Fundamentals of Business Economics and Management · Cost of Production

Average Cost and Marginal Cost: Meaning and Relationship

Q: What is the difference between average cost and marginal cost?

Average cost is total cost divided by total output, so it is the cost per unit across all units. Marginal cost is the extra cost of producing one more unit. Average looks at all units produced, while marginal looks only at the last one.

Q: Why is the AC curve U-shaped?

At low output, AFC falls sharply and inputs are used more efficiently, so AC falls. At higher output, diminishing returns raise AVC, and this rise outweighs the fall in AFC. AC then turns upward, giving a U shape.

Q: Why does MC cut AC at its minimum point?

When MC is below AC, each extra unit costs less than the average, so AC falls. When MC is above AC, AC rises. The turning point is where MC equals AC, which is the lowest point of AC.

Q: How do you calculate marginal cost with an example?

Subtract the earlier total cost from the later one and divide by the change in output. If TC is ₹150 at 3 units and ₹180 at 4 units, MC of the 4th unit is ₹30.

Updated 10 October 2026 · Fact-checked

Average cost is cost per unit: total cost divided by output. Marginal cost is the extra cost of producing one more unit. To solve questions, build a table of TC, find MC as the change in TC, then AC as TC ÷ Q. MC cuts AC at AC's minimum point.

Understand Average and Marginal Cost

Total cost tells you what a firm spends in all. But a manager also asks two smaller questions: what does each unit cost on average, and what does the next unit cost? Average cost and marginal cost answer these.

Average fixed cost (AFC) is fixed cost spread over output. Fixed cost does not change with output, so AFC keeps falling as output rises. Average variable cost (AVC) is variable cost per unit. Average total cost (ATC), also called average cost (AC), is total cost per unit. It equals AFC + AVC.

Marginal cost (MC) is the addition to total cost when output rises by one unit. Fixed cost does not change when output changes, so MC comes only from variable cost. That is why MC is the same whether you work from TC or from TVC.

Why is the AC curve U-shaped? At low output, AFC falls steeply and AVC also falls as inputs are used better, so AC falls. At higher output, the law of variable proportions makes extra units costlier, so AVC rises. Soon the rise in AVC is bigger than the fall in AFC, and AC turns up. The result is a U shape. MC is U-shaped for the same reason.

The relationship between the two is the most tested point. When MC is below AC, it pulls AC down. When MC is above AC, it pulls AC up. So MC cuts AC at its lowest point, where MC = AC. Think of marks: a new test score below your average lowers the average, and one above it raises the average.

Key formulas to remember

Total cost: TC = TFC + TVC; TFC stays constant at every level of output, including zero output.
Average fixed cost: AFC = TFC ÷ Q; Falls continuously as Q rises. It never touches zero.
Average variable cost: AVC = TVC ÷ Q; U-shaped in the short run.
Average total cost: ATC (AC) = TC ÷ Q = AFC + AVC; The gap between ATC and AVC is AFC, and it narrows as output rises.
Marginal cost: MC = ΔTC ÷ ΔQ = TCn − TC(n−1); When output rises by one unit at a time, MC is simply the difference between successive TC values. It also equals ΔTVC ÷ ΔQ.
AC-MC relationship: MC < AC → AC falls; MC = AC → AC is minimum; MC > AC → AC rises; MC cuts both AVC and ATC at their minimum points, coming from below.

How to solve Average and Marginal Cost questions

Use this method for any numerical or theory question on average and marginal cost.

1Write down what is given: TC, TVC, TFC, or per-unit values, and the output levels.
2If only TC is given, find TFC. It equals TC at zero output, or TC minus TVC at any output.
3Find TVC = TC − TFC if it is not given.
4Compute MC as the change in TC between successive output levels, divided by the change in output.
5Compute AFC, AVC and ATC by dividing TFC, TVC and TC by Q. Check that ATC = AFC + AVC.
6For a relationship question, compare MC with AC at each output level. Falling AC means MC is below it. Rising AC means MC is above it.
7Pick the option that matches your computed value and check the unit (₹ per unit, not total ₹).

Quickest way: Shortcut using TC differences and the AC-MC rule

When to use it: Use this when a question gives a short table or one or two data points and four close options.

For MC, subtract the previous TC from the current TC. Do not compute averages first.
For AC, divide TC by Q only for the one output asked.
For AFC, divide the fixed cost by Q. Fixed cost is TC at zero output.
For a theory question, remember that MC cuts AC at minimum AC. Eliminate any option showing MC cutting AC from above, or at the maximum.
Check the answer: ATC must be greater than AVC, and ATC − AVC must equal AFC.

Common mistakes in Average and Marginal Cost

Treating MC as TC ÷ Q.
Students mix up marginal with average because both are per-unit.
Fix: MC is the extra cost of the last unit. Always subtract successive TC values.
Thinking fixed cost affects MC.
TC includes fixed cost, so students feel MC must too.
Fix: TFC cancels out when you subtract. MC depends only on change in variable cost.
Saying MC cuts AC at its maximum or from above.
Students memorise that the curves meet but forget where.
Fix: MC falls first, rises faster, and cuts AC from below at AC's minimum point.
Believing AC rises whenever MC rises.
Both curves are U-shaped and rise together later, so the link looks direct.
Fix: AC rises only when MC is above AC. MC can be rising while still below AC, and then AC keeps falling.
Expecting AFC to be U-shaped or to turn upward.
Students apply the U shape to every average curve.
Fix: AFC is a falling curve (a rectangular hyperbola). Only AVC, ATC and MC are U-shaped.
Using TC at the first unit as TFC.
The table may not show output zero.
Fix: TFC is TC at zero output, or TC − TVC. If a table starts at Q = 1, check for a given TVC.

Worked examples

Example 1

A firm has total fixed cost of ₹60. Its total variable cost is ₹40 at 1 unit, ₹70 at 2 units, ₹90 at 3 units and ₹120 at 4 units. Find the marginal cost of the 4th unit and the average total cost at 4 units.

Show the solution

TFC = ₹60 at every output.
TC at 3 units = 60 + 90 = ₹150.
TC at 4 units = 60 + 120 = ₹180.
MC of 4th unit = 180 − 150 = ₹30.
ATC at 4 units = 180 ÷ 4 = ₹45.
Check: AFC = 60 ÷ 4 = 15 and AVC = 120 ÷ 4 = 30. AFC + AVC = 45, which matches.

Answer: MC of the 4th unit = ₹30 and ATC at 4 units = ₹45.

Example 2

A firm's total cost is ₹100 at 2 units, ₹130 at 3 units, ₹170 at 4 units and ₹220 at 5 units. Its average cost falls up to a point and then rises. At which output does MC first exceed AC, and what does this show?

Show the solution

AC at 2 units = 100 ÷ 2 = ₹50.
MC of 3rd unit = 130 − 100 = ₹30. AC at 3 = 130 ÷ 3 ≈ ₹43.33. MC is below AC, so AC falls.
MC of 4th unit = 170 − 130 = ₹40. AC at 4 = 170 ÷ 4 = ₹42.50. MC is still below AC (₹40 < ₹43.33), so AC keeps falling.
MC of 5th unit = 220 − 170 = ₹50. AC at 5 = 220 ÷ 5 = ₹44. AC rises, since ₹50 is above the previous AC of ₹42.50.
So MC first exceeds AC at the 5th unit.
The minimum AC lies at 4 units, where AC is ₹42.50. MC passes through AC around this point from below.

Answer: MC first exceeds AC at the 5th unit (₹50 against ₹42.50). This shows that AC rises only when MC is above AC, and AC is lowest at 4 units.

Exam tips

Most questions give a small TC or TVC table. Practise finding MC by subtracting rows, and finishing in under a minute.
Learn the three-line AC-MC rule: below pulls down, equal means minimum, above pulls up. Many theory MCQs test exactly this.
Know the shapes: AFC falls throughout, AVC, ATC and MC are U-shaped, and MC cuts AVC and ATC at their minimum points.
Check whether the question asks for a per-unit value or a total. Options often include both.
If TFC is not given, look for TC at zero output or use TC − TVC. Do not guess.

Practice questions from Cost of Production

Average and Marginal Cost in other exams

The same ground in other exams, if you are preparing for more than one or want another angle on it.

Average and Marginal Cost: frequently asked questions

What is the difference between average cost and marginal cost?

Average cost is total cost divided by total output, so it is the cost per unit across all units. Marginal cost is the extra cost of producing one more unit. Average looks at all units produced, while marginal looks only at the last one.

Why is the AC curve U-shaped?

At low output, AFC falls sharply and inputs are used more efficiently, so AC falls. At higher output, diminishing returns raise AVC, and this rise outweighs the fall in AFC. AC then turns upward, giving a U shape.

Why does MC cut AC at its minimum point?

When MC is below AC, each extra unit costs less than the average, so AC falls. When MC is above AC, AC rises. The turning point is where MC equals AC, which is the lowest point of AC.

How do you calculate marginal cost with an example?

Subtract the earlier total cost from the later one and divide by the change in output. If TC is ₹150 at 3 units and ₹180 at 4 units, MC of the 4th unit is ₹30.