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WACC Explained: How Companies Calculate Their Cost of Capital

Desk Dojo··5 min read

Every company funds itself with some mix of debt and equity, and each source comes at a cost. Weighted average cost of capital (WACC) blends those costs into a single rate based on the proportion of each in the company's funding. That rate becomes the benchmark for evaluating new investments.

Key takeaway: WACC is the minimum return a company must earn on its existing assets to satisfy both its lenders and shareholders.

What WACC Represents

Raising money has a price. Equity investors expect returns through dividends and stock price appreciation, while lenders expect interest payments. WACC captures both costs in a single rate, weighted by how much of the company's capital comes from each source.

If a project earns more than WACC, it creates value for shareholders. If it earns less, it destroys value. That is why WACC shows up as the discount rate in net present value (NPV) calculations and discounted cash flow (DCF) models.

The WACC Formula

WACC = (E/V x Re) + (D/V x Rd x (1 - T))
  • E: Market value of equity
  • D: Market value of debt
  • V: Total capital (E + D)
  • Re: Cost of equity
  • Rd: Cost of debt
  • T: Corporate tax rate

The term (1 - T) is the tax shield. Interest payments on debt are tax-deductible, so the effective cost of debt is lower than the stated interest rate. Equity has no such benefit.

Cost of Equity

The most common way to estimate the cost of equity is the Capital Asset Pricing Model (CAPM):

Re = Rf + B x (Rm - Rf)
  • Rf: Risk-free rate (typically the yield on government bonds)
  • B (beta): A measure of how volatile the stock is relative to the market
  • Rm - Rf: The equity risk premium, the extra return investors expect for holding stocks over risk-free assets

A company with a beta above 1 is more volatile than the market and will have a higher cost of equity. A beta below 1 means lower volatility and a lower required return.

Cost of Debt

The cost of debt is the interest rate a company pays on its borrowings, adjusted for the tax benefit:

After-tax cost of debt = Rd x (1 - T)

If a company borrows at 6% and its tax rate is 25%, the after-tax cost is 4.5%. The government effectively subsidizes part of the interest expense through the tax deduction.

WACC in Action

Consider a company with the following capital structure:

Component Market Value Weight Cost After-Tax Cost
Equity $600M 60% 10% 10%
Debt $400M 40% 6% 4.5%
Total $1B

The tax rate is 25%. Plugging into the formula:

WACC = (0.60 x 10%) + (0.40 x 6% x (1 - 0.25))
WACC = 6.0% + 1.8%
WACC = 7.8%

This company needs to earn at least 7.8% on new investments to cover its cost of capital. Any project with an expected return above 7.8% adds value. Anything below it does not.

How Capital Structure Affects WACC

Because debt is cheaper than equity after the tax shield, adding more debt lowers WACC. But only up to a point. Too much debt increases the risk of financial distress, which pushes both the cost of debt and cost of equity higher.

Here is how WACC shifts as the debt ratio changes, holding the base costs constant:

Debt Weight Equity Weight WACC
0% 100% 10.0%
20% 80% 8.9%
40% 60% 7.8%
60% 40% 6.7%
80% 20% 5.6%

In practice, the numbers do not stay this clean. As leverage rises, lenders charge higher rates and equity investors demand more return to compensate for the added risk. Somewhere in between sits the mix that minimizes WACC, and finding it is one of the central questions in corporate finance.

Why WACC Matters

WACC connects financing decisions to investment decisions:

  • Capital budgeting: WACC serves as the discount rate in NPV analysis. A positive NPV at the company's WACC means the project earns more than its cost of capital.
  • Valuation: DCF models discount a company's projected free cash flows at WACC to arrive at enterprise value.
  • Performance benchmarks: If a business unit earns less than WACC, it is not covering its cost of capital, even if it shows a profit on paper.

Conclusion

WACC gives you one number that reflects what it costs a company to fund itself. It depends on the mix of debt and equity, the cost of each, and the tax rate. Once you are comfortable with this formula, you have the discount rate behind most corporate valuation and capital budgeting work.

For the fundamentals behind discounting and present value, see our guide on time value of money.

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