Wheel Offset Calculator – From Backspacing to Offset, and Back Again

The picture: centreline against mounting surface

The dashed line is the wheel centreline. The solid amber line is the mounting surface. The gap between them is your offset, drawn to scale at 40 pixels per inch and capped at the edge of the diagram for very large offsets.

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How it works in three steps

How the number is worked out

Going deeper

Five wheels, five offsets — and why width is never optional

Every figure below was run through the calculator above, starting from its default wheel — 8 inches wide with 5 inches of backspacing — and changing one thing at a time. The formula never changes: offset equals backspacing minus half the wheel width, with the centreline sitting exactly halfway across the width. Positive means the mounting surface sits outboard of the centreline, toward the outer face of the wheel; zero means it sits exactly on the centreline; negative means it sits inboard, toward the inner edge. Front spacing, shown beside every result, is simply width minus backspacing.

The default row is the page’s own worked example: half the 8-inch width is 4 inches, and 5 minus 4 gives +1.00 inch, which is 1 times 25.4, or +25.4 mm. The front spacing is 8 minus 5, or 3.00 inches — the part of the wheel that sits outboard of the mounting surface. Those three numbers together — width, backspacing, offset — describe the wheel’s position completely, and any two of them determine the third: that is why the calculator offers both directions, offset from backspacing and backspacing from offset, and why the reverse mode’s arithmetic is the same subtraction run backwards (backspacing equals offset plus half the width).

The second row is the lesson the whole page exists to teach: widen the wheel without moving the mounting surface, and the offset changes even though nothing about the mounting moved. At 9 inches wide with the same 5 inches of backspacing, half the width is 4.50 inches and the offset falls to +0.50 inches (+12.7 mm), while the front spacing grows to 4.00 inches. The mounting surface is in exactly the same place relative to the inner edge — 5 inches in — but the centreline moved outboard by half an inch when the wheel widened, so the mounting surface is now only half an inch outboard of it instead of a full inch. Anyone who carries an offset from an 8-inch wheel to a 9-inch wheel without recomputing has carried a number that no longer describes the wheel: two wheels can share an offset but have different backspacing if their widths differ, and two wheels can share a backspacing but have different offsets for the same reason. Width must always be part of the calculation.

The zero and negative rows complete the sign’s meaning. At 9 inches wide with 4.5 inches of backspacing, the mounting surface sits exactly on the centreline: offset 0.00 inches, front spacing 4.50 inches, exactly half the wheel on each side. At 8 inches wide with 3 inches of backspacing, the offset is 3 minus 4, or −1.00 inch (−25.4 mm): the mounting surface sits inboard of the centreline, toward the inner edge, and the front spacing grows to 5.00 inches — most of the wheel now sits outboard of its own mounting surface. The sign is the whole story in one character: positive tucks the wheel inward toward the car, negative pushes it outward, and zero splits the difference exactly. The reverse row confirms the arithmetic from the other direction: an 8-inch wheel at a wanted offset of +2.00 inches needs backspacing of 2 plus 4, or 6.00 inches (152.4 mm), leaving a front spacing of 2.00 inches — a deeply tucked wheel, most of it inboard of the mounting surface.

Offset, backspacing and front spacing together

Three numbers describe where a wheel sits, and confusing any two of them is how fitment mistakes happen. Backspacing is measured from the inner edge of the wheel to the mounting surface. Front spacing is measured from the mounting surface to the outer edge, and it is simply width minus backspacing. Offset is measured from the centreline to the mounting surface, and it is backspacing minus half the width. The three are linked by the width itself: backspacing plus front spacing always equals the width (5.00 + 3.00 = 8.00 on the default wheel), and offset is the difference between backspacing and half the width, signed by which side of the centreline the mounting surface falls on. If any two of the three are known, the third follows exactly, and the calculator’s two modes are just the two directions of that link.

Why three numbers for one position? Because different people, different manufacturers and different fitment guides quote different ones, and a wheel described only by its offset is underspecified without its width. A +25.4 mm offset on an 8-inch wheel means 5.00 inches of backspacing and 3.00 inches of front spacing; the same +25.4 mm offset on a 9-inch wheel means 5.50 inches of backspacing and 3.50 inches of front spacing — the mounting surface sits half an inch further outboard in absolute terms, even though the offset number is identical. Conversely, a backspacing quoted without width tells you where the mounting surface sits from the inner edge but nothing about where the centreline is, and a front spacing quoted alone tells you nothing about the inner edge at all. Always carry the width with whichever number you are given, and convert before comparing: inches to millimetres at 25.4 per inch, in both directions, with the sign preserved.

What offset does to fitment, in the page’s own framing, is a matter of direction, not of approval: positive offset tucks the wheel further under the car, negative offset pushes it further out, and the consequences — clearance to suspension and brakes on the inside, clearance to arches and bodywork on the outside, and the change in track width and scrub radius — depend on the vehicle, the tyre, and the suspension, none of which this calculator knows. That is why the page guarantees nothing about fitment: it computes the geometry exactly from the numbers entered, and the question of whether a given geometry fits a given vehicle belongs to the vehicle’s fitment data, a test fit, and the manufacturer’s guidance — not to an offset number alone. A correct offset calculation is necessary for a correct fitment decision, and it is not sufficient for one, and the page is honest about both halves of that sentence.

Measuring, converting, and checking your numbers

Backspacing is the input most often measured wrong, so measure it the way the definition says: from the inner edge of the wheel — the edge that faces the car, not the outer face you see — straight across to the mounting surface, the flat face that bolts against the hub. Lay the wheel face down on a flat surface, lay a straight edge across the inner rim, and measure down to the mounting surface: that distance is the backspacing. Measure the width the same way the offset formula needs it: the full width of the wheel, edge to edge, in the same units as the backspacing. Mixing a width in inches with a backspacing in millimetres, or a width measured to the outer lips against a backspacing measured to the inner edge, corrupts the subtraction by exactly the mismatch — and the offset that comes out will be confidently wrong by that amount, with no warning, because the calculator cannot know which edge you measured from.

Conversion between inches and millimetres is exact and sign-preserving: multiply inches by 25.4 to get millimetres, divide millimetres by 25.4 to get inches, and carry the sign through unchanged. The page’s own conversions show the pattern: +1.00 inch is +25.4 mm, −1.00 inch is −25.4 mm, 0.00 inches is 0.0 mm, and a wanted offset of +2.00 inches needs 6.00 inches of backspacing on an 8-inch wheel, or 152.4 mm. The reverse mode reports backspacing in both units for the same reason the forward mode reports offset in both: whichever unit the wheel or the fitment guide quotes, the other is one multiplication away, and rounding should happen last, not first — convert the exact figure, then round for display, or a wheel ordered on a rounded conversion can arrive a millimetre or two from the intended position. A millimetre sounds small until it is the millimetre between a tyre and a suspension arm.

Three checks, each taking seconds, verify any offset calculation. First, backspacing plus front spacing must equal the width: 5.00 + 3.00 = 8.00 on the default wheel, and any pair that does not sum to the width contains an error in one of the three numbers. Second, the offset must equal backspacing minus half the width, with the sign matching the description: if the mounting surface is outboard of the centreline, the offset must be positive, and a positive offset with an inboard description (or vice versa) means a sign was dropped somewhere. Third, run the calculation in reverse: take the offset the forward mode gave, enter it in the reverse mode with the same width, and confirm the backspacing that comes back is the one you started with — 8 inches at +1.00 inch must return 5.00 inches, as the page’s own example does. A calculation that survives all three checks is geometrically sound; whether the geometry fits a particular vehicle is a separate question, for fitment data and a test fit, and no calculator can answer it from offset alone.

Staggered wheels — different widths front and rear — are where the width lesson from the table above becomes a practical necessity rather than a curiosity. Consider a car with 9-inch wheels at the rear and 8-inch wheels at the front, both ordered at the same +1.00 inch offset. The rear wheels, at 9 inches wide, need 5.50 inches of backspacing (1.00 + 4.50); the front wheels, at 8 inches wide, need 5.00 inches (1.00 + 4.00). Order both axles at 5.00 inches of backspacing and the rears will sit at +0.50 inches, not +1.00 — half an inch further outboard than intended, because the wider rear wheel’s centreline sits half an inch further outboard at the same backspacing. The verified figures confirm the pattern from the other direction too: a 10-inch wheel with 6 inches of backspacing also gives +1.00 inch (6 minus 5), the same offset as the 9-inch wheel at 5.50 and the 8-inch wheel at 5.00 — three different backspacings, one offset, three different widths. Offset without width is not a specification; it is half of one, and staggered setups are where the missing half bites.

Bolt pattern and centre bore are the two fitment quantities most often confused with offset, and neither is measured or computed on this page — which is itself the point. Bolt pattern (the number of studs and the diameter of their circle) decides whether the wheel bolts onto the hub at all; centre bore (the hole in the middle of the wheel) decides whether it centres on the hub correctly, with hub-centric rings bridging a larger bore to a smaller hub where appropriate. A wheel with perfect offset but the wrong bolt pattern does not fit, however exact the offset arithmetic; a wheel with perfect offset and bolt pattern but a centre bore smaller than the hub does not fit either. Offset, backspacing and front spacing answer one question — where the wheel sits laterally — and fitment as a whole answers several more, including bolt pattern, centre bore, brake clearance, tyre size, load rating, and arch clearance under suspension travel. This calculator answers its one question exactly; the other questions belong to the vehicle’s fitment data, the wheel manufacturer’s specifications, and a test fit, and no offset figure, however precisely computed, substitutes for them.

A last word on precision: offset is usually quoted in millimetres by manufacturers and in inches by many fitment guides, and the conversion at 25.4 per inch is exact — but the wheels themselves are manufactured to tolerances, and the measurements you take from a real wheel carry their own small errors. A backspacing measured to the nearest sixteenth of an inch carries an uncertainty of about 1.6 mm in the resulting offset, and a width measured to the nearest eighth carries half that into the half-width term. Those uncertainties do not make the calculation unreliable — they make the last decimal place unreliable, which is why the calculator displays offsets to two decimal places in inches and one in millimetres rather than pretending to more precision than the inputs support. When a fitment decision turns on a millimetre, measure twice, with the wheel off the car and the straight edge truly straight, and treat the calculator’s display as exact arithmetic on measurements that are themselves approximate. Precision in the formula and honesty about the measurements are both required; either alone is not enough.

When a fitment decision is genuinely close — a millimetre or two of clearance either way — the calculator’s role ends and the workshop’s begins. No offset figure, however exact, accounts for tyre sidewall bulge beyond the rim edge, for suspension movement under load, for manufacturing variation between two wheels of the same nominal specification, or for the difference between a wheel measured on a bench and a wheel bolted to a hub with a tyre mounted and inflated. The honest use of this page in a close case is to narrow the candidates: compute the offset and front spacing for each candidate wheel, discard every candidate whose geometry is plainly wrong for the vehicle, and test-fit the survivors — preferably with the tyre that will actually be used, at the pressures that will actually be run, through the full steering and suspension range. A test fit that clears everywhere is evidence; a calculation that clears on paper is a prediction; and where the two disagree, the workshop wins, because the workshop is measuring the real wheel on the real car.

Finally, a note on used and refurbished wheels, where the numbers on the listing may not be the numbers on the wheel. A wheel that has been repaired, widened, or re-barrelled may carry a width or offset different from its original specification, and a listing that quotes the original specification for a modified wheel is quoting a wheel that no longer exists. Measure a used wheel the way the measuring paragraph above describes — width edge to edge, backspacing from the inner edge to the mounting surface — and compute from the measurements, not from the listing’s specification, the manufacturer’s catalogue entry, or the specification of the wheel the car left the factory with. The same rule applies to wheels with spacers fitted: a spacer moves the mounting surface outboard by its own thickness, which changes the effective offset by exactly that thickness in the negative direction, and the calculation must use the effective mounting position, not the wheel’s bare offset. Measure what is actually bolted to the car, compute from that, and the offset figure will describe the wheel that is actually there.

Keep the measurements with the result: a wheel described as “8 inches, 5 inches backspacing, +1.00 inch offset” can be checked, ordered, and replaced exactly, while a wheel described only as “+1 inch offset” cannot. Record all three numbers, with their units, every time.

New guide: Wheel Offset Explained: ET, Backspacing, Bolt Patterns, and Fitment

A wheel that bolts onto the hub is not necessarily a wheel that fits the car.

Offset (ET) is measured in millimetres from the wheel centreline: positive offset moves the wheel inward, negative offset moves it outward, zero offset puts the mounting face on the centreline. Most modern passenger cars use positive offsets, typically ET35 to ET50. The full guide, Wheel Offset Explained: ET, Backspacing, Bolt Patterns, and Fitment, works through the method in detail, with worked examples and the cases that most often go wrong.

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