Short-run profit is the economic profit a perfectly competitive firm earns when the market price is above its average total cost at the profit-maximizing quantity (where MR = MC). It exists only temporarily, because free entry pushes price back down to break-even in the long run.
Short-run profit is what a firm earns when the price it gets for each unit is higher than its average total cost (ATC) at the quantity where marginal revenue equals marginal cost. The formula is profit = (P - ATC) × Q. On the standard side-by-side graph, it shows up as a rectangle between the price line and the ATC curve, stretching out to the profit-maximizing quantity.
The word "short-run" is doing real work here. In the short run, the number of firms in the industry is fixed, so an existing firm can keep that profit rectangle for a while. But in perfect competition there are no barriers to entry (EK PRD-3.A.1), so profit acts like a flashing neon sign that says "come join this industry." New firms enter, market supply shifts right, price falls, and the rectangle shrinks until price equals minimum ATC. At that point firms earn zero economic profit, which is also called normal profit. Short-run profit is temporary by design.
This concept lives in Topic 3.7 (Perfect Competition) in Unit 3, and it directly supports LO 3.7.C, which asks you to calculate economic profit or loss from a graph or table. It also feeds LO 3.7.B, because short-run profit is the trigger for the entire long-run adjustment story. If you can't identify whether a firm is profiting, breaking even, or losing money in the short run, you can't explain what happens to the industry next. The Unit 3 FRQ almost always involves drawing a perfectly competitive firm, labeling the MR = MC quantity, and shading or identifying the profit (or loss) area. Short-run profit is the payoff of the whole cost-curve apparatus you built in Topics 3.1 through 3.6.
Keep studying AP® Microeconomics Unit 3
Economic Profit (Unit 3)
Short-run profit in AP Micro means short-run economic profit, which counts implicit costs like the owner's forgone salary. A firm can show a positive accounting profit while earning zero economic profit, and the exam only cares about the economic version.
Normal Profit (Unit 3)
Normal profit is zero economic profit, the long-run resting point of perfect competition. Short-run profit is the temporary detour above that resting point, and entry is the force that drags the firm back to it.
Barriers to Entry (Unit 4)
The reason short-run profit can't survive in perfect competition is that nothing blocks new firms from entering. Flip that switch on, as in monopoly, and short-run profit can persist into the long run. This contrast is the bridge from Unit 3 to Unit 4.
Average Total Cost (ATC) (Unit 3)
ATC is the comparison line for profit. If price sits above ATC at the chosen quantity, the firm profits; below it, the firm takes a loss. The vertical gap between P and ATC, multiplied by quantity, is the profit rectangle you shade on the graph.
Multiple-choice questions test short-run profit in a few predictable ways. One classic stem starts an industry in long-run equilibrium, increases demand, and asks for the sequence of events. The answer is that price rises, existing firms earn short-run profit, new firms enter, supply shifts right, and price falls back to break-even. Another favorite tests whether you know that a lump-sum tax (a fixed cost) lowers profit but does not change the profit-maximizing quantity, while a change in a variable input cost shifts MC and does change output. On the FRQ, you'll typically draw the firm's graph, mark the MR = MC quantity, and identify the profit or loss area between price and ATC. Always calculate profit as (P - ATC) × Q, never by eyeballing total revenue alone.
Short-run profit means positive economic profit, where P > ATC. Normal profit means zero economic profit, where P = minimum ATC and the firm is exactly covering all costs, including opportunity costs. A firm earning normal profit is not failing; it's doing just well enough that owners have no reason to leave and outsiders have no reason to enter. In perfect competition, short-run profit always erodes toward normal profit in the long run.
A perfectly competitive firm earns short-run profit when price is above average total cost at the quantity where MR = MC, and the profit equals (P - ATC) × Q.
Short-run profit cannot last in perfect competition because there are no barriers to entry; new firms enter, supply increases, and price falls until economic profit is zero.
A lump-sum tax or an increase in rent (fixed costs) reduces short-run profit but does not change the profit-maximizing quantity, because it doesn't touch marginal cost.
An increase in a variable input cost shifts the MC curve up, so it changes both the firm's output and its profit.
Zero economic profit (normal profit) is the long-run outcome, and it means the firm is covering all explicit and implicit costs, not that it's broke.
It's the economic profit a firm earns in the short run when price exceeds average total cost at the MR = MC quantity. You calculate it as (P - ATC) × Q, and on a graph it's the rectangle between the price line and the ATC curve.
No. Because there are no barriers to entry, short-run profits attract new firms, market supply increases, and price falls until P = minimum ATC. In the long run, perfectly competitive firms earn zero economic profit, also called normal profit.
Short-run profit is positive economic profit (P > ATC), while normal profit is zero economic profit (P = minimum ATC). Normal profit still covers all costs including opportunity costs, so it's the long-run equilibrium, not a loss.
No. A lump-sum tax is a fixed cost, so it shifts ATC up but leaves marginal cost alone. The firm keeps producing where MR = MC at the same quantity, but its profit falls by the full amount of the tax. For example, a 500.
No. Zero economic profit means the firm covers all explicit costs plus implicit costs like the owner's best alternative earnings. The owners are doing exactly as well as they could anywhere else, which is why firms stay in the industry at break-even.
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