Wheel Offset Calculator

A wheel listing reads something like 18x9 ET35. The first two numbers are easy: eighteen inches across, nine inches wide. The third is the one buyers skim, and it decides more than either of the others. Offset is not a measure of how far a wheel sticks out. It says where the middle of the rim sits relative to the face that bolts to the car, and that single distance, combined with width, fixes two edges at once.

One edge heads for the fender, the other for the suspension, and a few lines of arithmetic show both before a single lug nut is turned. What the arithmetic cannot see matters just as much.

Where the wheel meets the car

Start with the definition, on which the sources agree. Summit Racing’s help pages call offset the distance between the centerline of the wheel and the hub mounting surface, measured in millimeters. Transport for NSW, the transport agency of New South Wales, defines it the same way. Zero offset puts the mounting surface on the centerline. Positive offset puts it in the outer half of the wheel, the usual arrangement on factory wheels for front-wheel-drive cars. Negative offset puts it in the inner half and produces the look often called deep dish.

The marking ET, as in ET35, comes from the German word Einpresstiefe. Continental’s technical databook calls the same thing offset depth and singles it out among wheel dimensions as a particularly important factor for the free movement of the tire in any wheel position. Offset decides whether things touch.

Width needs the same care, because a nine inch rim is not nine inches from lip to lip. Summit states that wheel width is measured from the inner surface of the lip, where the tire bead is mounted, and adds in capitals that it is not the overall width. Continental’s rim table agrees: the nominal figure is the distance between the flanges, 203 mm for an 8J rim and 228.5 mm for a 9J, and each flange of a J rim is 11 to 15 mm wide beyond that. The metal therefore measures roughly an inch more than the label. As a fitment column in LSX Magazine puts it, a 10 inch wheel measures 11 inches overall.

That inch matters for the older American measure, backspacing. It is the distance from the hub mounting surface to the innermost surface of the rim, taken in inches with a straightedge laid across the back of the wheel. Because the straightedge rests on the lip, backspacing includes the lip. The usual conversion adds one inch to the width, halves the result, and adds the offset converted to inches. Summit’s own chart follows that rule: an 8 inch wheel with 4.5 inches of backspacing has zero offset, and so does a 9 inch wheel with 5.0.

Wheel-Size.com traces the two measures to history. Backspacing served American cars whose owners mostly needed to know how much room there was on the inside, while offset arrived with European practice. They describe one wheel in two languages.

Two edges from one sum

The arithmetic follows directly from the definition. Convert the width to millimeters and halve it. The centerline sits inboard of the hub face by the offset, so the inner flange lies half the width plus the offset from the hub face, and the outer flange lies half the width minus the offset. Add the two distances and the width comes back, which is a useful check.

Take a common upgrade, from an 8 inch rim at ET45 to a 9 inch rim at ET35. Eight inches is 203.2 mm, so half is 101.6. The inner edge of the current wheel sits 101.6 plus 45, or 146.6 mm, inboard of the hub face, and the outer edge 101.6 minus 45, or 56.6 mm, outboard. Nine inches is 228.6 mm and half is 114.3, which puts the new inner edge at 149.3 mm and the new outer edge at 79.3 mm. The outer edge has moved 22.7 mm toward the fender. The inner edge has moved only 2.7 mm toward the suspension. Enter those two wheels in the calculator on this page and the same figures appear.

The lopsided result is the point of offset. An extra inch of width adds 12.7 mm to each side. Dropping the offset by 10 mm then slides the entire wheel outward by 10 mm, which takes nearly all of the growth away from the inside and piles it onto the outside. Keep ET45 on the wider rim instead and each edge moves 12.7 mm. Same width, same tire, a very different fit.

New South Wales defines track as the distance between the wheels on an axle, measured between the rim centerlines. Each wheel in the example has moved its centerline out by 10 mm, so the track has grown by 20 mm.

A spacer belongs in the same sum. A plate between hub and wheel pushes the mounting surface outward by its thickness, which amounts to subtracting that thickness from the offset. An ET45 wheel on a 10 mm spacer sits exactly where an ET35 wheel would.

What the steering and the bearing feel

Moving the centerline changes more than clearance. The steering axis meets the road at a point, and the distance from that point to the center of the tire’s contact patch is the scrub radius. Push the wheel outward and that distance grows in the positive direction. Nitto’s DrivingLine describes the result: the tire no longer pivots on its center but swings in an arc as it steers, which takes more effort and sends more of every bump back to the steering wheel. It puts a stock Jeep Wrangler at roughly 1 to 1.5 inches of scrub, and the writer’s own admittedly imprecise measurement at 2.5 inches on 38 inch tires with wheels at minus 38 mm.

Carmakers choose that geometry on purpose. A negative scrub radius compensates naturally when one brake circuit fails or when grip differs from side to side, and newer cars tend to have it. That may be why the New South Wales guidelines single out vehicles with diagonally split brake systems and require their front wheel offset and front track to remain as original. The guideline itself gives no reason.

The wheel bearing notices too. SKF’s calculation note treats the hub as two rows of rolling elements with the load arriving along a line through the wheel, and it shares that load between the rows according to where the line falls in the span between them. In its racing car example the line sits 27 mm into a 51 mm span and the rows carry almost the same, 620 and 572 newtons. In its truck example the line is 90 mm into a 100 mm span, and one bearing carries 22,000 newtons against 6,000 for the other. In that picture an offset change does one thing: it moves the line, millimeter for millimeter.

The same note gives bearing life as the ratio of rating to load raised to a power, ten thirds for the tapered rollers in its example. By that formula ten percent more load means about 27 percent less life. New South Wales says it without the algebra: wider wheels usually change the offset, increasing the loads on bearings, axles, suspension joints and steering tie rods.

None of this yields a number for a particular car without its steering axis and bearing positions, and the calculator does not attempt one. It reports where the rim goes.

Spacers and the center of the wheel

Spacers are the quick way to change effective offset, and the hardware is more varied than the word suggests. H&R lists thin plates from 3 mm that slip between wheel and hub and need longer wheel bolts, versions that replace the wheel studs with longer ones, and thick ones up to 65 mm that bolt to the hub and carry their own threads or studs for the wheel. It describes all of them as hubcentric, with a full contact hub like the factory one, for even load distribution and lateral support.

The fitting instructions for the slip-on kind show where the risk lies. The mating surfaces must be wire brushed clean, and the spacer must sit flush on the hub’s centering collar without play. The bolts must be longer by the thickness of the spacer, and the instructions set a minimum engagement: for an M12 x 1.5 thread at least 6.5 turns, about 10 mm of load-bearing thread, and for M14 x 1.5 at least 7.5 turns. After fitting, the wheel is turned by hand to confirm that the longer bolts touch nothing behind the hub, an ABS sensor for instance. Steel wheels are generally excluded.

Whether a spacer is allowed at all depends on where the car is registered. H&R sells its spacers with a parts approval or ABE and notes that this is valid only in Germany. New South Wales is blunter: spacers or adaptors must not be used between the mounting face and the wheel unless the manufacturer fitted them as original equipment. The same guideline caps the growth in track at 25 mm beyond the maker’s maximum for passenger cars and 50 mm for off-road four-wheel-drive and goods vehicles.

Where the arithmetic stops

Width and offset describe the outside of the barrel. They say nothing about the inside of the spokes, and that is where a brake caliper lives. Wilwood, for one, prints a wheel clearance diagram in its kit instructions, a profile of the caliper dimensioned from the wheel mounting surface. Its sheet for one front kit tells the installer to review the diagram before any disassembly begins and requires a minimum of 0.080 inch between wheel and caliper in all areas. It also warns that the diagram is for general fitment only and must not be relied on to make custom wheels.

Then there is the tire, which the sum ignores entirely. A tire is usually wider than the rim that carries it, and its width follows the rim: Continental gives roughly 5 mm of tire width for every half inch of rim width. Its tables allow an LT 265/70 R 17 to measure up to 288 mm across in service on an 8J rim, whose flanges are 203 mm apart. Rim-edge figures are therefore the least that a clearance will shrink, never the most.

The sum also describes a parked car with its wheels straight. Rules are written for the opposite case. New South Wales requires that wheel and tire stay within the bodywork with the wheels straight, and that they touch no part of the body or suspension under any operating condition, including full steering lock with the suspension fully compressed. Britain’s annual inspection lists a tire fouling a part of the vehicle as a major defect, although marks left by flexing or suspension movement are not counted.

Even the inputs deserve suspicion. Summit’s chart carries a note that actual offset may vary with the accuracy of the measurements taken, and the one-inch allowance for the lips is a convention with a range behind it. So treat the calculation as a filter that shows which wheels cannot fit and which deserve a closer look. For those, measure the car, read the brake maker’s diagram, and trial fit a bare wheel, because Summit warns that a wheel cannot be returned once a tire is mounted.