A pattern calls for yardage at one fabric width \u2014 find how much you need at a different width.
Assumes the design has no large one-way print or nap. Always round up and add a little for squaring the fabric.
Fabric comes in different widths, so a pattern that lists yardage for one width needs a different amount at another. The calculator scales the yardage by the ratio of the two widths. A pattern needing 3 yards of 45 inch fabric needs about 2.25 yards of 60 inch fabric, since 3 times 45 divided by 60 is 2.25.
This is an estimate, since the pieces have to be laid out on the fabric and a different width may not fit them as efficiently. Narrower fabric needs more, and the extra can be a lot if wide pieces no longer fit side by side.
Add about ten percent for safety, and check the pattern layout diagrams if there is one for your fabric width. Directional prints, nap, and stripes limit how you can place the pieces, so allow more for them.
If you are in doubt, lay out the paper pattern pieces on the floor in a rectangle the width of your fabric and measure the length, which gives a more accurate figure than any formula.