Price Ceilings

A price ceiling is a maximum legal price, like rent control capping rent at a certain dollar amount. It only changes anything if it is binding, meaning set below the equilibrium price. At that lower price, buyers want more than they did at equilibrium, and sellers are willing to supply less, since price is part of what makes production worth it. Quantity demanded now exceeds quantity supplied, and that gap is the shortage. The market cannot clear through price anymore, so it clears through something else: waiting lists, rationing, or first-come-first-served lines.

Work it with numbers. Qd = 40 - 2P and Qs = 4P - 20 give equilibrium where 40 - 2P = 4P - 20, so P = 10. Impose a ceiling at P = 6. Quantity demanded is 40 - 2(6) = 28. Quantity supplied is 4(6) - 20 = 4. The shortage is 28 - 4 = 24 units. Only 4 units actually trade, far below the 20 units that would trade at equilibrium, so all the surplus on units between 4 and 20 is lost as deadweight loss. If a ceiling is set above equilibrium price instead, it does nothing: the market already clears below the cap, so quantity and price stay at equilibrium.

This sits right alongside price floors, the mirror image, and both connect forward to tax incidence: all three wedge a gap between what buyers pay and what sellers receive, and all three shrink the quantity traded below the efficient level.