Converting from one width fabric to another is tricky. Sewing pattern manufacturers use sophisticated computer programs to determine the best possible layout for each size and width of fabric. Charts and formulas will only give you an estimate. Use your sewing pattern guide sheet to help you determine if the estimated yardage is sufficient. Also helpful is the width and length of the garment; a 80" skirt width would require at least 3 lengths of 36" wide fabric regardless of what the conversion chart indicated.