In mathematics, a percentage is a number or ratio as a fraction of 100. It is often denoted using the percent sign, “%”, or the abbreviation “pct. ” For example, 45% (read as “forty-five percent”) is equal to 45/100, or 0.45. A related system which expresses a number as a fraction of 1000 uses the terms "per mil" and "millage". Percentages are used to express how large/small one quantity is, relative to another quantity.