Marginal cost calculator

Enter a total cost function C(q), or two quantity and cost figures. You get marginal cost, average total, variable and fixed cost, and, with revenue, marginal revenue and marginal profit.

By the Calcist team · Checked against known answers · Updated October 29, 2026

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Estimates for planning, not financial, tax or legal advice. Figures use US rules unless the page says otherwise.

Next: Find the sales that cover your costs with the break-even calculator, or see how buyers react to price with the price elasticity calculator.

What is marginal cost?

Marginal cost is the extra cost of producing one more unit. It's the number that tells a business whether expanding output is worth it: if the next unit costs more to make than it brings in, making it lowers profit. Average costs answer a different question: what each unit costs on average so far.

How to calculate marginal cost

From data: MC = ΔC / Δq = (C₂ − C₁) / (q₂ − q₁)
From a cost function: MC(q) = C′(q) ≈ C(q + 1) − C(q)

ATC = C(q)/q · AVC = VC/q · AFC = FC/q = ATC − AVC
FC = C(0), VC = C(q) − FC
MR = R′(q), marginal profit MP = MR − MC

The calculator differentiates C(q) and R(q) symbolically and also shows the exact cost of the next unit, C(q + 1) − C(q), so you can see how close the derivative is.

Worked example

A workshop's cost is C(q) = 500 + 10q + 0.05q² dollars. At q = 100 units:

C(100) = 500 + 1,000 + 500 = $2,000. Fixed cost FC = C(0) = $500, so variable cost VC = $1,500.
MC(q) = C′(q) = 10 + 0.1q, so MC(100) = $20 per unit. The exact cost of unit 101 is C(101) − C(100) = $20.05.
ATC = 2,000/100 = $20, AVC = 1,500/100 = $15 and AFC = 500/100 = $5.

Here MC equals ATC, which is the bottom of the average total cost curve: below 100 units, extra units pull the average down; above it, they push it up.

Revenue works the same way. With R(x) = −0.03x² + 9x, marginal revenue is R′(x) = −0.06x + 9, so R′(100) = $3: the 101st sale brings in about $3 (exactly $2.97).

Average variable cost vs average total cost

Measure	Formula	What it tells you
Average total cost (ATC)	C/q	Full cost per unit, including overhead
Average variable cost (AVC)	VC/q	Running cost per unit, leaving out overhead
Average fixed cost (AFC)	FC/q	Overhead per unit; it always falls as output rises
Marginal cost (MC)	ΔC/Δq or C′(q)	Cost of the next unit

Limitations

Fixed cost is read as C(0). If your formula only holds for q > 0, enter the fixed cost from your books instead and use the two-point mode.
From two data points you get the average marginal cost across that range, not the exact cost of one particular unit.
Real costs jump in steps (a new machine, a second shift), which a smooth formula can't capture.

Sources

How we checked this

Before every update, this calculator is run in a real browser with these known answers, and it must match all of them:

C = 500 + 10q + 0.05q² at q = 100: MC = $20 ✓
Same cost: AVC = 1,500/100 = $15 ✓
R = 9q − 0.03q², C = 500 + 2q at q = 100: marginal profit $3 − $2 = $1 ✓
From (100, $2,000) to (110, $2,205): MC = $20.50 ✓
Equal quantities can't give a marginal cost ✓

Last checked October 26, 2026. Spotted something wrong? The math is done in your browser with the formula shown on this page, so you can check any result by hand.

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Questions people ask

What is marginal cost?

The additional cost of producing one more unit. From data it's the change in total cost divided by the change in quantity; from a cost function it's the derivative C′(q).

How do you calculate average variable cost?

Subtract fixed cost from total cost to get variable cost, then divide by the quantity: AVC = (TC − FC)/q.

What is marginal profit?

The extra profit from one more unit: marginal revenue minus marginal cost. While it's positive, producing more raises profit.

Why does marginal cost cross average total cost at its lowest point?

When the next unit costs less than the current average, it pulls the average down; when it costs more, it pulls the average up. So the average stops falling exactly where MC = ATC.

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