Marginal revenue (MR) is the additional revenue a firm earns from selling one more unit of output (MR = ΔTR/ΔQ). Every profit-maximizing firm produces where MR = MC; in perfect competition MR equals price, but for a monopoly or any firm with market power, MR is less than price.
Marginal revenue is the change in total revenue from selling one more unit, calculated as MR = ΔTR/ΔQ. It answers a simple question for the firm. If I sell one more, how much extra money comes in? Pair that with marginal cost (how much extra it costs to make one more) and you get the single most important rule in AP Micro. Produce every unit where MR > MC, stop at the quantity where MR = MC.
What MR looks like depends entirely on market structure. A perfectly competitive firm is a price taker, so selling one more unit always brings in exactly the market price. That means MR = P and the firm's demand curve is a horizontal line (EK PRD-3.A.3). A monopolist or monopolistically competitive firm faces a downward-sloping demand curve, so to sell one more unit it must lower the price on all units. The extra unit brings in the new price but loses a little revenue on everything it was already selling. That's why MR < P whenever demand slopes down, and why the MR curve sits below the demand curve (for a linear demand curve, MR falls twice as steeply).
MR is the hinge connecting Unit 3 (Topic 3.7, Perfect Competition) to all of Unit 4 (Topics 4.2 Monopolies, 4.3 Price Discrimination, and 4.4 Monopolistic Competition). EK PRD-3.B.6 states it directly. A monopoly's profit-maximizing quantity is found where MR = MC, and the price charged is greater than marginal cost. That P > MC gap is the whole story of Unit 4's inefficiency. Because the monopolist stops at MR = MC instead of producing out to where P = MC, output is too low, deadweight loss appears, and the market fails the allocative efficiency test that perfect competition passes (EK PRD-3.A.2). Learning objectives 4.2.A, 4.3.A, and 4.4.A all ask you to explain firm decision making and deadweight loss with graphs, and every one of those graphs starts by locating the MR = MC intersection. If you can't find MR on a graph, you can't find quantity, price, profit, or deadweight loss either.
Keep studying AP Microeconomics Unit 4
Profit Maximization (Units 3-4)
MR = MC is the universal profit-maximizing rule for every market structure on the exam. The structures differ only in what MR looks like. In perfect competition MR is flat at the market price; in monopoly and monopolistic competition it slopes down below demand. Same rule, different curve.
Price Elasticity of Demand (Units 2 and 4)
MR and elasticity are two views of the same thing. When demand is elastic, lowering price raises total revenue, so MR is positive. When demand is inelastic, MR is negative. That's why a monopolist always produces in the elastic region of its demand curve, a connection the exam loves to test.
Total Revenue (Units 2-3)
MR is the slope of total revenue. If a table gives you TR at each quantity, you find MR by taking the difference between consecutive TR values. This is the most common calculation move on table-based MCQs.
Allocative Efficiency (Units 3-4)
Efficiency requires P = MC. Since a monopolist sets MR = MC and MR < P, it ends up with P > MC and produces less than the allocatively efficient quantity. The vertical gap between demand and MR at the chosen output is literally the source of deadweight loss on the monopoly graph.
Marginal revenue shows up on basically every Unit 4 FRQ. The 2019, 2022, and 2023 FRQ Q1s all give you a profit-maximizing monopoly and ask you to draw a correctly labeled graph, which means demand, MR below it, MC, quantity at the MR = MC intersection, and price read up to the demand curve (not the MR curve, the classic point-loser). The 2017 FRQ Q3 layered MR onto an externality graph, so be ready to find MR even when other curves crowd the picture. MCQs hit it from several angles. They'll ask what a monopolist does when MR > MC (increase output), how the profit-maximizing quantity is found (MR = MC), and what MR < P implies about elasticity. You also need to compute MR from a total revenue table by subtracting consecutive TR values.
In perfect competition, MR and price are the same number, so it's tempting to treat them as identical everywhere. They're not. For any firm facing downward-sloping demand, MR < P, because selling another unit forces the firm to cut the price on all the units it was already selling. On the monopoly graph this means quantity comes from MR = MC, but price comes from the demand curve above that quantity. Reading price off the MR curve is one of the most common graph errors on FRQs.
Marginal revenue is the extra revenue from selling one more unit, calculated as the change in total revenue divided by the change in quantity.
Every profit-maximizing firm produces the quantity where MR = MC, regardless of market structure.
In perfect competition MR equals the market price because firms are price takers facing a horizontal demand curve.
For a monopoly or monopolistically competitive firm, MR is less than price because lowering price to sell more units cuts revenue on all previous units.
On a monopoly graph, find quantity where MR crosses MC, then go straight up to the demand curve to find the price.
If MR is always less than price, demand slopes downward, and a monopolist will only ever produce in the elastic region of demand where MR is positive.
Marginal revenue is the additional revenue from selling one more unit of output, MR = ΔTR/ΔQ. It's half of the MR = MC rule that determines profit-maximizing output for every firm in Units 3 and 4.
No, only in perfect competition. There, price takers face a horizontal demand curve, so each extra unit sells at the market price and MR = P. Any firm with market power (monopoly, monopolistic competition) has MR < P because it must lower price on all units to sell more.
To sell one more unit, the monopolist lowers price on every unit, not just the last one. The extra unit adds the new price but subtracts the price cut on all previous units, so the net gain (MR) is less than the price on the demand curve. With linear demand, MR falls twice as fast.
Total revenue is everything the firm earns (P × Q); marginal revenue is the change in total revenue from one more unit. On a table, MR is the difference between consecutive TR values. They're linked through elasticity, since TR rises when MR is positive and falls when MR is negative.
Increase output. Each additional unit adds more to revenue than to cost, so producing more raises profit. The firm keeps expanding until MR = MC, which is the profit-maximizing quantity. This is a frequently tested MCQ setup.