This calculation will give you the number of diameters which will determine the maximum sett for a fabric. The reason you want to know the Ashenhurst number (the maximum sett) is that it's his number that is used to make allowances for yarns, weave, shrinkage, finishing, purpose (e.g. upholstery or sheer curtains, etc.).
Using Ashenhurst's formula, you will find that the number of diameters will be more than if you wrapped the yarn on a ruler. I think the reason his number for diameters is higher is that it's a mathematical formula which he found to work. The two methods (Ashenhurst and wrapping) can be compared. You can use whichever method seems convenient to you. Surely, you would use Ashenhurst for a warp of sewing thread. It's likely that you would use the wrapping method for a large wool yarn. But, remember, you will get different numbers if you compare both methods on the same size thread. If you wrapped a yarn and counted the diameters, and you took 1/2 of that number for plain weave, you would need to divide that number by 0.7 to get a number close to Ashenhurst's.
An example might be a yarn wrapped 10 times on a ruler (=10 diameters/inch). Half of that is 5, for plain weave. Divide 5 by .7 to get a number that about equals Ashenhurst's maximum sett for plain weave.
5 divided by .7 = 7
The maximum sett would be 7, not 5.
Before I give you Ashenhurst's formula, you need to understand that you'll have to push a special key on your inexpensive calculator. It's the square root key and looks a lot like a check. See Figure.
To find the square root of a number, enter the number on your calculator then push the square root key.
