Multiply the length times the width of the original piece to calculate its area in square inches, feet or centimeters. For example, if the piece is 16 inches long and 10 inches wide, 16 x 10 = 160; its area is 160 square inches.
Use the same method to calculate the area of the smaller portion for which you need the percentage. For example, if the smaller portion is 8 inches long and 5 inches wide, 8 x 5 = 40; its area is 40 square inches.
Divide the smaller portion's area by the whole piece's area. For example, 40 / 160 = .25.
Therefore, the area of the smaller portion is 25 percent of the area of the original piece.