − backspacing
5. Outer lip moves out (mm) = (new width − current width) × 25.4 ÷ 2 − (new offset − current offset)
6. Inner lip moves in (mm) = (new width − current width) × 25.4 ÷ 2 + (new offset − current offset)

Half of the overall width is the distance from the centerline to either lip; the offset, converted to inches, is the distance from the centerline to the mounting face. Backspacing is their sum because the face sits outboard of the centerline when the offset is positive. In the comparison the flange allowance cancels out, since both wheels have lips of about the same width, so the moves depend only on the two widths and the two offsets.

Worked example

Three common questions, worked out with the same functions that run the calculator.

Offset to backspacing chart

The chart converts common offsets into backspacing for wheel widths from 6 to 12 inches, using the 1 in flange allowance that conversion charts use. Read across for a given width: every 25.4 mm of offset is exactly one inch of backspacing.

If your wheel's measured overall width differs from the nominal width plus 1 in, add half of the difference to the chart value. Offsets beyond these columns exist, but the conversion is the same straight line.

Common mistakes

• Comparing offsets of wheels with different widths. The same offset on a wider wheel still pushes the outer lip out by half the extra width, and the inner lip in by the same amount. Always compare width and offset together on the Compare wheels tab.
• Mixing up the sign. Going from +35 to +22 mm is a lower offset and moves the wheel out. Going more positive moves it in. Negative offsets are lower still.
• Using the nominal width for backspacing. Backspacing is measured to the lip, so the overall width counts. Leaving out the flange allowance makes the backspacing about half an inch short.
• Measuring backspacing from the wrong surface. Measure to the mounting pad that touches the hub, not to the face of the spokes or the center cap recess.
• Forgetting the tire. The wheel lip is not the outermost point once a tire is mounted: the sidewall usually bulges past it. Check the whole package with the wheel fitment calculator, and the overall diameter with the tire size calculator.