The floss tools has an option that gives the number of stitches for each color of floss (regardless of the material of the fabric) and an estimate of the amount of each color needed. It is based on the stated count of the project. If you do it using larger or smaller stitches, your amount will be different.
It is just an estimate. My personal experience is that it tends to under estimate the true floss needed. Also, depending on how you specify your backstitches, it could be substantially under the actual need.