The following video provides a simple visual that makes it easier to remember how the thread weighting system works. Just remember that the number refers to length and you’re less likely to become confused.
Also, keep in mind that the thickness of thread can vary by manufacturer and type. In other words, not all 50 weight threads are the same thickness.
Superior Threads has written an excellent article that explains the discrepancy. As it turns out, the ply (number of strands twisted together) has much to do with the thickness of the final thread.
So, what do they recommend you do when choosing between brands and types?
“Choose thread based on the type of fiber, look, feel, and thickness and not by the printed weight size.”
So, while weight size can serve as a guide, choose whichever thread seems right for the project based on how it looks and feels.