If you have a fabric calculator, you can use that to calculate how much you need. If you have an EQ program, you can use it to sketch out the quilt and the program will tell you the amounts of each fabric you need. Otherwise, you may have to estimate by studying the pattern and the amounts of the different fabrics which are required for the borders you want to repeat. Remember that each subsequent border will be larger than the one before it, so it's not just a matter of doubling those amounts. The longer borders will require more fabric than the original ones.