Elasticity measures the responsiveness of quantity demanded to a change in price. It is calculated by finding the percentage changes in quantity and price and taking the ratio. An elasticity over 1 means demand is elastic and sensitive to price, under 1 means inelastic demand not sensitive to price, and exactly 1 means unitary elastic demand where quantity and price move proportionately. Demand for necessities tends to be inelastic while luxuries have elastic demand more impacted by price changes.