The excess of the benefit a consumer gains from purchase of a good over the amount paid for the good. An individual demand curve shows the valuation put by a consumer on successive units of a good. Goods whose value to the consumer is higher than their price are bought; purchasing stops when their marginal utility is equal to their price times the marginal utility of income. Consumer surplus can be measured by the area below the demand curve but above the price. The surpluses of individual consumers from purchases of the same good can be aggregated for the market as a whole. Total consumer surplus from the purchase of two or more differentiated goods, in general, is not well-defined unless the marginal utility of income is constant.