For braids and weaves, make around 3" of a finished braid. Knot as you would if you were making it for "real". Mark the cord. Unravel and measure. Divide the amount used by 3 to get the length of cord per finished inch required for that braid. Don't forget to include the lengths of inner cords if there are any. Use this number as a multiplier next time you do a longer project to estimate required cord.
Always round up - better to cut excess than not have enough to work with. When planning a project with multiple knots and braids, refer to your notes for the individual knots and braids then add it all up!
Written by Kevin Gagne