I don't do much measuring anymore. I've come to be pretty sensitive to what's in hand just by the feel of it. But
@SDSmlf gives a good accounting of how to measure.
I got curious about my stock of patch material and pulled out a bag of various stuff. It's not all here! There's another bag somewhere

some of this stuff goes back over 40 years. Each piece in the picture is distinctively different in texture or thickness, etc.
Some of these were a total bust and some were pretty ok. Some with the same measurement were not equal in results.
BTW, a very common bit of advice seen on the forum(s) and given to new shooters who inquire about patch material is "Wally world pillow ticking". This is a good example of erroneous advice or conventional wisdom. In the picture below is a collection of Wally pillow ticking. None of it is the same!! Nor is it consistent when compared by same color.
#40 drill is the only one I've found currently available that is consistent bolt to bolt.
In the '80's I was using artist canvas that I purchased from a local artist supply shop. It was sold unsized off huge rolls in four weights by oz. I used one of those exclusively for a long time and it was excellent in every way. I had a huge supply and used it for many years. Of course I ran out and went to replenish it about 15 years ago and found that the quality had obviously changed. Weave was loose in all of it.
That's when I followed another shooters suggestion and started using #40. Hopefully it continues with the same quality!