How do I calculate how much fabric I need for bias binding?

Here is the formula for calculating bias fabric. The example uses inches and yards:

Multiply the length of the binding by the width of the binding. Then find the square root of this figure. There is a function on a calculator to do this for you. Be generous when calculating the original length figure to allow for 45° joins and corners.

For ex 100" of binding x cut at 3½" wide = 350". The square root of this number is 18.71 - so you would need a square of fabric at least 19". Philippa would probably use a slightly bigger square anyway just to be sure she had enough fabric, and not too many short lengths included in the sewn bias fabric strip.

If you want yardage and not a square of fabric then you divide your above number of 19" by 36 (36" in a yard) and get 0.52 of a yard. If you had been generous with your original calculations you could probably get away with half a yard of fabric, and certainly half a metre would do it. A yard, by the way, is 36" and a metre is 39" - or 100cm!