not another hemp necklace thread. but seriously this ones different.

hey i have some hemp necklaces and such that i have made and also i have a big ball of hemp. i am planning on making a belt soon, here are the plans if youre curious: http://thisyearsdozen.wordpress.com/2008/05/14/how-to-macrame-a-belt/

the question that i have is how do you convert the weight of the hemp twine into thickness.

the plans say 3.5mm twine, but all the sites ive looked at for buying this twine have the thickness listed by 'weight'. how would one convert from the weight in lbs to the thickness in mm, and vice versa.

either link a site that has all the weights listed, or just tell me what 3.5mm would be, idc.

thank you in advance, peace.