Offset and backspacing measure the same wheel from two different reference points
Both numbers answer one question: where does the hub mounting face sit relative to the rest of the wheel? They differ only in what they measure from. Offset, marked ET after the German Einpresstiefe, is the distance from the mounting face to the wheel's centreline, in millimetres, positive when the mounting face is outboard of the centreline. Backspacing is the distance from the mounting face to the inboard flange edge, in inches. ISO 3911 is the standard that fixes this terminology for wheels and rims.
Because they share a reference — the mounting face — you can convert between them as soon as you also know the wheel's width. A wheel with high positive offset tucks deep into the arch; a wheel with negative offset pushes the whole rim outward and gives the deep-dish look. Backspacing rises and falls with offset in exactly the same direction: more offset means more backspacing.
The wrinkle is that offset is referenced to the centreline of the flange-to-flange width, while the width printed on a wheel is the bead seat width, which is narrower. The industry convention is that flanges add about half an inch per side, so a wheel stamped 8 inches wide measures roughly 9 inches across the outside of the flanges. That one-inch allowance is where the + 1 in the formula comes from, and it is the piece that trips up people who try to convert without it.
Two wheels with identical offset but different widths do not sit in the same place. A 9-inch wheel at ET20 has its inner edge further in and its outer edge further out than an 8-inch wheel at ET20, because the extra inch of width splits half to each side. That is why fitment is always a two-variable problem.
The conversion, and what the two change numbers mean
Put the mounting face at zero and measure positive outward. The wheel's centreline sits inboard of the face by the offset, so it is at −ET. The overall width is W + 1, and the two flange edges sit half that width either side of the centreline. The inboard edge is therefore at −(ET/25.4) − (W+1)/2, and its distance from the mounting face — backspacing — is (W + 1)/2 + ET/25.4. Reverse it and ET = (BS − (W+1)/2) × 25.4.
The outboard edge sits at (W+1)/2 − ET/25.4 from the mounting face. Enthusiasts call this poke. Since the mounting face does not move — it is the hub — comparing two wheels means comparing these two distances.
Inboard change is the new backspacing minus the old. A positive number means the inner edge has moved toward the suspension: less room to the strut body, the upper control arm, the tie rod and the brake caliper. That is the direction that causes hard interference rather than cosmetic rubbing, and it is the one to measure first.
Outboard change is the new poke minus the old. Positive means the wheel and tire stick out further toward the fender lip. That direction causes rubbing at full lock and full compression, invites stone damage to the paint, and in some jurisdictions runs into rules requiring the tread to be covered by bodywork.
A useful check falls out of the algebra: the two changes always sum to the width change. Going from an 8-inch ET45 wheel to a 9-inch ET20 wheel moves the inner edge 0.484 in outboard and the outer edge 1.484 in outboard, and −0.484 + 1.484 = 1.000, the extra inch of width. If your two numbers do not sum to the width difference, you have made an arithmetic error.
Worked example: 8Jx17 ET45 to 9Jx17 ET20
A car came on 8-inch-wide 17s at ET45 and the owner wants 9-inch wheels at ET20. Work both wheels by hand.
- Current flange-to-flange width. 8 + 1 = 9.000 in, so half-width is 4.500 in.
- Current offset in inches. 45 ÷ 25.4 = 1.771654 in.
- Current backspacing. 4.500 + 1.771654 = 6.2717 in.
- Current poke. 4.500 − 1.771654 = 2.7283 in.
- New flange-to-flange width. 9 + 1 = 10.000 in, half-width 5.000 in.
- New offset in inches. 20 ÷ 25.4 = 0.787402 in.
- New backspacing. 5.000 + 0.787402 = 5.7874 in.
- New poke. 5.000 − 0.787402 = 4.2126 in.
- Inboard change. 5.7874 − 6.2717 = −0.484 in. Negative, so the inner edge actually moves 0.484 in away from the suspension — inboard clearance improves.
- Outboard change. 4.2126 − 2.7283 = +1.484 in, or 37.7 mm of extra poke per side.
The verdict: this swap is entirely a fender problem, not a suspension problem. Nearly an inch and a half of extra reach per side means the tire will almost certainly meet the fender lip unless the arches are rolled or flared, and track width grows by about three inches across the axle. Confirm the tire's own contribution to that with the tire size comparison calculator, because a wider tire on the same wheel adds a further bulge each side.
How to read the numbers before you spend money
Check the inboard number against a real measurement, not a rule of thumb. With the current wheel off, measure from the hub face to the closest suspension component along the plane the wheel sweeps — usually the strut body on a MacPherson car, the upper control arm on a double-wishbone, or the caliper on a big-brake conversion. Your new backspacing must be smaller than that clearance by a safe margin. A quarter inch is tight; half an inch is comfortable.
Check the outboard number at full lock and full bump, not parked. Steering rotation swings the front tire's leading edge toward the fender liner, and suspension compression brings the top of the tire up under the lip. Both happen at once on a mid-corner bump. Parked clearance tells you almost nothing.
Remember that moving a wheel outward loads the bearing harder. Every millimetre of extra poke lengthens the lever arm between the tire's contact patch and the bearing, raising the bending moment for the same vertical load. Manufacturers pick offset partly for that reason and partly to set the scrub radius, which affects steering feel, torque steer and how the car behaves under braking with unequal grip. A large offset change is a steering-geometry change, not just a look.
Do not treat a spacer as free offset. A spacer reduces effective offset only. If you need a higher offset than the wheel has — to tuck it in — no spacer will do it; you need a different wheel or a wheel with a machined register. Slip-on spacers are appropriate in thin sections where the studs still engage the full nut; anything approaching an inch should be a bolt-on adapter with its own studs and its own hub register.
Track width changes have knock-on effects on load transfer and on where the tire sits relative to the vehicle's roll centre. If the vehicle tows, factor the extra bearing and axle loading into the numbers you get from the towing capacity and payload calculator, and remember that wider track and wider tires generally cost fuel economy, which you can quantify afterwards with the fuel economy (MPG) calculator.
Offset to backspacing conversion chart
| Offset (ET, mm) | 8 in wheel (in) | 9 in wheel (in) |
|---|---|---|
| −50 | 2.531 | 3.031 |
| −25 | 3.516 | 4.016 |
| −12 | 4.028 | 4.528 |
| 0 | 4.500 | 5.000 |
| +12 | 4.972 | 5.472 |
| +20 | 5.287 | 5.787 |
| +25 | 5.484 | 5.984 |
| +35 | 5.878 | 6.378 |
| +45 | 6.272 | 6.772 |
| +55 | 6.665 | 7.165 |
At zero offset, backspacing is exactly half the flange-to-flange width — 4.500 in on an 8-inch wheel and 5.000 in on a 9-inch wheel. Note that a 9 in wheel at +20 and an 8 in wheel at +45 differ by nearly half an inch of backspacing but by almost an inch and a half of poke; backspacing alone never tells you the whole fitment story.
Fitment mistakes worth avoiding
- Comparing backspacing between wheels of different widths. Backspacing only positions the inner edge. Two wheels with identical backspacing and different widths put the outer edge in completely different places.
- Measuring width across the flanges. The stamped width is bead seat to bead seat. Measuring the outside of the flanges gives you a number about an inch too large and throws every conversion off by half an inch.
- Assuming a spacer fixes a rubbing problem. A spacer moves the wheel outward. It solves inboard interference with the strut and it creates or worsens outboard rubbing at the fender.
- Ignoring the hub bore. A wheel that fits geometrically can still be wrong if its centre bore is smaller than the hub, and a bore larger than the hub needs hub-centric rings so the studs are not carrying the vehicle's weight in shear.
- Forgetting brake clearance. Offset positions the wheel laterally; caliper clearance is a radial and a barrel-profile problem. A wheel with correct offset can still foul a large caliper on its inner barrel or spoke face.
- Changing offset on one axle only. Different track widths front and rear alter the balance of the car under cornering and braking, sometimes usefully and sometimes not. Do it deliberately, not by accident.
Where offset sits among the other fitment checks
Offset and backspacing settle the lateral position of the wheel. They do not settle whether the wheel fits, because five other dimensions have to agree: bolt pattern, centre bore, wheel diameter against the brakes, load rating against the vehicle's corner weight, and the tire's approved rim width range against the wheel you are buying. A calculator can resolve the first of those; the rest require the vehicle in front of you or a manufacturer's specification.
Offset also has a chassis-engineering meaning that goes beyond clearance. Together with the steering axis inclination and the wheel's position, it sets the scrub radius — the distance at the road surface between where the steering axis intersects the ground and the centre of the contact patch. Reducing offset increases positive scrub radius, which increases steering kickback over bumps, changes how the car pulls under braking when grip differs side to side, and increases the steering effort a driver feels. Manufacturers choose factory offsets partly to keep scrub radius near zero. A 25 mm offset change is enough to notice.
Older American wheels were specified in backspacing because a tape measure and a straightedge across the flanges gave it directly, with no reference to a centreline you cannot see. European wheels adopted offset because it is a single signed number that is independent of the measurement technique. Both survive, so you will keep meeting both — which is the whole reason for this conversion.
Once fitment is settled, the consequences of a wider track and heavier wheel-and-tire package show up elsewhere: unsprung mass rises, rotational inertia rises, and both hurt acceleration and braking response. Check the braking side with the braking stopping distance calculator, which shows how much of stopping performance is grip rather than hardware.
Key terms
- Offset (ET)
- Distance in millimetres from the wheel's mounting face to its flange-to-flange centreline. Positive when the mounting face is outboard of the centreline, which tucks the wheel into the arch.
- Backspacing
- Distance in inches from the mounting face to the inboard flange edge. Directly measurable with a straightedge laid across the back flanges and a ruler down to the hub face.
- Poke
- How far the outer flange stands proud of the mounting face. It equals half the flange-to-flange width minus the offset, and it is what determines fender clearance.
- Hub-centric
- A wheel whose centre bore locates precisely on the hub's machined register, so the hub carries the vehicle's weight and the studs carry only clamping load. Lug-centric wheels rely on tapered seats instead.
- Scrub radius
- The distance at ground level between the steering axis and the centre of the contact patch. Offset changes move it, which changes steering feel and behaviour under uneven braking.