How do you measure wheel offset?

When you're trying to determine what new wheels will fit your car knowing how to measure the wheel offset on your current wheels makes the job much easier.

Wheel offset is a measurement in millimetres of where the mounting surface of your wheel sits in relation to the true centre point of the outer wheel rim and this dictates how far in or out of your car's body the outer rim and tyre will sit. A high positive value or high offset wheel will be sunk into your car's bodywork, whereas a low offset or high negative offset wheel will be more pokey out. 

Normally the offset is stamped on a wheel, usually on the back of a spoke or the wheel hub and marked either as an ET number (ET is short for EinpressTiefe which means Offset in German) written like ET25 for an offset of +25mm or simply as a number on its own. Offset values can be positive or negative, so if you see a number prefaced by plus or minus like +35 or -15 this will typically be your wheel's offset value.

But sometimes a wheel will have a damaged marking or no marking at all. It can also be worth double-checking the wheel offset even if this is marked as some wheels may have had extra machining, had inner or outer barrels swapped on a 3-piece wheel or can have the wrong information stamped on them.

So here's a quick guide to help you get an accurate wheel offset measurement every time.

Step 1: Remove Your Wheel

Begin by laying the wheel face down on a flat surface so the face that bolts to the hub is facing upwards as shown here. The tyre can stay fitted as this will sit centrally on the wheel, so even if it sticks out, it will do so by the same amount on both sides and your offset measurement is still valid.

You'll need a straight edge which won't bend (a long spirit level works well) and a tape measure or a ruler like a steel rule where the zero mark is right at the end. 

Step 2: Your Wheel Width & Centre Line

Take your straight edge (spirit level, rule, etc.) bridging across the rim of the wheel and measure from the bottom of this to the floor. This height gives your overall wheel and tyre's width.

Note that it's normal for this measurement to be larger than the nominal width of your wheel size (eg an 8.5" wide wheel), as wheel size this is taken from inside the bead seat, not the outer edge.

Once you have the width (in our example 244mm) you divide this by 2 which gives you your centre point for the wheel (122mm). This would be ET0 where the mounting face is perfectly central in the wheel.

You should then measure your back spacing, from the bottom of your straight edge to the hub mounting face. Ours measured as 175mm.

Step 3: Your Wheel Offset Measurement

Now take that back spacing measurement and take off your centre point measurement from it.

For us, that's 175mm minus 122mm which gives a difference of 53mm. 

That difference is your offset, so this wheel is an ET53.

To be double certain, you can also measure from the mounting face to the floor as shown here and take this front spacing measurement away from the centre point measurement. This measured 69mm in our example, so 122 minus 69 is 53 again which confirms the offset of ET53.

Step 4: Is It Positive or Negative?

As above, a wheel offset can be positive or negative and you don't want to get those 2 confused or your wheel specs could end up way out from what you want.

So if you ended up with a positive number from the calculations above, you have a positive offset wheel. If it was a negative number, it's a negative offset wheel

As a final example, a theoretical 300mm wide wheel with a 150mm centre point. If the wheel face measures 180mm from the back, then this is an ET +30, whereas if it measured 120mm this would be a negative offset ET-30.

Hopefully, now you should be able to use this to work out your wheel offset (ET).