Wheel offset & backspacing calculator

Convert offset (ET) to backspacing and back, or compare your current wheel with the one you’re considering: see how far the new wheel moves toward the suspension and the fender, and whether it should fit.

Start from your car factory wheels

Make Model Year

Rim width (″) ET Offset (mm) Backspacing (″)

Rim width in inches as stamped on the wheel (8J = 8″) · offset in mm (ET45 = 45; negative for deep-dish wheels) · Type the offset or the backspacing: the other one follows.

Check the clearance

The outer edge sits 11 mm further out: check the fender clearance at full suspension compression and full steering lock.

Inside, toward the suspension

+1 mm

1 mm closer to the suspension

Outside, toward the fender

+11 mm

11 mm further out

Measurement	Current	New	Diff
Rim width	191 mm 7.50″	203 mm 8.00″	+13 mm
Offset (ET)	45 mm	40 mm	−5 mm
Backspacing	153 mm 6.02″	154 mm 6.07″	+1 mm
Front spacing	63 mm 2.48″	74 mm 2.93″	+11 mm

Backspacing: mounting face to the inner edge. Front spacing: mounting face to the outer edge. Both count half an inch for each rim flange.

Top view

CurrentNew

How wheel offset works

Positive, zero and negative offset

With positive offset the mounting face sits outside the rim’s centre line, tucking the wheel in — almost every modern car. Zero offset puts it on the centre line. Negative offset puts it inside, pushing the wheel out: the deep-dish look of many truck and off-road wheels.

Width and offset together

A wheel 1″ wider at the same offset grows about 12.7 mm on each side. To keep the inner edge where it was, lower the offset by the same 12.7 mm — the extra width then all goes outward. This is why the calculator shows both edges, not only the offset.

The 5 mm rule

A new wheel that sits within about 5 mm of the original on both sides usually fits like stock. Between 5 and 15 mm, check the gap to the strut, spring and brake lines inside, and to the fender lip outside at full compression and full lock.

Look up your car’s factory wheel widths and offsets by make and model, its bolt pattern, or compare tire sizes for the new wheels in the tire size calculator.

Backspacing vs offset

Two ways to say where the wheel sits

Offset is measured from the rim’s centre line to the mounting face, backspacing from the inner edge to the mounting face. Offset alone does not say how deep the wheel reaches inward: at ET0 a 9″ wheel has 5″ of backspacing, a 7″ wheel only 4″. Backspacing is the number to hold against the strut and the spring.

Converting one into the other

Backspacing = half the overall width + offset, and offset = backspacing − half the overall width. The overall width is the rim width plus about 1″ for the two flanges, so a 17x8 ET35 wheel has 4.5″ + 35 mm = 149.3 mm (5.88″) of backspacing.

How to measure backspacing

Lay the wheel face down, put a straightedge across the inner rim lip and measure from it down to the mounting face. Read the rim width off the stamp (8J = 8″), enter both above, and the calculator gives the offset of an unmarked wheel.

Backspacing to offset chart

Offset (ET) in millimetres for common rim widths and backspacing in inches. Every ½″ of backspacing is 12.7 mm of offset; for other values, type them into the calculator above.

Rim width	3.5″	4″	4.5″	5″	5.5″	6″	6.5″
	Backspacing
6″	0	+13	+25	+38	+51	+63	+76
6.5″	−6	+6	+19	+32	+44	+57	+70
7″	−13	0	+13	+25	+38	+51	+64
7.5″	−19	−6	+6	+19	+32	+44	+57
8″	−25	−13	0	+13	+25	+38	+51
8.5″	−32	−19	−6	+6	+19	+32	+44
9″	−38	−25	−13	0	+13	+25	+38
9.5″	−44	−32	−19	−6	+6	+19	+32
10″	−51	−38	−25	−13	0	+13	+25
10.5″	−57	−44	−32	−19	−6	+6	+19
11″	−63	−51	−38	−25	−13	0	+13
12″	−76	−64	−51	−38	−25	−13	0

Wheel offset and backspacing FAQ

What is wheel offset?

Offset (ET, from the German Einpresstiefe) is the distance in millimetres from the centre line of the rim to the face that bolts to the hub. Positive offset puts that face toward the outside, so the wheel sits further in; negative offset pushes the wheel out. Most modern cars use positive offsets between ET30 and ET50.

What is backspacing?

Backspacing is the distance from the mounting face to the inner edge of the wheel, in inches or millimetres. It decides how close the wheel gets to the suspension: more backspacing, less room. This calculator converts it to offset and back, counting half an inch for each rim flange.

How do I convert offset to backspacing?

Add the offset to half the wheel’s overall width, which is the rim width plus 1″ for the two flanges. A 17x8 wheel at ET35: (8 + 1) ÷ 2 = 4.5″ = 114.3 mm, plus 35 mm makes 149.3 mm, about 5.88″ of backspacing. A negative offset is taken away: the same wheel at ET−12 has 102.3 mm, about 4.03″.

How do I convert backspacing to offset?

Take half the overall width away from the backspacing. A 17x9 wheel with 4.5″ of backspacing: (9 + 1) ÷ 2 = 5″, so the offset is 4.5 − 5 = −0.5″, or −12.7 mm (about ET−13). Type the backspacing into the calculator above and it fills in the offset.

How much can wheel offset change?

As a rule of thumb, a new wheel should sit within about 5 mm of the original on both the inside and the outside. Up to 15 mm can work after checking the clearance; beyond that it usually rubs the suspension or the fender. A wider wheel moves both edges at once, so check both numbers, not only the offset.

Does a lower offset make the wheel stick out?

Yes. Lowering the offset by 10 mm moves the whole wheel 10 mm outward: 10 mm more room on the inside, 10 mm more poke on the outside. A wider wheel at the same offset grows by half its extra width on each side.

What else must match besides the offset?

The bolt pattern (PCD) and the centre bore must suit the car, and the wheel must be rated for its load. The offset only decides where the wheel sits; check your car's bolt pattern and factory offsets on its page in our size database.