Wheel Offset Calculator

What does a wheel offset calculator do?

A wheel offset calculator tells you where a new wheel will sit compared with the one you have now: how many millimeters the outer face moves toward the fender, and how much closer the inner lip gets to the brakes and suspension. It also converts the offset stamped on the wheel (ET, in millimeters) into backspacing (in inches), the number most US truck and off-road catalogs quote.

Enter the rim width and offset of both wheels above. Add the tire section widths (the 245 in 245/40R18) if you want to see where the tire sidewalls end up too. The result reads in plain language, for example “outer edge sits 12 mm (0.47 in) further out”.

Key terms on this page. Offset (ET): distance from the rim’s centreline to the hub mounting face, in mm. Backspacing: distance from the mounting face to the inner rim lip, in inches. Front spacing: mounting face to outer lip. Poke: how far the wheel or tire sticks out toward or past the fender. Hub-centric: a wheel located on the hub’s center pilot rather than only by the lug nuts. Scrub radius: the distance at the road between the tire’s center and the line the wheel steers around.

What is wheel offset: positive, zero and negative?

Offset is the distance between the wheel’s mounting face and its centreline. UN Regulation No. 124, the international standard for replacement passenger-car wheels, defines this “inset” as “the distance from the attachment face of the disc to the centre line of the rim”, and notes it can be positive, zero or negative. The same regulation requires the inset to be permanently marked on the wheel, which is why you find “ET45” or similar cast on the back of a spoke.

• Positive offset (ET+): the mounting face is outboard of the centreline, toward the street side. The wheel tucks in under the body. Typical for front-wheel-drive cars and crossovers.
• Zero offset (ET0): the mounting face is exactly on the centreline.
• Negative offset (ET−): the mounting face is inboard of the centreline. The wheel sits further out, which gives the “deep dish” look on lifted trucks.

Heavy-truck wheels use different words for the same idea. Wheel maker Accuride explains that commercial wheels which can run as duals use an offset measurement, while single wheels use inset (wheel centerline inboard of the hub mounting surface) or outset (centerline outboard). If you shop for truck wheels, check which convention a catalog uses before comparing numbers.

What do +35, +40 and negative offsets mean? (8 in wheel table)

The number is millimeters: +35 means the mounting face sits 35 mm outboard of the rim centreline, +40 means 40 mm, and −44 means 44 mm inboard. On its own the number tells you little; what matters is the change from your car’s original offset. The table compares common offsets on the same 8 in wide wheel against an ET45 baseline, computed with this calculator:

So a +40 wheel on a car that came with +45 pokes out just 5 mm, while a −44 “deep dish” wheel on the same hub would sit 3.5 in further out, which is why heavily negative offsets usually go with lifted trucks, fender flares and wide-track builds. A “good” offset is the one that keeps the tire inside the fender and clears the brakes and suspension: start from the original ET on your placard or owner’s manual and change it in small steps.

How do you convert offset to backspacing?

Backspacing = (rim width + 1 in) ÷ 2 + offset ÷ 25.4. The extra inch is a convention, not a law of physics. The rim width on a size such as 18x8.5J is the nominal width between the flanges (FMVSS 120 defines rim width as “the nominal distance between the rim flanges”), but backspacing is measured to the outside edge of the inner flange. The flanges add roughly half an inch on each side, so the overall lip-to-lip width of a typical 8.5 in wheel is about 9.5 in. If you can, measure your own wheel with a straightedge across the back lip: flange shapes vary by maker.

backspacing (in) = (width + 1) ÷ 2 + ET ÷ 25.4  ·  ET (mm) = (backspacing − (width + 1) ÷ 2) × 25.4

This table gives backspacing in inches for common widths and offsets, computed with the same code as the calculator:

How far will my new wheels stick out?

Two things move the wheel’s edges: the change in offset and the change in width. Lowering the offset shifts the whole wheel outward by the difference. Adding width grows the wheel equally on both sides, half the extra width on each (12.7 mm per side for every extra inch). The calculator combines both:

A plus sign on the inner edge means less room between the wheel and the suspension; a minus sign means more. A plus sign on the outer edge means more poke. Read the two together: a wider wheel with a lower offset (row four) can keep almost the same inner clearance while pushing the face nearly an inch further out.

The tire adds its own width. The calculator assumes the tire is centred on the rim, so a 245 mm tire on a wheel that replaces a 225 mm tire adds 10 mm per side before any offset change.

How much clearance do wheels need?

There is no single US number: check your vehicle manual, the door-jamb placard and the wheel maker’s fitment data. In the US, FMVSS 110 makes carmakers list the original tire size on the placard and FMVSS 120 governs rim size marking for heavier vehicles, but neither sets an aftermarket offset limit for passenger cars.

The clearest published benchmark is international. Annex 10 of UN Regulation No. 124 says that when a replacement wheel’s inner contour is tighter than the carmaker’s, the check should, “as a rule”, find at least 3 mm of clearance to brake components (with new pads), 4 mm to suspension parts such as control arms, 4 mm to steering parts such as tie rods, and 2 mm between balance weights and the vehicle. Treat those as a minimum test benchmark, not a target: tires flex and suspensions move.

Does wheel offset affect handling and the speedometer?

Offset changes steering geometry, but it does not change your speedometer. Moving the wheel outward changes the scrub radius: the distance at the road between the tire’s contact patch and the steering axis. Carmakers tune scrub radius with the original offset, so a large change can add steering kickback over bumps, tramlining (the car following ruts) and extra load on wheel bearings. In our view, a few millimeters is rarely noticeable, while a change of 20 mm or more deserves a careful test drive and an alignment check.

Your speedometer counts wheel revolutions, so only a change in the tire’s overall diameter alters the reading. If you change tire size with your new wheels, run both sizes through the tire size calculator, then check the result with a GPS reading as explained in speedometer calibration and GPS vs car speedometer. The online speedometer works for that check from a passenger seat.

Hub-centric rings, spacers and lug-centric wheels

A hub-centric wheel centres on the hub’s pilot diameter; a lug-centric wheel relies on the tapered lug nuts. UN R124 treats the “spigot mounting diameter” (the center bore) as a fitment dimension alongside inset and bolt pattern. Aftermarket wheels often have a larger center bore to fit many cars, so hub-centric rings fill the gap. Rings do not change offset.

Wheel spacers do. A 15 mm spacer moves the mounting face outward by 15 mm, so an ET45 wheel on a 15 mm spacer behaves like ET30. Enter the effective offset in the calculator to see the result, and follow the spacer maker’s torque and stud-length instructions.

How accurate is this calculator?

The geometry is exact for the numbers you enter; the uncertainty is in the inputs. The 1 in flange allowance is a convention, real tires bulge differently by brand and rim width, and suspension movement changes clearance as you drive. The calculator uses one tested formula (src/lib/offset.ts) and shows millimeters and inches side by side so you can compare with any catalog. It’s a planning tool, not a fitment guarantee: always confirm with your vehicle manual, the wheel maker and a physical test fit.