I often check my wpi during spinning because I want to end up with a specific yarn thickness – usually fingering or sport weight, occasionally DK or worsted weight. The rule of thumb is ½ the wpi of your singles will be the wpi of a 3 ply; for 2 ply it’s 2/3. I developed a card that shows the various wpi diameters as solid black lines and I hold my singles up to that along with a sample I have made. I also let a fresh singles self ply and check that thickness. This approach seems to work fairly well. I have learned to spin a bit finer than what I want, because of the way wool blooms after washing. If my goal is 16 wpi, then I aim for 18.
After washing and drying the final skein, I want to see if I met my goal before I start swatching. I have used the dowel method and the ruler method but each has problems. It’s hard to do with a skein and on a wound ball, you can really only sample the 2 ends. My spinning isn’t quite that consistent. The visual comparison method on a skein works okay and you can sample several strands from across it. But today I hit on something you could do on a skein that samples more strands and gives a more representative value.
Take 6 to 8 strands (or whatever seems appropriate) from across the skein – something that looks like at least ½ inch. Lay them over your index forefinger, pushing the strands together to they touch but aren’t crammed. Try not to stretch the strands. Measure how wide they are with a ruler. The number of strands divided by their width gives you the yarn thickness – wpi if you measure in inches, wpc if in centimeters. The strands can come from across the skein and if you do this with several sets and average, I think you end up with a far better idea of how thick your yarn really is.
In the picture below, there are 8 strands that measure 1/2 inch –> 8/.5 = 16 wpi. The angle in the picture is funny because I was trying to hold everything in place with one hand and take the picture with the other.