A while back someone posted that 40 weight thread is coincidentally .40 mm thick. I took this to be obviously incorrect. Not only is the measurement misleading the conclusion was arrived by laying a two pieces of 40 weight thread on a ruler and wa-la there you have it! The reason I questioned this other than the obvious kindergarten method of measuring was it seemed incorrect to me. So I contacted Ackermann to see if they had a more accurate measurement. The USA warehouse didn't have this information and referred me to Germany. Germany responded by giving me this information. Quote: Poly 40 weight thread is .24685 mm in thickness. Rayon is a little thinner. So now when you try to calculate anything using 40 thread you have the accurate measurement. This would be very useful when zooming in on a stitch file and measuring densities.
So use your new found knowledge well. And have a good week )