Quick answer
Marginal cost = change in total cost ÷ change in quantity. Producing 30 more units (100 to 130) raised total cost from $1,000 to $1,300, so marginal cost is $300 ÷ 30 = $10.00 a unit.
Enter total cost and quantity before and after a change. The calculator returns the cost of each extra unit.
Related tools: Marginal Revenue Calculator: Extra revenue from selling additional units. Opportunity Cost Calculator: Value given up by choosing one option over another. Price Elasticity of Demand Calculator: Price elasticity of demand by the midpoint (arc) method.
How to use the Marginal Cost Calculator
- Enter total cost before ($) (0 or more).
- Enter total cost after ($) (0 or more).
- Enter quantity before (0 or more).
- Enter quantity after (0 or more).
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Total cost before | A number, in $ | ≥ 0 |
| Total cost after | A number, in $ | ≥ 0 |
| Quantity before | A number | ≥ 0 |
| Quantity after | A number | ≥ 0 |
Formula and method
Formula
marginal cost = ΔC ÷ ΔQ
IEEE-754 double precision; shown to the decimals indicated.
Cost per additional unit between two output levels.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Total cost before = 1000; Total cost after = 1300; Quantity before = 100; Quantity after = 130.
| Result | Value |
|---|---|
| Marginal cost | $10.00 |
Example 2
Inputs: Total cost before = 5000; Total cost after = 5400; Quantity before = 50; Quantity after = 60.
| Result | Value |
|---|---|
| Marginal cost | $40.00 |
The first line is the headline result (marginal cost). IEEE-754 double precision; shown to the decimals indicated.
Why firms use it
The extra unit is worth making only if the price or marginal revenue exceeds its marginal cost. Here each extra unit costs $10 while the average cost at 130 units is $10.77, so average and marginal cost differ and decisions should use the marginal figure.
Where this estimate stops applying
The result is an average over the range you enter; the true cost of a single unit can differ, and fixed costs that step up (a new machine) distort it. The two quantities must be different.
How the result changes with total cost before
Holding every other input at the first worked example, this table changes only total cost before. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Total cost before | Marginal cost |
|---|---|
| 500 | $26.67 |
| 1000 | $10.00 |
| 2000 | -$23.33 |
| 4000 | -$90.00 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Quantity after: entering "5" shows "The quantities must be different." beside that field and no result.
- Total cost before: left empty shows "Enter total cost before." beside that field and no result.
Choosing between the Marginal Cost Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Marginal Revenue Calculator](/business/marginal-revenue/) | total revenue before, total revenue after and quantity before | Marginal revenue |
| [Opportunity Cost Calculator](/business/opportunity-cost/) | return of the option you gave up and return of the option you chose | Opportunity cost |
| [Price Elasticity of Demand Calculator](/business/price-elasticity-demand/) | price before, price after and quantity before | Price Elasticity of Demand |
| [GDP Calculator](/business/gdp-calculator/) | consumption, investment and government spending | GDP |
| [Income Elasticity of Demand Calculator](/business/income-elasticity-demand/) | income before, income after and quantity before | Income Elasticity of Demand |
Background: business metrics
Business ratios depend on exactly which costs and revenues you include, so read each page's definition. Two common checks: break-even units = fixed costs ÷ (price − variable cost per unit), so $50,000 of fixed costs with a $10 price and $5 variable cost breaks even at 10,000 units; and net profit margin = net profit ÷ revenue, so $20,000 profit on $100,000 revenue is 20%.
Return on investment ignores time: 20% over five years is far less impressive than 20% in one, so compare returns over the same period.
Assumptions and limitations
- Quantities must differ
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for quantity after and total cost before to confirm this.
Frequently asked questions
Is marginal cost the same as average cost?
No. Average cost divides total cost by all units, marginal cost looks only at the extra ones.
Why do marginal costs eventually rise?
Capacity limits and overtime make additional output costlier.
What if fixed costs change?
Include only the costs that change with output.
What does the Marginal Cost Calculator calculate?
Extra cost of producing additional units. It returns Marginal cost.
What formula does the Marginal Cost Calculator use?
marginal cost = ΔC ÷ ΔQ. Cost per additional unit between two output levels.
What do I need to enter in the marginal Cost Calculator?
You enter Total cost before ($), Total cost after ($), Quantity before and Quantity after.
How is the result rounded or limited?
IEEE-754 double precision; shown to the decimals indicated. Quantities must differ.
What is an example of a marginal Cost result?
With total cost before 1000, total cost after 1300, quantity before 100 and quantity after 130, the marginal cost is $10.00.
Related calculators
- Marginal Revenue Calculator — Extra revenue from selling additional units.
- Opportunity Cost Calculator — Value given up by choosing one option over another.
- Price Elasticity of Demand Calculator — Price elasticity of demand by the midpoint (arc) method.
- GDP Calculator — Gross domestic product by the expenditure approach.
- Income Elasticity of Demand Calculator — Income elasticity of demand by the midpoint (arc) method.
- Price Elasticity of Supply Calculator — Price elasticity of supply by the midpoint (arc) method.
Sources
- OpenStax, Principles of Accounting, Volume 2: Managerial Accounting (Rice University)
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.