Elasticity is a measure of how responsive buyers and sellers are to changes in price and other market conditions. Price elasticity of demand specifically measures the responsiveness of quantity demanded to a change in price. It is calculated as the percentage change in quantity demanded divided by the percentage change in price. Demand can be perfectly inelastic, inelastic, unit elastic, or elastic depending on whether the quantity changes less than, equal to, or more than the price change. Determinants of price elasticity include whether a good is a necessity vs luxury and the availability of substitutes. Income elasticity measures responsiveness of demand to income changes.