When Gross Margin Lies: Measuring the True Cost of a Customer

In 2010, I built a simple internal model for something that kept bothering me:

Two customers could generate similar sales and similar accounting profit… but create completely different pressure on the business.

Why?

Because one paid fast. The other trapped cash for months.

Most companies looked only at sales and gross margin.

I wanted to measure something different:

“How much capital does this customer force the company to finance?”

So I started connecting:

• inventory holding days • customer payment delays • supplier credit days

to estimate the real funding gap created by each customer.

Then I applied the company’s cost of capital to that gap.

The result was interesting:

Some “good” customers were actually extremely expensive to carry financially.

Not because they negotiated lower prices. Not because margins were low.

But because cash stayed trapped inside their operating cycle for too long.

At the time, I built the model using annual averages and customer ratios because the company did not have clean operational time-series data.

Ideally, the model should have used actual transaction timing:

• when inventory entered the warehouse • when it was sold • when the customer actually paid • when suppliers were actually paid

using real daily cash movements instead of approximations.

But even the simplified version changed the discussion.

It shifted the focus from: “Who buys the most?” to: “Who consumes the most liquidity?”

That distinction matters much more than many businesses realize.