In order to express a number as a percentage, one converts it from a mixed number or fraction to a decimal, moves the decimal point two places to the right and adds the percent sign. By definition, percentage is the rate per 100. Moving the decimal point is a shortcut for multiplying by 100.
When using percentages in calculations, it is easiest to convert the number back to a decimal before completing the operation. The conversion involves moving the decimal point two places to the left and removing the decimal. To calculate a percentage of a total, multiply the total by the resulting decimal number. Unless the percentage is greater than 100, the resulting number is always smaller than the total.
Percentages are useful in daily life. When dining out, the guideline for tipping is 18 to 20 percent of the total bill. People can calculate a 20 percent tip by multiplying the total of the bill by 0.20. They can also adjust the amount of the tip to compensate for service received. Tax rates also use percentages. Shoppers can calculate a sales tax rate of 6 percent by multiplying the total by 0.06. The consumer pays an additional six cents for every dollar he spends.
The use of percentages converts portions to whole numbers. Scientists often deal with ratios that are too small to report as a rate per 100. They use parts per million or parts per billion in a similar way to that in which percentages are used.