How much ribbon does it take to make a kumihimo necklace?

There is no exact formula for the amount of ribbon to be used in a braid. Not all ribbons are alike, so you can only use a "rule of thumb": the thicker the fiber, the longer you need to make it. Using rattail, I have found that you can make each of the eight strands twice the length of what you want the final braid to be. For ribbon, I generally try to have the eight strands three times the final length. The tricky part comes when you mix fibers. A braider will invariably need more of the thinner fiber than anticipated. Even when you are braiding with all of the same fiber, don't be surprised if you run out of one quite a bit before you run out of others. Don't know why: it just happens. Remember that it's always better to have too much than not enough!