💰 Costs of Production
Every firm faces costs when producing goods and services. Understanding the three types of cost and how they change with output is essential for analysing business decisions and market outcomes.
The three cost categories: Fixed costs (FC) do not change with output (e.g. rent, insurance, salaries of permanent staff). Even if the firm produces nothing, it still pays these.
Variable costs (VC) change directly with output (e.g. raw materials, energy, wages of hourly workers). The more you produce, the higher VC.
Total cost (TC) is the sum of fixed and variable costs: TC = FC + VC.
Why does this matter?
In the short run, at least one factor of production is fixed (e.g. factory size), so the firm has fixed costs. In the long run, all factors are variable — the firm can change everything, including its scale of operation.
- FC stays constant regardless of how much (or little) is produced
- VC starts at zero and increases as output rises
- TC starts at the level of FC (because VC = 0 when Q = 0)
- The gap between TC and FC is always equal to VC
- Short run = at least one fixed factor; long run = all factors variable
Real-world example: A bakery: Fixed costs: monthly rent (£2,000), oven lease (£500), insurance (£200) = £2,700 per month — these stay the same whether the bakery makes 100 or 1,000 loaves.
Variable costs: flour, sugar, butter, packaging, electricity for ovens — these increase with every extra loaf baked.
Total cost = £2,700 + variable costs for that month.
Exam tips: Common exam marks come from stating relationships clearly:
- Show FC as a horizontal line on a cost diagram — it does not change with output
- TC and VC have the same shape because TC = FC + VC
- Explain why VC increases with output: more inputs are needed (materials, labour, energy)
- Common mistake: saying FC = 0 when output = 0. Wrong — FC still exists at zero output
📊 Average and Marginal Cost
While total costs show the overall picture, firms make decisions based on cost per unit (average cost) and the cost of one more unit (marginal cost). These concepts explain pricing, output decisions, and efficiency.
Four key per-unit costs: Average total cost (ATC) = TC ÷ Q — the cost per unit of output.
Average variable cost (AVC) = VC ÷ Q — the variable cost per unit only.
Average fixed cost (AFC) = FC ÷ Q — keeps falling as output rises (spreading fixed costs).
Marginal cost (MC) = the extra cost of producing one more unit. This is the most important cost curve for decision-making.
Why is ATC U-shaped?
At low output levels, AFC is very high because fixed costs are spread over only a few units, so ATC is high. As output rises, AFC falls rapidly and pulls ATC down. Eventually, AVC rises faster due to diminishing marginal returns and pushes ATC back up. The result is the classic U-shape.
The MC–ATC rule (Exam essential): MC intersects ATC at its minimum point.
When MC < ATC → ATC is falling (the next unit costs less than the average, so it pulls the average down).
When MC = ATC → ATC is at its minimum (the turning point).
When MC > ATC → ATC is rising (the next unit costs more than the average, pulling it up).
Analogy: If your average score is 70% and you score 60% next, your average falls. Score 80% and it rises. The marginal result determines the direction of the average.
Diminishing marginal returns
In the short run, adding more of a variable factor (e.g. workers) to a fixed factor (e.g. factory size) eventually leads to smaller increases in output per extra worker. This is diminishing marginal returns. As a result, each additional unit of output becomes more costly to produce, which causes marginal cost to rise.
- ATC = AFC + AVC (always)
- MC cuts ATC at its minimum point
- MC also cuts AVC at its minimum (which occurs before ATC's minimum)
- AFC always falls — it never rises
- The gap between ATC and AVC equals AFC (and this gap shrinks as output rises)
- Diminishing marginal returns → MC rises → eventually pulls ATC upward
Exam tips: When drawing cost curves, always show MC crossing ATC at its lowest point.
MC is steeper than ATC — it rises and falls faster.
Label all curves clearly: MC, ATC, AVC. Include AFC only if required.
Productive efficiency occurs at the minimum of ATC (lowest cost per unit).
Do not confuse diminishing returns (short run, one factor fixed) with diseconomies of scale (long run, all factors variable).