Marginal Cost Calculator

Marginal cost (MC) is the additional total cost incurred by producing one more unit of output. The formula is MC = ΔTC / ΔQ, the change in total cost (TC) divided by the change in quantity (Q) between two production points. Example: total cost rises from $1,000 at Q=100 to $1,200 at Q=120, so MC = ($1,200 − $1,000) / (120 − 100) = $200 / 20 = $10 per additional unit. This figure is descriptive — it describes the cost pattern in the data you enter, not a business decision recommendation.

Marginal cost (MC) measures the cost of an ADDITIONAL unit (ΔTC/ΔQ), while average cost measures the cost PER unit of all output: ATC = TC / Q (average total cost) and AVC = VC / Q (average variable cost). MC is marginal (at the edge), ATC/AVC are averages (in the middle). So MC can fall while ATC is still rising, or vice versa — they answer different questions about cost structure.

There is a fixed relationship: when MC < ATC, average cost is FALLING; when MC > ATC, average cost is RISING; and when MC = ATC, ATC is at its MINIMUM — MC cuts the ATC curve exactly at its minimum point. The same relationship holds for AVC. Example: with the data above, ATC1 = $1,000/100 = $10 and ATC2 = $1,200/120 = $10, so MC = $10 = ATC — meaning both points are at the minimum point of average cost.

At the start of production, adding inputs usually lowers the cost per additional unit (efficiency, division of labor). But as capacity nears its limit — machines on overtime, queues, less productive additional inputs — the cost of each additional unit starts to rise. This fall-then-rise pattern is what makes the MC curve U-shaped. The table mode in this calculator helps you see that trend from your own (Q, TC) data series.

Yes, with a conceptual caveat. In the short run some costs are fixed (rent, machines), so the analysis separates AVC and ATC; in the long run all costs are considered variable and the firm can change its plant scale. The formula MC = ΔTC/ΔQ is the same in both frameworks — only the cost components treated as fixed differ. This calculator does not model long-run choices; it computes MC from the cost data you provide.

Choose one of four modes: (1) Basic MC — enter (TC1, Q1) and (TC2, Q2) to get MC with ΔTC/ΔQ steps; (2) Multi-point table — paste a Q,TC data series (one point per line) to see MC per increment, the rising/falling trend, and the lowest MC; (3) MC vs ATC/AVC comparator — enter MC, Q, TC, and VC to see whether average cost is falling, rising, or at its minimum; (4) Solver — find TC2 from a targeted MC and ΔQ, or find Q2 from MC and TC2. All calculations run in your browser.

This calculator COMPUTES marginal cost from total cost data (MC = ΔTC/ΔQ). The Lerner Index calculator instead TAKES marginal cost as input to measure market power with the formula L = (P − MC) / P — its solver mode only inverts the Lerner formula itself (MC = P × (1 − L)), it does not compute MC from cost data. So: need MC from cost data → use this calculator; need to measure market power from price and MC → use our Lerner Index calculator.