What is WACC and how do you calculate it?

WACC comes up in almost every valuation conversation, and it is rarely asked just once. They start with the formula, then pull on every part of it.

The name is a mouthful — weighted average cost of capital — but the idea behind it is simple.

A company needs money to operate. It gets that money from lenders and from shareholders. Both groups expect something in return. WACC is the blended return the company has to earn to keep them both happy.

If a business earns less than its WACC, it is destroying value. If it earns more, it is creating value. That is why this number matters so much.

Start with what "capital" means

Capital is just money that has been put into a business so it can operate.

It comes from two places, and they behave very differently.

Debt is borrowed money. The company promises to pay interest and repay the loan. Lenders do not own anything, and they get paid before anyone else. Because their position is safer, they accept a lower return.

Equity is money from shareholders. They own a piece of the company. If it does badly, they may get nothing. Because their position is riskier, they demand a higher return.

So debt is cheaper than equity. Hold on to that, because it comes up later.

The formula

WACC = (E/V × cost of equity) + (D/V × cost of debt × (1 − tax rate))

Where E is the value of equity, D is the value of debt, and V is the two added together.

Do not be put off by the letters. It is just a weighted average. If a company is funded 70 percent by equity and 30 percent by debt, you take 70 percent of the cost of equity and 30 percent of the cost of debt, and add them.

The only unusual part is that last bit — multiplying the cost of debt by one minus the tax rate. We will get to why.

Now take each piece in turn.

Part 1: The cost of equity

This is the hardest piece, because shareholders do not send an invoice. Nobody tells you what return they expect. You have to estimate it.

The standard way is a model called CAPM, the capital asset pricing model.

Cost of equity = risk-free rate + (beta × equity risk premium)

Three ingredients. Take them one at a time.

The risk-free rate

This is what you could earn with essentially no risk at all. In practice, people use the interest rate on a long-dated government bond — usually the 10-year or 20-year — from a stable government.

The idea is that a national government is about as close to certain to pay you back as anything gets. So this is your baseline. Any investment riskier than that has to offer more.

Match the currency. If you are valuing a European business in euros, use a European government bond, not an American one.

The equity risk premium

Stocks are riskier than government bonds. So investors demand extra return for holding them.

That extra amount is the equity risk premium. It is the difference between what the whole stock market returns and what a government bond returns. Historically it has sat somewhere in the mid single digits — around 5 to 6 percent.

You do not calculate this yourself. You take it from a published source.

Beta

Beta measures how much a particular stock moves compared to the market as a whole.

• A beta of 1 means the stock moves in line with the market.
• A beta of 1.5 means it swings 50 percent more than the market, up and down.
• A beta of 0.6 means it moves less than the market.

A supermarket has a low beta. People buy food in a recession. A luxury car maker has a high beta, because people stop buying expensive cars when times are hard.

Higher beta means more risk, which means shareholders demand a higher return.

The part interviewers actually test: unlevering and relevering

Here is where most candidates get caught.

You cannot just take the beta you see quoted for a company and use it. That number reflects two different things mixed together:

1. How risky the underlying business is.
2. How much debt the company has borrowed.

Debt makes a company's share price swing more, because interest has to be paid whether business is good or bad. So a company with lots of debt shows a higher beta, even if the business itself is no riskier.

You only want the business risk. So you do this:

Step 1. Find several similar companies and look up each one's beta. These are "levered" betas, because they include the effect of debt.

Step 2. Strip out the debt effect from each one. This is called unlevering. The formula is:

Unlevered beta = levered beta ÷ [1 + (1 − tax rate) × (debt ÷ equity)]

Step 3. Take the middle value of those unlevered betas.

Step 4. Add back the debt level of the company you are actually valuing. This is called relevering, and it uses the same formula in reverse.

Now you have a beta that reflects the right business risk and the right amount of debt.

If you can explain why this is necessary, in plain words, you are ahead of most candidates.

Part 2: The cost of debt

This one is easier, because lenders do tell you what they charge.

The cost of debt is what the company would pay to borrow today — not what it agreed to pay years ago.

If a company issued a bond five years ago at 4 percent, but similar companies now borrow at 7 percent, then 7 percent is the real cost of debt. The old rate is history.

You find it by looking at the yield on the company's existing bonds, or by estimating from its credit rating.

Why you multiply by (1 − tax rate)

Interest payments are tax deductible. That means they reduce the profit you pay tax on.

Here is what that means in money. Say a company pays $100 of interest, and the tax rate is 25 percent. That $100 of interest reduces its taxable profit by $100, which saves it $25 in tax.

So the interest really cost $75, not $100. The government effectively paid the other $25.

That is why you multiply by one minus the tax rate. It is called the tax shield, and it is a real advantage of borrowing.

Part 3: The weights

Now you decide how much of each to use.

Two rules.

Use market values, not book values. Book equity is an accounting figure from the past and has nothing to do with what shareholders actually expect today. Use the market capitalisation — share price times shares.

Use the target capital structure. If a company is temporarily carrying unusual debt levels, use the level it plans to run at over time, not today's snapshot.

Why WACC matters

WACC is the rate you use to discount unlevered free cash flow in a DCF.

That pairing is not optional. Unlevered cash flow is measured before interest, so it belongs to lenders and shareholders together — and WACC is the blended cost of both groups. They match.

If you use cash flow that is already after interest, you must use the cost of equity instead. Using WACC there counts the benefit of debt twice.

The trap question

Almost every interviewer eventually asks some version of:

"Debt is cheaper than equity. So should companies just borrow as much as possible?"

The answer is no, and here is why.

Adding a bit of debt does lower WACC, because debt is cheaper and comes with a tax shield.

But as debt grows, the company becomes riskier. Shareholders now sit behind a bigger pile of lenders, so they demand a higher return — the cost of equity rises. Lenders also start worrying about getting paid, so the cost of debt rises too.

Push far enough and the company risks not being able to pay at all, which brings its own costs.

So the relationship is a curve, not a straight line. WACC falls, reaches a low point, then rises. Companies aim for somewhere near that low point.

Saying it out loud

"WACC is the blended return a company needs to earn to satisfy everyone who funded it, weighted by how much each group provided.

The cost of equity comes from CAPM: the risk-free rate, plus beta times the equity risk premium. For beta you take comparable companies, unlever each one to remove the effect of their debt, take the median, then relever at your company's target capital structure.

The cost of debt is what the company would pay to borrow today, multiplied by one minus the tax rate, because interest is tax deductible.

You weight the two using market values, not book values.

And WACC is what you use to discount unlevered free cash flow, because that cash belongs to lenders and shareholders alike."

What the interviewer is really checking

Do the rate and the cash flow match?

This is the single biggest thing. Unlevered cash flow goes with WACC. Levered cash flow goes with the cost of equity. Getting this wrong is the clearest possible sign you have never built a model.

Is beta a definition or a process?

"Beta measures volatility relative to the market" is a textbook line. Where you got it, why you unlevered it, and what capital structure you relevered at — that is the answer.

Do you understand the tax shield?

Be ready to explain in plain words why interest being tax deductible makes debt cheaper than the headline rate suggests.

Do you know it is an estimate?

WACC is one of the two assumptions a DCF is most sensitive to. Strong candidates say so, and mention that the output is shown as a range rather than a number.

Will you fall for the debt trap?

"Debt is cheaper, so why not use only debt?" is asked constantly. Handle it well and the technical section usually moves on.

Common mistakes

Using book values for the weights.

Always market values. Book equity is an accounting leftover and bears no relationship to what shareholders expect.

Skipping the unlevering step.

Taking a comparable company's beta straight off the screen imports their debt levels into your company's discount rate. This is the mistake interviewers are specifically listening for.

Forgetting the (1 − tax rate) on the cost of debt.

Small term, very visible when it goes missing.

Using the old coupon rate as the cost of debt.

What a company agreed to pay years ago is not what it costs to borrow now. Use today's rate.

Saying more debt always lowers WACC.

It lowers it up to a point, then rising risk pushes both the cost of equity and the cost of debt up. The relationship is a curve with a low point.

Calculating WACC to three decimal places.

Every input is an estimate. Presenting 8.437 percent implies a precision that does not exist, and interviewers notice.

Using terms you cannot explain.

If you say CAPM, be ready to unpack it. If you say equity risk premium, be ready to say what it is. Using words as decoration is worse than using simpler ones.