How to Calculate the Percentage of Something

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Percentage calculations come into play in many real-world situations, such as determining how much to tip in a restaurant and figuring out the discounted prices of clothing. Here’s how to do these calculations.

Know the Basics

Before going gung-ho on performing calculations, know the basics about percentages. The word “percent” means “out of 100” or “for every 100.” The sentence “It rained 14 percent of the time” is much less cumbersome than the statement “It rained 14 days out of every 100,” but it means essentially the same thing. In math equations, the % symbol is used to represent percentages. To write a percentage as a decimal, move the decimal point two spaces to the left. To write a decimal as percentage, move the decimal point two spaces to the right. Further, to convert a fraction to a percentage, simply multiply by 100; to convert a fraction to a decimal, divide by 100 and then reduce the fraction, if necessary, according to Helping With Math. Read on for some examples of how percentage calculations can come in handy in everyday life.

Calculating Percentages Based on Ratios

Ratios with denominators other than 100 are often used in calculating percentages. Imagine that 14 out of 50 students in a class received an “A” on a test. What percentage got an “A”? To find the percentage represented by the ratio, divide the numerator by the denominator, then multiply by 100. Because the ratio is 14 out of 50, the percentage is calculated by dividing 14 by 50 to get 0.28, and then multiplying 0.28 by 100 to get 28 percent. In this example, 28 percent of the 50 students got an “A.”

Working Backwards from a Percentage

Working backwards can be a useful method for calculating loan interest, tips and taxes, or in any other situation wherein the initial total and the percentage are known but the numerical value of the percentage is not. First, convert the percentage into a decimal by multiplying the percentage by .01. Then, multiply the initial total by the decimal. For instance, to calculate daily interest of 3 percent on a $15 personal loan, multiply 3 by .01 to get .03. Then multiply 15 by .03 to get .045. This means $0.45 is the amount of interest earned each day. To add a 20 percent tip to a $50 restaurant bill, multiply 20 by .01 to get .20, or .2. Then multiply $50 by .2 to get $10. $10 is the amount of the tip.

Calculating Discounts from the Opposite of the Percent

Retail stores frequently put out racks of clothing with signs saying “30 percent off” or “40 percent off,” and the original price tags are still attached to the clothing. To find the discounted price of such a garment, first determine the opposite of the discount percent. If a shirt is on sale for 30 percent off, for example, the opposite is 70 percent. Convert the opposite percent into a decimal, so that 70 percent then becomes .7, in this example. Finally, multiply the original price by the decimal. If the shirt originally cost $20, multiply $20 by .7. to get $14. The discounted price is $14.