Demand function is a function expressing the relationship between quantity demanded of a commodity and its determinants.

In other words demand function relates to prices and quantity. It tells us how many units of a product we can buy a certain price. Market demand function is calculated by getting the average of individual demand function in a market.

A demand function may appear like this;

Taking for example a commodity x its' demand function will be;

Q(X) =f{ P(X), P(C), Y, P(s)}

Where Q(X) is quantity demanded of product X, P(X) is the price of X, P(C) is the price of complimentary product to X, Y is income of an individual and P(s) is the price of a substitute of commodity X.