Why using your bank interest rate to measure working capital cost is misleading

When small businesses think about the cost of working capital, they usually start with the easiest number they can find: their bank interest rate.

It makes sense. If borrowing costs 8%, then cash tied in receivables or inventory must cost 8% too.

Simple. Clean. Easy to calculate. And incomplete.

Because working capital is not funded only by debt. It is funded by the capital structure of the business. That means debt, equity, retained earnings, and sometimes owner capital. And each of those carries its own cost. This distinction matters more than most businesses realize.

Take a simple example.

A company has €100,000 tied in receivables. Its customers pay in 90 days.

Management looks at its overdraft facility: 7%. So they assume delayed collections cost them 7%. But that number only reflects the cost of debt. What about the owner’s capital?

What about retained earnings that could have been deployed elsewhere?

What about the risk of delayed or missed payment?

Those are real economic costs. They just don’t show up as bank charges. That is where Weighted Average Cost of Capital becomes useful. WACC forces you to look at the blended cost of all capital. Not just borrowed capital.

Suppose:

Cost of debt = 7% Cost of equity = 18% Capital structure = 50% debt, 50% equity

The blended capital cost becomes 12.5%. That changes the economics immediately. Suddenly, the €100,000 trapped in receivables is not costing €7,000 a year. It is costing €12,500. Same receivables. Different decision framework. And that difference matters.

Because operational decisions depend on it. If you underestimate working capital cost:

you may offer customer credit too easily.

you may hold too much inventory.

you may underprice urgency.

you may accept slower collections than the business can afford.

This is one of the hidden reasons businesses feel profitable on paper but constantly feel pressure on cash. Margins look fine. Sales look healthy. But capital stays trapped. And trapped capital has a real cost. Not just the rate the bank charges.

The bigger point is this: working capital is not just an accounting metric. It is capital allocation. And capital allocation should always be measured against the full cost of capital. Not the easiest number on the loan agreement.

Most businesses never calculate this. They should.