Supply and demand are equated in a free market through the price mechanism. If buyers wish to purchase more of a good than is available at the prevailing price, they will tend to bid the price up. If they wish to purchase less than is available at the prevailing price, suppliers will bid prices down. The price mechanism thus determines what quantities of goods are to be produced. The price mechanism also determines which goods are to be produced, how the goods are to be produced, and who will get the goods—i.e., how the goods will be distributed. Goods so produced and distributed may be consumer items, services, labour, or other salable commodities. In each case, an increase in demand will lead to the price being bid up, which will induce producers to supply more; a decrease in demand will lead to the price being bid down, which will induce producers to supply less. The price system thus provides a simple scale by which competing demands may be weighed by every consumer or producer.

The tendency to move toward the equilibrium price is known as the market mechanism, and the resulting balance between supply and demand is called a market equilibrium.

As the price of a good rises, the quantity offered usually increases, and the willingness of consumers to buy the good normally declines, but those changes are not necessarily proportional. The measure of the responsiveness of supply and demand to changes in price is called the price elasticity of supply or demand, calculated as the ratio of the percentage change in quantity supplied or demanded to the percentage change in price. Thus, if the price of a commodity decreases by 10 percent and sales of the commodity consequently increase by 20 percent, then the price elasticity of demand for that commodity is said to be 2.

In algebraic form, elasticity (E) is defined as E = %Δy/%Δx; y is elastic with respect to x if E is greater than 1, inelastic with respect to x if E is less than 1, and “unit elastic” with respect to x if E is equal to 1. Several other types of elasticities are frequently used to describe well-known economic variables. These include, but are not limited to, the income elasticity of demand, the cross-price elasticity (the elasticity of the price of a good with respect to the price of another good), the elasticity of substitution between different factors of production (for example, between capital and labour), and the elasticity of intertemporal substitution (for example, the elasticity of consumption in the future relative to consumption in the present).

Several other types of elasticities that are frequently used to describe well-known economic variables have acquired their own special names over time. These include, but are not limited to, the income elasticity of demand, the cross-price elasticity (the elasticity of the price of a good with respect to the price of another good), the elasticity of substitution between different factors of production (for example, between capital and labour), and the elasticity of intertemporal substitution (for example, the elasticity of consumption in the future relative to consumption in the present).

The demand for products that have readily available substitutes is likely to be elastic, which means that it will be more responsive to changes in the price of the product. That is because consumers can easily replace the good with another if its price rises. The demand for a product may be inelastic if there are no close substitutes and if expenditures on the product constitute only a small part of the consumer’s income. Firms faced with relatively inelastic demands for their products may increase their total revenue by raising prices; those facing elastic demands cannot.

Supply-and-demand analysis may be applied to markets for final goods and services or to markets for labour, capital, and other factors of production. It can be applied at the level of the firm or the industry or at the aggregate level for the entire economy.