Wheel offset is a critical part of properly fitting wheels, and I often see questions regarding offset. Below is a post I wrote years ago describing offset and how to measure it.

Wheel offset is the difference between the true center of the wheel and the mounting surface. Positive offset is when the mounting surface is closer to the outside of the wheel; negative offset is when the mounting surface is closer to the inside of the wheel. Negative offset will push the wheel outwards.
Most wheels have a positive offset to allow more room for the brakes and suspension parts (i.e., 20x9 +55). Negative offset wheels are put on Hondas to make the car look funny.

To measure the offset:
1)- place the wheel flat on the floor and place a straightedge across the wheel. This will work for wheels mounted with tires too.

2)- measure the overall width of the wheel/tire (from the underside of the straightedge to the floor):




3)- measure the distance to the mounting face of the wheel.


4)- Divide the overall width of the wheel/tire by two to find the center of the wheel. Subtract the distance measured to the mounting surface by the distance to the center of the wheel. This difference is the wheel offset.

For my wheels (GSM Intro Forged 22" x 10.5") the overall (with tires) measured 10.88". The distance to the mounting face measured 6.13".
The offset is [6.13" - (10.88"/2)] = 0.69"
Multiply inches by 25.4mm/inch to convert to millimeters
0.69" x 25.4mm/inch = 17.5mm

Considering the floor in my garage was poured before dirt was invented, I would estimate the offset is between 17 and 18mm.

Note: Offset is often questioned when fitting a car with large brakes. This can be misleading since the problem with large brake fitments can be with interference with the huge calipers, and not especially the offset. One wheel with an 18mm offset may fit, while another with the same offset may not. It is the shape of the "spokes" that is critical when fitting over larger calipers on big brake cars. There must be enough space behind the spokes so they won't hit the calipers. My I-Forged rims have about 1/16" clearance!
This dimension is hard to measure unless you actually fit the wheel on the hub (front wheels only), so it is recommended that you get a guarantee from the wheel vendor that the rims will fit your car before dropping the cash!

 

I see the second photo had a bad link (Photobucket has gone downhill)
Here it is :

 

Front wheel Offset 20”, 9"J, 49 offset
Rear Wheel Offset 20”, 10"J, 46 offset