Needle sizing and yarn weights

Needle sizing and yarn weights vary from country to country,  this guide should  sort out the confusion    

UK 4ply  = US sportsweight
gauge over 10 cm 23-26 sts
Average needle size 3.25 mm- 3.75mm

UK Double knitting ( DK) = US worsted
gauge over 10 cm 21-24 sts
Average needle size 3.75mm-4mm

UK Aran = US Fisherman/Medium
gauge over 10 cm 16-20 sts
Average needle size 4.5 mm-5.5mm