Welllll...actually I'm at about 37" now, but I ordered the 28 yard quantity and had about 8'-10' of cord left over when I made the 37" girthbelt seen in the pic.
Basically the way I figure out how much I need is get a waist circumference measured with a cloth tape at the navel, add 2" to that length, and then multiply it by the number of rows I want on my girthbelt. Something I should mention is that I did notice that 20 rows of this bolo gave me a girthbelt that was 5.25" wide. On this particular girthbelt, I added an additional row because the wearer is 5'9", and he can always cut off a row if it looks too wide when he actually tries it on.