Here’s a question: how many strands of floss thread would equal a specific size of pearl cotton thread? Floss thread comes with 6 strands of threads bundled together. You can separate the bundle and stitch with 1, 2, 3, 4, 5, or all 6 threads together. Pearl cotton thread is not divisible. You use it as a “single”.
I don’t dye floss anymore but I’ve tried to compare my hand-dyed pearl cotton thread sizes with strands of floss. Following is a chart to help you compare the two types of threads. (This is just a guess, I have no real way of measuring the diameters of the threads.)