Shortage and surplus describe plans at a price

Subtract the smaller planned quantity from the larger one.

At a price below equilibrium, quantity demanded exceeds quantity supplied. Buyers compete for too few units, inventories disappear, and price tends to rise. At a price above equilibrium, sellers offer more than buyers plan to purchase. Unsold inventories create pressure to reduce price. These are tendencies in the basic model. Contracts, menu costs, or regulation can slow actual adjustment.

Equilibrium coordinates plans. It does not mean that every buyer obtains all desired units or every seller is pleased with the price. At the equilibrium price, the total quantity buyers plan to purchase equals the total quantity sellers plan to offer. No unplanned inventory change creates pressure for the market price to move in the basic model.

Shortage and surplus are measured at a specified price, not across two different rows. Write both planned quantities at that price, identify which is larger, and subtract. A shortage is Q_d-Q_s when positive. A surplus is Q_s-Q_d when positive. The words describe a gap between plans, not the physical number of goods on a shelf.

If at $6 buyers want 90 units and sellers offer 55, the shortage is 90-55=35 units. Do not report 90, 55, or their sum. At the equilibrium price the shortage and surplus are both zero because the planned quantities are equal.

Price pressure follows the side that cannot carry out its plan. During a shortage, some buyers bid more or search harder, encouraging a higher price and additional quantity supplied while reducing quantity demanded. During a surplus, sellers cut price or reduce production, encouraging more quantity demanded and less quantity supplied. Both movements close the gap along the existing curves when no determinant has changed.

Read a row before naming the imbalance

At $9, buyers plan 74 units and sellers plan 92. The market has an 18-unit surplus, and downward price pressure follows. Calling 74 a shortage confuses quantity demanded with the difference between plans. Calling the situation a shortage because some buyers still want the good at lower prices ignores the price specified in the row.

Regulation can prevent the price adjustment. A binding price ceiling may hold price below equilibrium, causing the shortage to persist. A binding price floor may hold it above equilibrium, causing a persistent surplus. The imbalance logic is the same. The legal rule blocks the ordinary price movement.

Do not treat equilibrium as automatically efficient in every market. With externalities, market power, asymmetric information, or public goods, private demand and supply can omit relevant social values. Equilibrium still means private plans match at a price. Efficiency requires the correct marginal social benefit and cost.

During a shortage or surplus, do not shift a curve merely because price is adjusting. The price change causes movements along both existing curves until plans match. A curve shifts only if a determinant changes during the process. This distinction keeps the adjustment mechanism separate from the event that may have created the initial imbalance.