To calculate a percentage, you first need to know the basic rules for percentages. A percent is simply a ratio or fraction with 100 as its denominator (base). So, if you have 50% of something, that means you have 50/100 or 1/2 of it. To find the percentage of something, you need to multiply the number by the decimal form of the percent (e.g., 70% = 0.70).