Wheel Offset Calculator results show how far a new wheel moves toward the fender and the strut, using both wheel widths, ET values, any spacer and the clearances you measure today.
Spacer and Tires
Tire width is the first number of the size, such as 245 in 245/40R18.
Measured Clearance
Measure with the current wheels: from the inner lip to the strut or spring, and from the outer lip straight up to the fender edge. Use a negative fender value if the lip already pokes out.
Offset and Backspacing Chart
| Offset | Backspacing | Poke vs Now | Toward Suspension |
|---|---|---|---|
| ET−44 | 83 mm | +101.7 mm | −76.3 mm |
| ET−25 | 102 mm | +82.7 mm | −57.3 mm |
| ET−12 | 115 mm | +69.7 mm | −44.3 mm |
| ET0 | 127 mm | +57.7 mm | −32.3 mm |
| ET+12 | 139 mm | +45.7 mm | −20.3 mm |
| ET+20 | 147 mm | +37.7 mm | −12.3 mm |
| ET+25 | 152 mm | +32.7 mm | −7.3 mm |
| ET+30 | 157 mm | +27.7 mm | −2.3 mm |
| ET+35 | 162 mm | +22.7 mm | +2.7 mm |
| ET+40 | 167 mm | +17.7 mm | +7.7 mm |
| ET+45 | 172 mm | +12.7 mm | +12.7 mm |
| ET+50 | 177 mm | +7.7 mm | +17.7 mm |
| ET+55 | 182 mm | +2.7 mm | +22.7 mm |
Wheel Offset Calculator for Poke and Clearance
The Wheel Offset Calculator compares the wheel on your car now with the wheel you plan to buy. It shows how far the outer lip moves toward the fender and how far the inner lip moves toward the strut. It also converts both wheels to backspacing and checks the change against the room you actually measured. Buyers use it before ordering aftermarket wheels, adding spacers, or picking up a used set with a different ET.
Wheel width goes in inches as stamped on the wheel, such as the 8 in 18x8J. Offset, spacer, tire width and clearance go in millimeters, and the Units menu can show distances in inches at 25.4 mm per inch. Measure inner clearance from the current wheel’s inner lip to the strut or spring. Measure fender clearance straight up from the outer lip to the fender edge, and enter it as negative if the lip already sits outside.
What the ET Number on a Wheel Means
Offset is the distance in millimeters from the wheel’s centerline to its hub mounting face. SAE J1982, the SAE nomenclature standard for passenger car and light truck wheels, calls it inset, zeroset or outset. European wheels print it as ET, from the German word Einpresstiefe, and some Japanese wheels are cast with Inset beside the number. A positive value such as ET45 puts the mounting face toward the outer face, which tucks the wheel in.
How Poke and Inner Clearance Are Found
Every result starts from the wheel’s centerline, with half the nominal width on each side of it. The offset sets where the hub face lands, and a spacer pushes the whole wheel outward by its thickness. So a spacer subtracts from the inner distance and adds to the outer one.
$$d_{in} = \frac{W \times 25.4}{2} + ET – S$$
$$d_{out} = \frac{W \times 25.4}{2} – ET + S$$
Here W is the nominal wheel width in inches, ET is the offset in millimeters, and S is the spacer thickness in millimeters. Subtracting the current wheel’s values from the new wheel’s values gives the change on each side.
A common mistake is entering the overall lip-to-lip width, which runs about an inch wider than the stamped size. The stamped width is measured bead seat to bead seat, and that is the figure the formula expects.
Worked Example: 8-Inch ET45 to 9-Inch ET35
Take a current 8-inch wheel at ET45 and a new 9-inch wheel at ET35 with no spacer. The inner lip moves from 146.6 mm to 149.3 mm behind the hub face, so it sits 2.7 mm closer to the strut. The outer lip moves from 56.6 mm to 79.3 mm, which is 22.7 mm more poke. With 20 mm of inner room and 15 mm of fender room measured today, 17.3 mm stays at the strut, but the lip ends 7.7 mm past the fender.
The extra inch of width adds 12.7 mm to each side on its own. The 10 mm offset drop then moves the whole wheel outward, so the outer side gains far more than the inner side. Track width grows by twice that 10 mm shift, so this axle gets 20 mm wider.
Offset to Backspacing Formula
Backspacing is the distance in inches from the hub face to the inner edge of the wheel, and truck wheels are often sold by it. The Mickey Thompson tech bulletin on backspacing and offset converts offset using the nominal width plus 1 inch for the two flanges. The Wheel Offset Calculator applies that same allowance to both wheels.
$$BS = \frac{W + 1}{2} + \frac{ET}{25.4}$$
Front spacing is the overall width, W + 1, minus the backspacing. In the example, the 8-inch ET45 wheel has 6.27 in (159 mm) of backspacing, and the 9-inch ET35 wheel has 6.38 in (162 mm) with 3.62 in (92 mm) of front spacing.
The 1-inch allowance is an industry convention, so a straightedge across the back of the wheel, as Discount Tire’s backspacing guide describes, gives the exact figure. The chart below applies the formula to common sizes in inches.
| Offset | 7 in wheel | 8 in wheel | 9 in wheel | 10 in wheel |
|---|---|---|---|---|
| ET+55 | 6.17 in | 6.67 in | 7.17 in | 7.67 in |
| ET+45 | 5.77 in | 6.27 in | 6.77 in | 7.27 in |
| ET+35 | 5.38 in | 5.88 in | 6.38 in | 6.88 in |
| ET+20 | 4.79 in | 5.29 in | 5.79 in | 6.29 in |
| ET0 | 4.00 in | 4.50 in | 5.00 in | 5.50 in |
| ET−12 | 3.53 in | 4.03 in | 4.53 in | 5.03 in |
| ET−25 | 3.02 in | 3.52 in | 4.02 in | 4.52 in |
How Tire Width Changes Sidewall Position
A wider tire grows by half its width change on each side, and the calculator adds that to the wheel’s centerline shift. Going from a 245 to a 255 adds 5 mm per side. With the 10 mm outward shift, the inner sidewall moves 5 mm away from the strut and the outer sidewall moves 15 mm out. For the diameter and speedometer side of a new size, use the Tire Size Calculator.
A tire’s published section width only applies on its measuring rim. Tire Rack’s rim width guidance puts the change at about 0.2 in, or roughly 5 mm, for every 0.5 in of rim width. The sidewall rows use published widths, so the same 245 moved from an 8-inch to a 9-inch wheel can gain close to 10 mm by itself. On a tight fit, allow about 2.5 mm more per side for each extra half inch of rim.
Finding the ET That Keeps Your Current Fitment
The last card of the Wheel Offset Calculator works backward from where your current wheel sits. It solves for the offset that keeps the outer lip in place, and for the one that keeps the inner lip in place. Both use the new width and include any spacer.
$$ET_{poke} = \frac{W_{new} \times 25.4}{2} + S – d_{out,now}$$
$$ET_{inner} = d_{in,now} – \frac{W_{new} \times 25.4}{2} + S$$
In the example, ET+58 keeps today’s poke and ET+32 keeps today’s inner room. The gap between them equals the extra width in millimeters, since a wider wheel cannot hold both lips where they were. Any ET between the two splits that extra width between the fender side and the strut side. Pick the end your car has less room on, then check the other side’s result.
Spacers and Fender Coverage Rules
The fender card subtracts the outer lip change from the fender clearance you measured, and a negative result means the lip ends past the fender. Some states check this at inspection. Pennsylvania rejects a vehicle under 67 Pa. Code § 175.80 when the tread extends past the wheel housing, including any fender flares.
Spacers face their own limits. 67 Pa. Code § 175.65(h) bars spacers thicker than 1/4 inch, about 6.35 mm, when they are used to widen the track. The calculator accepts spacers up to 100 mm because the geometry is the same at any thickness, not because every thickness is road legal.
Input Limits and Where the Math Stops
The Wheel Offset Calculator accepts wheel widths from 3 to 20 in, offsets from −150 to +150 mm, and tire widths from 100 to 500 mm. Entries outside its limits are flagged, and results pause until they are fixed. When the inner room left drops below zero, the alert switches to a suspension contact warning. Every figure is static lip geometry, so real clearance still varies with spoke-to-caliper fit, suspension travel, steering lock and load.
Wheel Offset Calculator Input Mistakes
The most frequent slip is dropping the minus sign on a negative offset, since ET−12 entered as 12 moves the result by 24 mm. Another is typing a backspacing figure into the offset field, even though backspacing is in inches and offset is in millimeters. Some users also enter the aspect ratio or the full tire size instead of the section width, which is the 245 in 245/40R18.
Common Wheel Offset Questions
Is a wheel spacer the same as a lower offset?
For wheel position, a spacer and a lower offset give the same result millimeter for millimeter. An ET45 wheel on a 15 mm spacer sits exactly where an ET30 wheel of the same width would. The spacer adds parts, though, so it needs hub-centric fitment and studs or bolts long enough for full thread engagement. It also moves the contact patch outward exactly as a lower offset would.
How much can I change offset from stock?
No standard sets a single safe limit, and published shop advice ranges from staying within 5 mm of factory to about 15 mm. The real limit is the clearance your car has, which is why the Wheel Offset Calculator asks for measured inner and fender room. Forum builders often treat fingertip clearance, about 10 mm, as a comfortable fender gap, and an alignment check after a large change is sensible.
How do I find the offset of a wheel with no markings?
Lay a straightedge across the back of the wheel and measure down to the hub mounting face to get backspacing. Subtract half of the nominal width plus 1 inch, then multiply by 25.4 to get millimeters. An 8-inch wheel with 6.27 in of backspacing sits 1.77 in beyond its 4.5 in centerline, which is ET45. A negative answer means the wheel has negative offset.
Does changing wheel offset affect handling?
Yes, because moving the wheel centerline outward lengthens the scrub radius by the same amount. Modern Tire Dealer notes that a 5 mm offset change is enough to feel on most cars. A wider track can add stability, but it also puts more leverage on the wheel bearings. The Scrub Radius Calculator shows how that shift changes your front-end geometry.