What the number represents
The weighted average cost of capital is the return that a firm must earn to keep every provider of capital — lenders and shareholders — satisfied. Below it, the firm is shrinking the value it was given. Above it, it is creating value. That is the whole standard.
It is called a cost, but it behaves like a hurdle. It is the price of admission for any project the firm chooses to fund.
The pieces, and where they come from
WACC blends the after-tax cost of debt with the cost of equity, weighted by how much of each a firm uses. Each component is an estimate, and each estimate comes with a choice.
- Cost of debt: the yield the firm would pay today on new borrowing, not the coupon on an old issue. Interest is tax-deductible, so the after-tax cost is what matters.
- Cost of equity: usually built from the risk-free rate, a market risk premium, and a beta. The beta is measured from history but applied to the future — the weakest link in most models.
- Weights: target or market weights, not book values. The question is how the firm intends to be financed, not how it was financed years ago.
The levers that move it
Managers who understand WACC stop treating it as weather. Financing choices move it. So do operating choices. A business that becomes more cyclical becomes riskier to both lenders and shareholders, and its capital costs rise. A business that diversifies into steadier cash flows can lower its hurdle.
That is why capital structure is a strategy question, not an accounting one. Adding debt lowers WACC at first because interest is tax-deductible. Add too much and the risk of distress raises both the cost of debt and the cost of equity, and the average turns back up.
There is no single correct WACC. There is a defensible one, documented with assumptions someone else could challenge.
The mistakes that cost the most
The common errors are quiet but expensive: using a book cost of debt instead of today's market rate, forgetting the tax shield, using the company's beta for a project in a different industry, and changing the discount rate to force a project to clear the hurdle.
The last one is the most dangerous, because it is invisible. The rate is a fact about the market, not a dial to be turned until the answer is yes.
How to use it well
Set the rate before you see the project. Write down the assumptions. Use a range, not a point, and see whether the decision survives both ends. Then apply it consistently, so that every project in the firm competes on the same terms.
Sources and further reading