Percentages are a function of mathematics that enables you to compare two or more unequal things by expressing both as portions of 100. You can also express the number as a fraction or a decimal. For example, 10 percent is 10 out of 100, or 1/10 or .10. Calculate percentages by dividing the fraction's numerator by its denominator, as in 16/64 = 16 divided by 64, or 1/4, or .25 or 25 percent (%). Find the percentage of a portion of an object by dividing the area of the portion by the area of the whole original object.

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.