Marginal Cost Calculator
Calculate the marginal cost of production — the change in total cost when one additional unit is produced. Enter the change in total cost and the change in quantity to find the cost of the next unit.
How to use this tool
- Enter total cost at quantity 1, quantity 1, total cost at quantity 2 and quantity 2 in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your marginal cost per unit and the full breakdown beneath it.
⚠ This tool provides general estimates for education only and is not financial, tax or legal advice. Figures may not reflect your situation — verify with a qualified professional.
Formula
MC = ΔTC ÷ ΔQ = (TC2 − TC1) ÷ (Q2 − Q1)
Where TC is total cost and Q is quantity produced.
How it works
Marginal cost (MC) is the additional cost incurred when producing one more unit of output. It is calculated as the change in total cost divided by the change in quantity produced over that interval.
In the short run, marginal cost typically falls initially due to economies of scale, then rises as diminishing returns set in. A profit-maximizing firm produces up to the point where marginal cost equals marginal revenue (MC = MR).
Worked example
Production Rises from 100 to 110 Units
- Change in total cost (ΔTC) = $1,080 − $1,000 = $80.00
- Change in quantity (ΔQ) = 110 − 100 = 10 units
- Marginal cost = $80 ÷ 10 = $8.00 per unit
The marginal cost is $8.00 per unit — producing each additional unit in this range costs $8.00.
Common mistakes to avoid
- Dividing total cost by quantity (average cost) instead of the change in total cost by the change in quantity — MC is a marginal (incremental) concept, not an average.
- Including fixed costs in the change in total cost when they do not actually change with output — only variable cost components change at the margin; fixed costs are sunk for incremental production decisions.
- Assuming MC is constant across all output levels; in practice, MC typically falls initially (economies of scale) and then rises (diminishing marginal returns), making a single MC figure valid only for a specific output range.
Key terms
- What is marginal cost?
- Marginal cost is the incremental cost of producing one additional unit of a good or service. It equals the change in total cost divided by the change in quantity.
- Why does marginal cost typically form a U-shape?
- Initially, fixed costs are spread over more units and specialization improves efficiency, so MC falls. Eventually, diminishing returns to variable inputs cause MC to rise, creating the classic U-shape.
- What is the relationship between MC and AVC?
- Marginal cost intersects average variable cost (AVC) and average total cost (ATC) at their minimum points. When MC is below AVC, AVC is falling; when MC is above AVC, AVC is rising.
- What is the profit-maximization rule using MC?
- A competitive firm maximizes profit by producing at the output level where MC equals the market price (P = MC). More generally, any firm maximizes profit where MC = MR (marginal revenue).
Frequently asked questions
- Why does a profit-maximizing firm produce where MC = MR?
- At any output where MR exceeds MC, producing one more unit adds more to revenue than to cost, increasing profit. At any output where MC exceeds MR, the last unit costs more than it earns. Profit is maximized at the quantity where these two are equal.
- How is MC related to the supply curve?
- For a competitive firm, the marginal cost curve above the average variable cost curve is the firm's short-run supply curve — it shows the minimum price required to induce the firm to supply each additional unit of output.
- Can marginal cost be negative?
- Theoretically yes in rare cases (e.g., a firm that must pay to dispose of a by-product sees producing more actually reduce total cost). In most production contexts, MC is positive because additional inputs cost money.