Determine the total quantity. Expressed as a fraction, the denominator, or bottom number, of the ratio is the total amount of things being counted. For example, if you have 13 cheese pizzas and six mushroom pizzas, the total quantity is 13 + 6, or 19.
Determine which ratio to express. The numerator, or top number of the fraction, will represent a distinct population within the total quantity in the denominator. In the example, use 13 to express the ratio of cheese pizzas or 6 to express the ratio of mushroom pizzas.
Write the ratio as a fraction. In the example, express the ratio of cheese pizzas with 13/19 -- meaning that 13 of 19 pizzas are cheese. To represent mushroom pizzas, write 6/19.
Ratios are commonly expressed in fractional form or in the formats 13:19 or 13-to-19.
Express the ratio as a percentage by dividing the numerator by the denominator, then multiply it by 100. In the cheese pizza example, 13/19 = 0.68. Multiply 0.68 by 100 to yield 68 percent.