Marginal Cost Calculator

Marginal cost is the cost of one more unit

Marginal cost is the extra cost of producing one more unit of output, and it is the number that decides whether the next unit is worth making. A bakery does not ask what a loaf costs on average, it asks what the 1,001st loaf costs, because that is the loaf it has to sell or lose. Average cost spreads fixed rent over every loaf, while marginal cost measures the cost of the next loaf alone.

The distinction matters because the two move in opposite directions. As a bakery adds ovens, shifts and delivery runs, the average cost per loaf falls as fixed costs spread out, while the marginal cost of each extra loaf rises once capacity starts to strain. The OpenStax Principles of Economics chapter on short run costs lays out the same split: average cost measures the cost of the average unit, marginal cost measures the cost of the next unit only.

For everyday decisions the definition is plain enough. A cup of coffee costs you the beans, the milk, the cup and the labor to make one more cup, and that is what you compare against the price of the cup. If the extra cost of the next cup is 60 cents and you sell it for 4 dollars, the extra cup is worth making. If the extra cost is 4.20 dollars, the next cup loses money even though the shop as a whole might still be profitable.

The formula: change in cost over change in output

Marginal cost is a rate of change, and the rate is measured between two points rather than at a single one:

• MC = (TC2 - TC1) / (Q2 - Q1)
• TC1 and TC2 are total cost at two output levels
• Q1 and Q2 are the output levels, in units

Multiply out the algebra and you get the definition in words: the change in total cost divided by the change in quantity. The units work out to dollars per unit, the same unit as a price, which is why the two can be set side by side. Note the minus signs: when total cost rises, the marginal cost is positive, and when total cost falls as output rises, the marginal cost is negative, which is the economics of a business that gets more efficient with scale.

The textbook illustration from OpenStax uses a hair salon where total cost rises from 320 dollars at 40 haircuts to 400 dollars at 60 haircuts. The change in cost is 80 dollars and the change in output is 20 haircuts, so the marginal cost of each of those haircuts is 80 / 20 = 4 dollars per haircut. A second OpenStax example starts with one widget costing 32.50 dollars and two widgets costing 44 dollars, so the marginal cost of the second widget is 44 - 32.50 = 11.50 dollars. Both numbers come from dividing a change in cost by a change in quantity, and the calculator on this page does the same division for your own figures.

One important detail: the two output levels should be close together if you want the marginal cost of a single unit. Averaging over a wide range blurs the step you care about, the way measuring the average height of everyone in a stadium tells you nothing about the tallest person in it.

Worked example: a bakery adds 300 loaves a week

Take a neighborhood bakery with fixed costs of 1,800 dollars a month for rent, insurance and the loan on the ovens. Last month it baked 1,000 loaves for 4,000 dollars of total cost. This month it baked 1,300 loaves and total cost rose to 4,855 dollars. The change in cost is 4,855 - 4,000 = 855 dollars and the change in output is 1,300 - 1,000 = 300 loaves, so the marginal cost of a loaf is 855 / 300 = 2.85 dollars per loaf.

Now the other numbers in the table, all built from the same two points. Average total cost was 4,000 / 1,000 = 4.00 dollars a loaf last month and 4,855 / 1,300 = 3.73 dollars this month, so the average fell as volume rose. Variable cost per loaf at the higher output is (4,855 - 1,800) / 1,300 = 2.35 dollars, the part of cost that moves with each loaf. At a price of 4.75 dollars a loaf, revenue is 4.75 x 1,300 = 6,175 dollars and profit is 6,175 - 4,855 = 1,320 dollars. Each extra loaf adds 4.75 - 2.85 = 1.90 dollars of contribution, and the break-even point where fixed cost is covered is 1,800 / (4.75 - 2.35) = 750 loaves.

Read the three cost numbers together and the story is clear. The average cost of 3.73 dollars looks healthy, but it is the 2.85 dollar marginal cost that says anything about the next loaf. The bakery can afford to keep baking, because 2.85 dollars of extra cost sits well below the 4.75 dollar price, and 1,300 loaves is far past the 750 loaf break-even point. If a flour supplier pushed the marginal cost of a loaf up to 4.90 dollars, the average cost would still be under 4 dollars and the shop would still be profitable in total, yet every additional loaf would now lose 15 cents. That is the trap marginal cost catches and average cost hides.

Average cost, marginal cost and the shape of the curve

The bakery numbers can be laid out as a schedule so you can watch the two curves cross. Here fixed cost is 500 dollars and total cost climbs as the shop approaches capacity.

Average total cost falls from 9.00 dollars to 7.33 dollars as fixed cost spreads over more loaves, then climbs again once the marginal cost of the next loaf runs above the average. Marginal cost starts at 4.00 dollars for the first hundred loaves and reaches 12.00 dollars for the last hundred, because the shop is adding shifts, oven time and delivery runs that each carry their own cost. The marginal cost curve crosses the average cost curve at its lowest point, near 300 loaves, and that crossing is the textbook signal to stop expanding output unless price has risen to match.

For a price taker, such as a bakery selling wholesale at a market price, the rule that falls out of this schedule is the one in the OpenStax chapter on output decisions for competitive firms: produce up to the point where price equals marginal cost. Below that point each unit adds profit, above it each unit subtracts. A business that sells at a fixed price of 8 dollars a loaf on this schedule would stop somewhere between 300 and 400 loaves, where the marginal cost of the next loaf crosses 8 dollars.

Comparing marginal cost with price and with marginal revenue

When a firm can change its own price, price stops being a constant and the comparison becomes marginal cost against marginal revenue, the extra revenue from selling one more unit. At a constant price the two are the same number, which is why the simpler price test works for most small businesses.

Run the lawn care case through the same arithmetic. Fixed cost of 310 dollars a month covers the truck payment and insurance. At 30 lawns total cost is 565 dollars and at 45 lawns it is 737.50 dollars, so the change in cost is 172.50 dollars over 15 extra lawns, a marginal cost of 172.50 / 15 = 11.50 dollars a lawn. The variable cost per lawn is (737.50 - 310) / 45 = 9.50 dollars, which is the fuel, the mower fuel can and the crew time. At 42 dollars a lawn the extra profit per lawn is 42 - 11.50 = 30.50 dollars, and the break-even point is 310 / (42 - 9.50) = 10 lawns. A crew only cuts lawns above that number, and the margin says why: 30 lawns is already a comfortable business.

The mobile coffee cart is the case where fixed cost eats the most. With 900 dollars of fixed cost, total cost of 1,080 dollars at 200 cups and 1,149.60 dollars at 260 cups, the marginal cost of a cup is 69.60 / 60 = 1.16 dollars, and variable cost per cup is (1,149.60 - 900) / 260 = 0.96 dollars. At 4.75 dollars a cup the cart earns 1,235 dollars of revenue and 85.40 dollars of profit, and the break-even point is 900 / (4.75 - 0.96) = 238 cups. The margin of 3.59 dollars a cup is the most comfortable of the three, yet the cart needs nearly 240 cups a month before it earns anything at all. Marginal cost and the break-even point answer different questions, and a business needs both.

Where the numbers come from: labor, power and materials

A marginal cost figure is only as honest as the inputs behind it, so it helps to know what the big cost lines look like right now. Three federal sources cover most of them.

Three federal series cover most of the inputs you need, and each one moves on its own schedule.

Labor. The federal minimum wage is 7.25 dollars an hour under the Fair Labor Standards Act, and the Department of Labor minimum wage page still shows that figure as the floor for covered nonexempt workers in 2026, with many state and local rates above it. At the federal floor, one 8 hour shift costs 7.25 x 8 = 58.00 dollars. If that shift bakes 40 loaves, the labor cost carried by each loaf is 58.00 / 40 = 1.45 dollars, and that figure belongs in the variable cost side of your marginal cost calculation.

Power. Electricity is a classic variable cost for anyone running equipment all day. The EIA Electric Power Monthly, Table 5.6.A puts the average US residential price at 18.34 cents per kilowatthour in June 2026, against 17.47 cents a year earlier. A 1,500 watt space heater running 8 hours draws 12 kilowatthours, which at 18.34 cents is 12 x 0.1834 = 2.20 dollars of power for one shift. If a business added a second heater for the same 12 kilowatthours, that 2.20 dollars is the marginal cost of the extra heat, with no fixed cost attached.

Materials. Input prices move too, and the Bureau of Labor Statistics producer price index showed producer prices for final demand up 3.0 percent over the year ending December 2025, after a 3.5 percent rise in 2024. A material that costs 0.40 dollars per unit at the start of that year carries 0.40 x 1.03 = 0.412 dollars at the end of it, so a marginal cost computed on January prices is 3 cents a unit stale by December. Update the cost side of the calculation before you change the price side.

Where a two point marginal cost estimate goes wrong

The formula is simple, and most of the errors come from choosing the wrong two points or forgetting what belongs in each one.

• Do not use total cost when you mean variable cost. If fixed cost changed between the two points, for instance because you signed a new lease, that jump lands in your marginal cost and makes it look like a huge cost of production. Use two points inside the same cost structure.
• Do not divide total cost by total output. That is average cost. The marginal figure needs the change in the numerator and the change in the denominator.
• Do not compare costs across different periods. A January total and a June total blend price changes and volume changes into one number that describes neither.
• Do not drop step costs. A new oven is a jump in fixed cost, not a rise in the cost per loaf. If you spread the oven over a year, its depreciation belongs in your fixed cost estimate, not in the marginal cost of the next loaf.
• Do not read a negative marginal cost as free money. Falling marginal cost usually means the extra volume came with a setup that pays off, such as a bulk discount on materials or a second shift filling an already staffed shift.

One more caution about small businesses: marginal cost is a per unit number, and a per unit number hides a lot. A bakery that adds 300 loaves a month also adds staff hours, delivery miles, packaging and spoilage. Compute the marginal cost of a loaf, then multiply by the 300 loaves and check that the total matches your actual spending, because a per unit figure that is wrong by a few cents becomes a real hole at volume.

For the neighbouring jobs, the break-even calculator covers the fixed cost side of the same arithmetic, the cost of goods sold calculator handles what you carry, and the markup calculator turns cost into a shelf price. This page stays on the one question of what the next unit costs.