In simple terms
A friendly intro before the formal notes — no formulas yet.
Looking Inside the Firm
Every firm faces two flows: money going out (costs) and money coming in (revenue). This topic gives you the tools to measure both per unit, then combines them into one rule that tells the firm exactly how much to produce to make the most profit possible.
Picture a pizza kitchen. The oven and the rent are fixed — they cost the same whether you bake 10 pizzas or 100. The flour, cheese and delivery drivers are variable — the more pizzas, the more you spend. As you cram more cooks into one kitchen they start bumping elbows, so each extra pizza gets more expensive to make: that is diminishing marginal returns pushing marginal cost up. You keep baking one more pizza as long as the money it brings in (marginal revenue) is bigger than what it costs to make it (marginal cost). Stop the moment the two are equal — that is your profit-maximising output.
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Split costs into fixed (don't change with output) and variable (do change with output).
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Turn totals into per-unit figures: average and marginal cost. Diminishing returns make the MC curve turn upward.
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Do the same on the revenue side: total, average and marginal revenue.
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Apply the golden rule — produce where MC = MR — then read off whether the firm makes normal profit, abnormal profit, or a loss.
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Key formulas
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Full topic notes
Formal explanation with the rigour you need for the exam.
Short-run production and the law of diminishing marginal returns
Economists distinguish two time periods. In the SHORT RUN at least one factor of production is fixed — typically capital, such as the size of a factory or the number of machines. In the LONG RUN every factor is variable: the firm can build a new plant, buy more machines, or shut sites entirely. The behaviour of costs is completely different in the two periods, so always know which one a question is asking about.
Consider a firm in the short run adding more workers (the variable factor) to a fixed amount of capital. At first, extra workers allow specialisation and fuller use of the machinery, so each additional worker adds a lot to output — marginal product rises. But because capital is fixed, there comes a point where workers start to get in each other's way and share equipment: each additional worker adds less to output than the one before. This is the LAW OF DIMINISHING MARGINAL RETURNS — as successive units of a variable factor are added to a fixed factor, the marginal product of the variable factor eventually falls. It is a short-run law, and it is the reason marginal cost eventually rises.
Short run — at least one factor is fixed (giving rise to fixed costs).
Long run — all factors are variable (no fixed costs; the firm changes its whole scale).
Diminishing marginal returns — a short-run phenomenon: extra variable input added to a fixed input eventually raises output by less and less.
Falling marginal product means rising marginal cost — this is the bridge from production to cost curves.
Costs: fixed, variable and total
Total cost splits into two parts. Total fixed cost (TFC) does not change with the level of output — rent, insurance, business rates and loan repayments must be paid even if the firm produces nothing. Total variable cost (TVC) rises as output rises — raw materials, power and hourly wages all increase the more the firm makes. Adding them gives total cost.
(Total Cost = Total Fixed Cost + Total Variable Cost). Note that TFC is a horizontal line, while TVC and TC both rise with output.
Average and marginal cost, and the shape of the curves
To decide on output, firms convert totals into per-unit figures. Average cost tells us the cost per unit; marginal cost tells us the cost of one more unit. There are three average measures and one marginal measure to master.
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The averages-and-marginal relationship is the same one you meet with test scores: if your next (marginal) test score is below your current average, the average falls; if it is above, the average rises; and your average is at its lowest exactly when the marginal score crosses it from below. That is why the MC curve must pass through the bottom of the ATC and AVC curves.
AFC falls continuously as output rises — the fixed lump is spread over more and more units (it approaches, but never reaches, zero).
AVC and ATC are both U-shaped: they fall while increasing returns dominate, then rise once diminishing returns set in.
MC is also U-shaped, for exactly the same reason — it is the mirror image of marginal product.
MC cuts both AVC and ATC at their minimum points. When MC is below an average it pulls the average down; when MC is above it, it pulls the average up; so they can only cross at the lowest point of the average curve.
The vertical gap between ATC and AVC at any output equals AFC — and that gap narrows as output rises because AFC is falling.
The long run: economies and diseconomies of scale
In the long run there are no fixed factors — the firm can vary everything, including the size of its plant. The relevant curve is now the long-run average total cost (LRATC) curve, which shows the lowest cost per unit achievable at each scale of production. It is U-shaped, but for a completely different reason from the short-run curves: it reflects economies and diseconomies of scale, not diminishing returns.
Economies of scale (downward part of LRATC): as the firm grows, cost per unit FALLS. Sources include purchasing (bulk-buying discounts), technical (larger, more efficient machinery), managerial (specialist managers), financial (cheaper borrowing) and risk-bearing (diversification) economies.
Constant returns to scale (flat part): a range of output over which cost per unit is roughly unchanged.
Diseconomies of scale (upward part): beyond some size, cost per unit RISES, usually because a very large organisation becomes hard to coordinate, communicate within and motivate.
Minimum efficient scale (MES) — the lowest output at which LRATC is minimised; the smallest size a firm must reach to be cost-competitive.
Do not confuse the two U-shapes. The SHORT-RUN ATC curve is U-shaped because of the law of diminishing marginal returns (a fixed factor). The LONG-RUN ATC curve is U-shaped because of economies then diseconomies of scale (all factors variable). Using 'diminishing returns' to explain the long-run curve, or 'diseconomies of scale' to explain the short-run curve, is a frequent and easily avoided error.
Revenues: total, average and marginal
On the other side of the ledger is revenue — the money the firm receives from selling its output. As with costs, we work with totals, averages and marginals.
(Total Revenue) | (Average Revenue — equal to price, so the AR curve is the firm's demand curve) | (Marginal Revenue)
AR always equals price, so the average revenue curve is simply the demand curve facing the firm.
Under perfect competition the firm is a price taker — it sells all it wants at the market price — so price is constant and P = AR = MR (a horizontal line).
Under imperfect competition (monopoly, monopolistic competition, oligopoly) the firm faces a downward-sloping demand curve; to sell more it must lower price, so MR < AR and MR falls faster than AR.
When MR is positive, TR is still rising; TR is maximised where MR = 0; when MR is negative, TR is falling.
The profit-maximisation rule: produce where MC = MR
We can now combine the two sides. A profit-maximising firm produces the output at which marginal cost equals marginal revenue, MC = MR (with MC rising through MR at that point). The logic is decisive: if MR > MC, the next unit adds more to revenue than to cost, so producing it raises total profit — keep expanding. If MR < MC, the next unit costs more than it earns, so producing it lowers total profit — cut back. Only where MC = MR is there no way to increase profit by changing output. This rule holds in EVERY market structure.
MR > MC → expand output (each extra unit adds to profit).
MR < MC → reduce output (each extra unit subtracts from profit).
MR = MC → profit-maximising output (nothing more to gain).
This is NOT the same as maximising total revenue (that is where MR = 0) or maximising profit per unit (minimum ATC). Maximising TOTAL profit means MC = MR.
Normal profit, abnormal profit, and the break-even and shut-down points
Economists measure profit against economic cost, which includes the opportunity cost of the owner's resources (implicit costs) as well as explicit out-of-pocket costs. This gives three outcomes at the profit-maximising output, all read by comparing average revenue (price) with average total cost.
Normal profit — economic profit is exactly zero: AR = ATC. The firm is covering ALL its costs, including opportunity cost, so it earns just enough to stay in the industry. This is the break-even point.
Abnormal (supernormal / economic) profit — AR > ATC: the firm earns more than the minimum needed to stay, attracting entry where barriers are low.
Economic loss — AR < ATC: the firm is not covering all its costs and would, in the long run, leave the industry.
Shut-down point (short run) — the firm keeps producing at a loss only while AR ≥ AVC, because it is still covering its variable costs and contributing towards fixed costs. If AR falls below minimum AVC, it should shut down, since producing then adds to the loss.
Long run — a firm must cover ATC to survive; persistent economic losses force exit, while abnormal profits attract entry (in competitive markets).
A brief overview of the four market structures
The cost and revenue tools you have just built are applied, in later lessons, to four market structures. They are distinguished by the number of firms, whether the product is identical or differentiated, how high the barriers to entry are, and how much power the firm has over price. Here is the map.
In every one of these structures the firm still chooses output where MC = MR — what changes between them is the shape of the demand (AR) curve the firm faces and therefore how much price-setting power it has and whether abnormal profit survives into the long run.
Perfect competition — very many small firms, an identical (homogeneous) product, no barriers to entry, and perfect information. Each firm is a price taker (P = AR = MR). Abnormal profits are competed away in the long run, leaving only normal profit.
Monopoly — a single dominant firm (or one with substantial market power), high barriers to entry, and considerable price-setting power. Can sustain abnormal profit in the long run because entry is blocked.
Monopolistic competition — many firms selling differentiated products (branding, quality, location), with low barriers to entry. Firms have some price-setting power in the short run, but entry erodes abnormal profit to normal profit in the long run.
Oligopoly — a few large, interdependent firms dominate, behind high barriers to entry. Each firm's decisions depend on rivals' likely reactions, so outcomes range from price wars to collusion. Product may be homogeneous (e.g. oil) or differentiated (e.g. cars).
Common mistakes examiners penalise
Profit-maximising at the wrong point — profit is maximised where MC = MR, NOT where total revenue is highest (MR = 0) and NOT where average profit per unit is greatest (minimum ATC). This is the single most-penalised error in the topic.
Mislabelling fixed and variable costs — rent, insurance and loan repayments are FIXED (unchanged by output); raw materials, power and hourly wages are VARIABLE. Do not call a cost 'fixed' just because its per-unit value is constant.
Confusing normal and abnormal profit — normal profit means economic profit is ZERO (AR = ATC), not that the firm makes 'a normal amount of money'. Abnormal profit is the surplus ABOVE that, when AR > ATC.
Confusing diminishing returns with diseconomies of scale — diminishing marginal returns is SHORT RUN (a fixed factor, explains the short-run MC/ATC U-shape); diseconomies of scale is LONG RUN (all factors variable, explains the LRATC U-shape).
Using the wrong shut-down test — the short-run shut-down rule compares AR with AVC (shut down if AR < AVC), not with ATC. A firm making a loss can still be right to keep producing if it covers its variable costs.
Forgetting to show working or units on Paper 3 — the M (method) marks are for the working itself; a correct final number with no method shown, or a number with no $ or unit, throws away marks the marking engine would otherwise award.
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
A firm has total fixed costs of $60. The table shows its total variable cost at each level of output.
| Output (Q) | TVC ( |---|---|
| 0 | 0 |
|---|---|
| 1 | 30 |
| 2 | 50 |
| 3 | 66 |
| 4 | 88 |
| 5 | 120 |
(a) Calculate total cost (TC) at 3 units. (b) Calculate average total cost (ATC) at 3 units. (c) Calculate the marginal cost (MC) of the 4th unit. (d) Between which units does the law of diminishing marginal returns first begin to affect this firm's costs? Justify your answer.
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(a) TC at 3 units: TC = TFC + TVC = 66 = **
A firm sells at a constant price of $12. At 100 units its total cost is $900; at 101 units its total cost is $908. Determine the marginal revenue and marginal cost of the 101st unit and state whether the firm should produce it to maximise profit. [4]
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Marginal revenue of the 101st unit: The firm sells at a constant price of $12, so each extra unit adds exactly the price to total revenue. Therefore MR = P = $12. (Formally, MR = ΔTR / ΔQ. TR at 100 = 100 × 1,200; TR at 101 = 101 × 1,212; ΔTR = $12 for one extra unit, so MR = $12.)
A firm produces 500 units. Its price (average revenue) is $10, its average total cost is $8.50 and its average variable cost is
(a) Calculate the firm's total economic profit or loss. (b) State whether it is earning normal profit, abnormal profit or a loss, and explain. (c) If price fell to $5, should the firm shut down in the short run? Justify using the shut-down rule.
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(a) Profit per unit = AR − ATC = 8.50 = $1.50. Total profit = profit per unit × Q = $1.50 × 500 = $750 abnormal (economic) profit.
How it all connects
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Glossary
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Quick check
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Revision flashcards
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Law of diminishing marginal returns
In the SHORT RUN, as successive units of a variable factor (e.g. labour) are added to a FIXED factor (e.g. capital), the marginal product of the variable factor eventually falls. It is a short-run law and it is what drives marginal cost upward.
Key takeaways
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Short run — at least one factor is fixed (giving rise to fixed costs).
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Long run — all factors are variable (no fixed costs; the firm changes its whole scale).
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Diminishing marginal returns — a short-run phenomenon: extra variable input added to a fixed input eventually raises output by less and less.
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Falling marginal product means rising marginal cost — this is the bridge from production to cost curves.
Practice — then mark it
The whole point: a real Cambridge question, marked mark-by-mark.
Get a Paper 3 quantitative answer marked: calculate MR, MC and apply the MC = MR rule
Get a Paper 3 quantitative answer marked: calculate MR, MC and apply the MC = MR rule
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