This chart was assembled from numerous resources online dealing with average hand and arm sizes for men, women, and children. And now for your designing pleasure, they are all available here in a handy reference chart.
Have you ever worked a gauge swatch only to find that the stitch count is correct, but the rows are too short or too high? Why is that? Shouldn’t you be able to make both stitch and row gauge with the same hook size? The answer lies in the Golden Loop.
These measurements will give you a rough idea of how big to make your hats in various sizes.