I knit a lot of hats for both adults and kids so I commonly use 3.75mm,4mm,4.5mm and 5mm. I use these when using dk or worsted. For chunky (which I am not crazy about) used only occasionally it is 6-6.5mm. Remember though that if knitting hats on circulars you will also nee corresponding double- pointed needles for your decreases. Hope this helps.LdyM said:Hi all, I'm considering investing in 16" circulars. I cannot buy ALL the sizes, and as I'm just beginning to use patterns that require them, which ones are the most commonly used?