Marginal Cost Calculator

What does one more cost?

You are making something. Somebody asks whether you can make one more.

What does that one cost you?

Not your average cost per unit. Not a share of the rent. The actual extra money that leaves the business because you made one more than you were going to.

That is marginal cost, and it is the number almost every production and pricing decision should be built on. It is also the number most businesses do not know, because their accounts are organised around totals and averages instead.

Two boxes here. The arithmetic takes a second. What the answer means takes the rest of this page.

Two boxes and a unit selector

1. Change in total cost. How much more you spent.
2. Change in quantity. How many more units that bought you.

Beside the quantity is a selector: units, pairs, decades or dozens. It multiplies whatever you type, so 5 dozens becomes 60 units.

That is useful if your production is counted in batches. Enter 5 with dozens selected and you get the cost per single unit, not per dozen, which is nearly always the number you want.

Both figures must describe the same step. If costs rose by 550 when you went from 40 units to 50, then 550 and 10 go in the boxes. Not your total cost, and not your total output.

The arithmetic, and what the selector is for

Marginal cost = change in total cost ÷ change in quantity

That is the whole formula. Extra money divided by extra units.

The unit selector sits inside the denominator, so the full calculation is:

Marginal cost = change in total cost ÷ (quantity entered × unit multiplier)

Spend 600 to produce 5 more dozens and the tool divides 600 by 60, giving 10.00 per unit.

Worth checking the selector before you trust an answer. Leave it on units when you meant dozens and your marginal cost comes out twelve times too high, which is the sort of error that gets a product cancelled.

A small factory, batch by batch

Here is a business making something in batches of ten. The total cost column is what it spends in all, at each level of output.

Read the marginal column on its own. It goes 60, 50, 45, 45, 55, 75, 110, 160.

Down, then up.

The cheapest units this business will ever make are the ones between 20 and 40, at 45 each. The last ten cost 160 each, which is 3.6 times what the cheapest ten cost.

Same factory, same product, same week. The only thing that changed is how many were being made.

Why it falls and then climbs

That shape turns up so reliably that it has a name. Economists call it the U-shaped marginal cost curve, and both halves have ordinary explanations.

The falling half is things getting into their stride. Machines run closer to their efficient rate. Staff stop switching between jobs. You buy materials in larger lots and pay less. Setup time gets spread across more output.

All of which is why the first ten units cost 60 each and the next ten cost 50.

The rising half is running out of room. This is the interesting one, and it has a specific cause.

At some point something in the business is at its limit. The machine cannot go faster. The floor space is full. And the only way to make more is to start doing things the expensive way: overtime at premium rates, a night shift, a rush order on materials, a subcontractor, hiring people you have not had time to train properly.

So the cost of each extra unit climbs, and it climbs steeply, because you are no longer using the efficient method. You are using whatever method is still available.

Economists call this diminishing returns, which sounds abstract. On a factory floor it is much simpler than that: you have one good machine and adding a second shift to it costs more per unit than the first shift did.

The practical upshot is that your cheapest output is somewhere in the middle. Producing too little wastes your fixed capacity. Producing too much means paying premium rates for the last stretch.

The mistake that loses money on every sale

Look at the table again, at 60 units.

Total cost is 4,300 and average cost is 71.67 per unit. That is the number that comes out of most accounting systems, and it is the number most businesses price from.

Now look at the marginal column at 70 units: 110.00.

So if a customer asks for ten more and you quote your average cost plus a margin, you have priced at something around 71.67 for units that cost you 110 to make.

You lose 38.33 on every single one, and the order looks profitable on the spreadsheet because the spreadsheet is using an average.

That is not a rounding error. That is a business taking on work that makes it poorer, one order at a time.

The distinction in one line:

• Average cost is what everything you have already made cost you, per unit. It is a description of the past.
• Marginal cost is what the next one will cost. It is the only one that answers a decision.

They agree only when marginal cost is flat, which the table shows is not the normal case.

So whenever the question is "should I make more", the average is the wrong number. Always.

Where the fixed costs went

Notice the first row of the table. Zero units made, and the total cost is 1,000.

That is the rent, the insurance, the loan payment, the salaried staff. It happens whether you produce anything or not.

And it does not appear in the marginal cost column anywhere. Not once.

Which is correct, and it catches people. Marginal cost measures the change in total cost, and fixed costs do not change. They cancel out of every single subtraction in that column.

This has a consequence that feels wrong until you sit with it. If your factory is running and the rent is already paid, then for a short-run decision about one more order, the rent is irrelevant. It is already spent. The only question is whether the extra revenue beats the extra cost.

Which is why a business will sometimes take an order below its average cost and be right to do so. If marginal cost is 45 and someone offers 60, that order contributes 15 toward the fixed costs you were going to pay anyway.

Two cautions on that, and they matter.

It only holds in the short run. Over time the fixed costs have to be covered by something, and a business selling everything above marginal cost and below average cost is going out of business slowly.

And it only holds if the cheap order does not displace a full-price one, or teach your other customers what you are willing to accept.

What to do with the number

Three things, in rough order of how often they come up.

Deciding on an order. Compare the marginal cost against what the order pays. Above it, the order adds money. Below it, the order costs you money regardless of what the total looks like.

Finding your efficient output. Run the calculation at several output levels and look for where the marginal cost bottoms out. On our factory that is somewhere around 30 to 40 units, and it is the cheapest place to operate.

Setting a price floor. Marginal cost is the absolute lowest price that makes any sense at all, and only in the short run. Your real floor has to cover a share of the fixed costs too. The margin calculator takes a cost and a target margin and returns the price that delivers it.

And then there is the fourth thing, which is the one this pairs with.

Knowing what one more costs only answers half a question. The other half is what one more earns, and that is not simply the price, because selling more often means charging less. The marginal revenue calculator handles that side, and the two together give you the rule that decides how much to make.

This is a measurement of figures you supply rather than an assessment of a business, and nothing here is financial advice.

Questions people ask

How is marginal cost calculated?

Change in total cost divided by change in quantity. If costs rose 550 when output rose by 10 units, marginal cost is 55 per unit.

How is it different from average cost?

Average cost is total cost divided by total units, which describes what you have already made. Marginal cost is what the next unit costs. They agree only when marginal cost is flat.

Do fixed costs count?

No, and that is not an omission. Marginal cost measures the change in total cost, and fixed costs do not change, so they cancel out of the calculation entirely.

Why does marginal cost fall and then rise?

It falls as production gets efficient and volume discounts arrive. It rises when something hits its limit and further output needs overtime, rush orders or subcontractors.

What is the unit selector for?

Converting batches into single units. Enter 5 with dozens selected and the tool divides your cost change by 60, giving a cost per single unit.

Should I ever sell below average cost?

In the short run, if the price beats marginal cost, the order contributes toward fixed costs you are paying regardless. It is not sustainable long term, and it is dangerous if it displaces full-price work.

What is the lowest price I can charge?

Marginal cost is the absolute floor, and only briefly. A price that covers marginal cost and nothing else leaves the fixed costs unpaid.

How do I know how much to produce?

Compare marginal cost against marginal revenue. Produce while the extra revenue exceeds the extra cost, and stop where they meet.

References

Marginal cost as the change in total cost divided by the change in quantity, the U-shaped marginal cost curve arising from increasing and then diminishing returns to a variable input, and the rule that output should expand while marginal revenue exceeds marginal cost, are standard results in microeconomic theory. The distinction between costs that vary with output and fixed costs that do not, and the treatment of cost of goods sold as the direct cost of the goods actually sold in a period, follow Internal Revenue Service small business guidance. The classification of production and operating costs on the income statement follows Regulation S-X.

1. Internal Revenue Service, Publication 334: Tax Guide for Small Business. https://www.irs.gov/publications/p334
2. Internal Revenue Service, Publication 538: Accounting Periods and Methods. https://www.irs.gov/publications/p538
3. United States Securities and Exchange Commission, Regulation S-X, Rule 5-03: Statements of Comprehensive Income (17 CFR 210.5-03). Text quoted in SEC staff correspondence at https://www.sec.gov/Archives/edgar/data/0001019361/000101936112000010/filename1.txt
4. US Chamber of Commerce, Pricing Markups Explained: Definition and Similar Terms. https://www.uschamber.com/co/start/strategy/what-are-pricing-markups