Some examples of common demographics include age, gender, income level, occupation and so on. These labels are used to describe the socioeconomic characteristics of any given population, and they're usually expressed as a percentage. That allows you to compare each demographic's proportion to the entire population, no matter what the size of said population.
TL;DR (Too Long; Didn't Read)
Divide the target demographic by the entire population, and then multiply the result by 100 to convert it to a percentage.
In order to calculate demographic percentages, you need two pieces of information: How many people belong to the particular demographic you're measuring, and how many people belong to the entire population.
Example: Imagine that you've been asked to calculate the demographic percentage of men, in a population of 700 people where 315 of them are men.
Divide the number of people in your demographic (in the example, that's 315 men) by the entire population (in this case, 700 people). That gives you:
Multiply the result from Step 2 by 100 to convert it into a percentage:
So in this example, men make up 45 percent of the population.
Here's another example, using a different demographic: age. Imagine that in a population of 1,350 people, 460 of them are between 25 and 34 years of age. What percentage of the population does that represent? You already have all the information you need, so you can go straight to dividing:
Divide the target demographic (in this case, the 460 people that are between 25 and 34 years of age) by the entire population:
Multiply the result from Step 1 by 100 to convert it into a percentage:
So people between 25 and 34 years of age make up 34 percent of the population being considered.