Wheel Offset/Backspace Calculator will help you to find the wheel offset and the backspace, the important measurements for positioning the wheel/tyre assembly inside the wheel well. It will help you to determine which wheels are adequate for your car. This calculator also gives the option to compare wheels, to compare the offset and the backspace of one or more additional wheels to the base wheel.
Below the calculator there are detailed instructions how to use this calculator and about the wheel offset and backspace.
Wheel Offset/Backspace Calculator
This button will be enabled when you've got dimensions for at least one more wheel, besides the first entered in the calculator.
After you click on this button, the last calculated wheel dimensions will be added for the comparision in the "Comparing Wheels" form below.
To compare another two wheels click on the button "Clear Compared Wheels". To start over, with a new data for the base wheel - please refresh the page.
Try out our Wheel/Rim Size Calculator which shows the acceptable wheel width range for any given tyre size.
Tyre size for rim size/width calculator tells you what tyre sizes to go for, for the size of your rim.
Tire size converter can help you to convert the inch tire size (Standard, English measuring system) to the metric tire size and the other way around. The converter also displays adequate actual tires with the dimensions which are closest to the calculated tire size, if such exist.
The calculator results should be verified with the wheel manufacturer. This is for informative purposes only. The tyresizecalculator.com is not responsible for any miscalculations of this calculator or any errors that may arise with its usage.
How to use the wheel offset/backspace calculator?
Our calculator is divided into two sections:
Section "enter overall wheel width" gives you an option to input the overall wheel width directly into the form field "measured overall width" or you can enter the declared wheel width and the lip width and the calculator will calculate the overall wheel width based on these two measurements. You can choose between these two options. Default lip width is set to a ½ inch.
Section "enter backspace or offset" allows you to choose which measurement you want to enter: backspace or offset. If you wanted to find out the offset you would choose to enter the backspace, conversely if you wanted to find out the backspace you would choose to enter the offset.
After you click the button "Calculate ..." all wheel dimensions will be presented, in the table below the calculator.
This calculator also gives the option to compare wheels, to compare the offset and the backspace for two additional wheels to the base wheel - the first wheel entered into the calculator which is the basis for the comparision. After you've got dimensions for the second wheel, the "Compare this wheel" button will be enabled, so you can click on it and add that wheel for a comparision into the form "Comparing wheels". In that form, besides the data for the base wheel, every compared wheel, with its dimensions, is contained in a section "Compared wheel x" and it has the column "Diff" which shows the calculated differences for its dimensions, compared to the base wheel.
The offset of a wheel is what locates the tyre and wheel/rim assembly in relation to the suspension. More specifically, it's the measured distance between the mounting pad (the mounting surface of the wheel) and the center line of the rim. Sometimes It's used interchangeably with the term "backspace".
Nearly every wheel has the offset stamped on it somewhere, usually ingraved on the wheel as a part of the wheel markings (e.g., 6½ J x 15 H2 5/112 ET39 - ET39 means 39mm offset) and its measurement unit is millimeter (mm). The wheel offset can have zero, positive or negative value.
The wheel has "positive offset" when the mounting pad is outboard of the centerline, towards the street, located closer to the face of the wheel in relation to the centerline.
The wheel has "negative offset" when the mounting pad is on the inboard side of centerline, towards the suspension, located closer to the back side of the wheel in relation to the centerline.
The wheel has "zero offset" when the mounting pad position is in line with the position of the centerline.
e.g., If the wheel is declared as 6 inches (1 inch = 25.4mm) wide with a ½ inch lip width, it is: 152.4mm + 2 x 12.7mm = 177.8mm wide overall. With the declared positive offset of 40, the wheel's mounting pad is located 40mm from the center line, towards wheel's front face, being 40mm from the center line position which is at 88.9mm.
This means that the mounting pad location measures 48.9mm (88.9 - 40) from the front side plane of the wheel (the wheel frontspace) and 128.9mm (88.9 + 40) from the back side plane of the wheel (the wheel backspace). These measurements refer to the shortest distance between measurement points.
A wheel that has a positive offset or greater backspace has to slide further into the wheel well to be mounted, it's closer to the breaks and the suspension of a car. Similarly, the one with a negative offset or lesser backspace will tend to jut closer, or even beyond the fenders.
A similar concept to an offset, a backspace is simply the space between the wheel's mounting surface (the mounting pad) and the inboard flange (lip) of a wheel. Therefore the backspace depends on both the overall width of the wheel and the offset of the wheel. It shows where exactly the mounting pad is in relation to wheel's overall width. Terms: the offset and the backspace are often used interchangeably. Both terms determine where the wheel sits within the wheel well, how much of the wheel will protrude inboard, towards the suspension components or jut out, towards the fender. A wheel with the positive offset has more backspace, conversely a wheel with the negative offset has less backspace.
A little heads-up: the backspace is not measured from the inboard bead seat but from the very edge of the rim including the metal thickness (the back side rim lip or the rim flange) to the mounting pad.
e.g., A 6 inches wheel with the mounting pad positioned right in the center of the wheel would measure a 3½ inch backspace if it has a ½ inch wall (lip width). The overall width of the wheel would be: 6in + 2 x ½in = 7in.
How to measure the wheel backspace?
For this measurement you will need a straight edge and a metric ruler. Lay the wheel flat on the ground with the back facing upward. Next, lay the straight edge across the wheel and measure the distance from the back of the mounting surface (pad) to the straight edge.
Another, simpler way, if you have the offset, is to measure the wheel's overall width. Input that value and the value for the offset, from the offset designation on the wheel, into our wheel offset/backspace calculator to get the backspace. Also, the overall width can be calculated from the declared wheel width and the lip width. This can also be done by our calculator.
Declared/rated wheel width
The width of a wheel is the distance from the inboard bead seat to the outboard bead seat (the bead seat is the place where a tyre sits inside the wheel). The rated width of the wheel doesn't include the lip (the flange) on each side of the wheel.
Overall wheel width
The overall width of a wheel includes the lip width (or the flange width - the wall thickness on the sides of the rim) on each side of the wheel. The overall wheel width is the sum of the rated wheel width and the width of lips.
e.g., If the rated wheel width is 6in and the lip width is ½in, the overall wheel width is: the rated wheel width + 2 x lip width = 6in + 2 x ½in = 7in.
Centerline of the wheel
The centerline of the wheel marks the center of the wheel's width. This is the line which vertically divides the wheel into halves, in the direction of the wheel movement.
Mounting pad (plate, hub face, hub mounting face/surface)
The mounting pad of the wheel is the mounting surface of the wheel, the flat surface on the back side of the wheel's plate which contains the holes for bolts which tighten the wheel to the car's axle pad.
Why are these measurements: offset and backspace, important?
If you intent to change the wheels on your car, replacing them with the new wheels, make sure that the new, custom wheels have offset/backspace that is as close as it can be to the offset/backspace of the original wheels - the ones that you know work well - because a very different offsets may cause problems.
If you have too little backspace (negative offset), then your tires may rub the fenders. However, too much backspace (positive offset) and a wheel is more likely to rub the brake (the brakes not fitting well within the new wheels) or the suspension components.
As you can see, the backspace and the offset are closely related. Many manufacturers have done the work for you by using both measurements in their product descriptions. If you want to calculate them on your own, our calculator: wheel offset/backspace calculator can help you.