Most people asking whether a business works are asking about the total: revenue in, costs out, something left over. That is a fair question and it is the wrong one to start with, because a total tells you what happened and not what to do next.

The useful question is about one. One customer, one job, one subscription, one installation. What did it bring in, what did it cost to serve, and what did it cost to go and get? If one more of them makes the business better off, growth is the answer. If one more makes it worse off, growth is how the business dies faster, and there are plenty of businesses that grew enthusiastically into insolvency because nobody had done this arithmetic.

Choosing the unit

The unit is whatever you sell more of when you grow. This sounds obvious and is where most of the errors live.

A consultancy that sells retainers has a unit of one retained client per month. A consultancy that sells projects has a unit of one project. If it sells both and picks whichever unit flatters the figure, it has learned nothing. The test is whether the unit is the thing that scales: if the business doubled, what would there be twice as many of?

Get this wrong and every number downstream is wrong in the same direction. It is worth ten minutes.

The three numbers

What it brings in. Revenue for one unit, over its whole life, not just the first sale. A client who signs once and stays four years is a different unit from a client who signs once and leaves, even though the first invoice is identical. This is lifetime value.

What it costs to serve. Only the costs that move with the unit. Your accountant's fee does not change when you take on one more client; the contractor you hire to do the work does. The difference is the whole point of the exercise, and it is what gross margin measures.

What it costs to acquire. The money and time spent getting that customer, including the ones you spent it on who did not buy. This is customer acquisition cost, and it is the number most owners have never calculated, because the effort feels like overhead rather than a cost of sale.

Subtract the second two from the first. That is what one unit is worth. Everything else in the business — rent, software, your own salary, the accountant — is paid out of the sum of those, which is the argument for why the fixed costs are a separate question and why operating leverage is the page about them.

What the number is for

It settles four arguments that otherwise go round forever.

Whether to spend on getting customers. If a unit is worth $4,000 over its life and costs $600 to acquire, spending more on acquisition is obviously right and the only real constraint is cash. If it is worth $700 and costs $600, the business is doing a great deal of work to stand still.

Whether to raise prices. A thin unit margin means price is doing almost none of the work, and a small increase moves the whole business. This is the arithmetic behind value-based pricing: the margin is what pays for everything, and a 10% price rise on a 20% margin is a 50% increase in what the business keeps.

Which customers to keep. Almost every business has a set of clients whose unit economics are negative and who are retained out of habit, gratitude, or fear. They are usually the ones who take the most attention, which is the Pareto principle pointing the wrong way.

Whether to grow at all. A business with good unit economics and no growth has a marketing problem. A business with bad unit economics and lots of growth has a much more serious problem that the growth is currently hiding.

Why it gets avoided

Because the cost side is genuinely hard, and because the answer is often unwelcome.

Allocating your own time is the usual sticking point. If you spend six hours a month on a client and do not pay yourself for those hours, the unit looks profitable and is not — you have simply moved the cost somewhere the accounts cannot see it. Price the hours at what you would have to pay someone competent to do them. The gap between that figure and the profit is the part of the business that only works because you are doing it for free, and it is the same gap that shows up as owner dependency when it comes time to sell.

The other reason is that the arithmetic settles arguments people would rather keep having. It is much more comfortable to believe that the business will work at scale than to establish that each individual sale loses money and that scale multiplies the losing.