How to Calculate WACC: A Step-by-Step Guide

The Weighted Average Cost of Capital (WACC) is the single most important discount rate in corporate finance. Every DCF valuation, capital budgeting decision, and strategic investment hinges on it. It represents the minimum return investors require—equity holders and creditors alike. Underestimate WACC and you'll overvalue the company; overestimate and you'll reject profitable projects. Getting it right is non-negotiable.

WACC is the weighted average of a company's cost of equity and after-tax cost of debt, where the weights are the proportions of equity and debt in the capital structure. The formula is deceptively simple, but the components—especially the cost of equity—demand careful estimation.

In this guide, we'll walk through the mechanics of WACC, the pitfalls that trip up most analysts, and a complete worked example.

The WACC Build: From Inputs to Blended Discount Rate

The WACC Formula

WACC has two components: the cost of equity (Re) and the after-tax cost of debt (Rd × (1 - Tc)), weighted by their proportion in the capital structure.

WACC = (E / V) × Re + (D / V) × Rd × (1 - Tc)

Where:
  E = Market value of equity
  D = Market value of debt
  V = E + D (total firm value)
  Re = Cost of equity (calculated using CAPM)
  Rd = Cost of debt (yield to maturity on debt or weighted average coupon)
  Tc = Corporate tax rate

Breaking this down:

• (E / V) × Re: The weighted cost of equity. Equity is riskier than debt, so its cost is higher. Equity holders demand a return that compensates for this risk.
• (D / V) × Rd × (1 - Tc): The weighted after-tax cost of debt. The tax adjustment reflects that interest is deductible, reducing the true cost of debt to the company.

The sum is WACC, the overall hurdle rate for the company.

Cost of Equity (CAPM Walk-Through)

The cost of equity is calculated using the Capital Asset Pricing Model (CAPM). This is not the return you expect; it is the return investors require based on the risk they are taking.

Re = Rf + β × (Rm - Rf)

Where:
  Rf = Risk-free rate
  β = Beta (systematic risk)
  Rm - Rf = Equity Risk Premium (ERP)

The Risk-Free Rate (Rf)

The risk-free rate is the yield on a government bond with no default risk. For US companies, use the 10-year US Treasury yield. For other countries, use the government bond yield in the company's home currency.

As of early 2026, typical risk-free rates range from 3.5% to 5.0% for developed markets, depending on the yield curve and monetary policy.

A Critical Point on Timing: Use the risk-free rate on the valuation date, not a historical average. If you are building a model in May 2026, use the 10-year Treasury yield from May 2026. Stale risk-free rates are a common source of valuation errors.

Beta (β)

Beta measures how much a stock's returns move relative to the overall market. A beta of 1.0 means the stock moves exactly with the market. A beta greater than 1.0 is more volatile (riskier); a beta less than 1.0 is more stable (less risky).

Where to Find Beta: Beta is published by data providers like Bloomberg, Yahoo Finance, and CapitalIQ. However, published betas are often "levered" betas—they reflect the company's current capital structure including debt. If you are valuing a company with a different capital structure than the market assumes, you must unlever and re-lever beta.

Levering and Unlevering Beta:

Unlevered beta (or asset beta) represents the risk of the company's operations with no debt. Levered beta (or equity beta) includes the risk of the debt.

// Unlever beta (remove the effect of debt)
= Levered β / [1 + (1 - Tax Rate) × (Debt / Equity)]

// Re-lever beta (apply your target capital structure)
= Unlevered β × [1 + (1 - Tax Rate) × (Debt / Equity)]

Example: A software company has a published levered beta of 1.3, a tax rate of 25%, and a current debt-to-equity ratio of 0.5. Your target capital structure is 40% debt (D/E = 0.67).

// Unlever published beta
= 1.3 / [1 + (1 - 0.25) × 0.5]
= 1.3 / 1.375
= 0.945 (unlevered)

// Re-lever for your target capital structure
= 0.945 × [1 + (1 - 0.25) × 0.67]
= 0.945 × 1.5025
= 1.42 (re-levered)

Equity Risk Premium (ERP)

The equity risk premium is the additional return investors demand for taking the risk of owning equities instead of risk-free government bonds. It is the market's reward for systematic (non-diversifiable) risk.

There is significant debate in academia about the "correct" ERP. Some argue it should be 5%, others 6.5%, and a few advocate for up to 8%. Most practitioners use 5.5% for US companies as a middle ground. The ERP is relatively stable over long periods but can shift with market sentiment. Use the same ERP for all companies in your analysis to maintain consistency.

Putting It Together: A Cost of Equity Calculation

Suppose you are valuing a mid-cap software company. You gather the following:

• Risk-free rate (10-year Treasury): 4.5%
• Levered beta from a comparable company: 1.35
• Comparable company's debt-to-equity: 0.4
• Target company's debt-to-equity: 0.5
• Tax rate: 25%
• Equity risk premium: 5.5%

First, unlever the published beta:

Unlevered β = 1.35 / [1 + (1 - 0.25) × 0.4] = 1.35 / 1.30 = 1.038

Then, re-lever for the target capital structure:

Re = 1.038 × [1 + (1 - 0.25) × 0.5] = 1.038 × 1.375 = 1.427

Now calculate the cost of equity:

Re = 4.5% + 1.427 × 5.5% = 4.5% + 7.85% = 12.35%

The company's cost of equity is 12.35%. This is the minimum return equity investors require.

Cost of Debt (Yield to Maturity vs Coupon)

The cost of debt is the rate the company pays on its borrowings. This is not the coupon rate on the debt; it is the yield to maturity (YTM)—the market's assessment of the company's credit risk.

For Public Debt

If the company has traded bonds, use the yield to maturity of the most recent issuance or the weighted average YTM across all outstanding bonds.

Why YTM, not coupon? The coupon is fixed at issuance; the YTM changes daily with market conditions. If a company issued a bond at 4% coupon five years ago but is now considered risky, the bond trades at a discount and the YTM is 6%. Using the old 4% coupon understates the cost of debt.

For Private Debt or Loans

If the company has no public debt, estimate cost of debt from:

1. Credit rating: If the company has a credit rating (S&P, Moody's, Fitch), use the average YTM for that rating.
2. Leverage multiples: Use the company's debt-to-EBITDA or interest coverage ratio to estimate a comparable credit spread.
3. Peer analysis: Find similar private companies' loan terms (e.g., a bank loan at LIBOR + 300 bps = 4.5% + 3% = 7.5%).

Capital Weights (Book Value vs Market Value)

This is where many analysts make a critical error: using book values instead of market values.

Why Market Value?

WACC is a forward-looking metric. It represents the return investors require today. Market value reflects current expectations and risk; book value reflects historical cost and is often stale.

Example: A real estate company's land is carried on the balance sheet at £50 million (historical cost from 30 years ago). Today it is worth £200 million. Using book value in WACC would drastically overweight debt, understating the cost of capital. Market value reflects the true economic reality.

Calculating Market Values

Worked Example:

A publicly-traded software company has:

• 100 million shares outstanding
• Stock price: £45 per share
• Total debt outstanding: £2 billion (mix of bonds and loans, all recently issued at current rates)

E = 100 million × £45 = £4.5 billion
D = £2 billion
V = £4.5 billion + £2 billion = £6.5 billion

Weights:
E / V = £4.5 / £6.5 = 69.2%
D / V = £2 / £6.5 = 30.8%

The company is 69% equity-financed and 31% debt-financed.

Tax Adjustment

Interest expense is tax-deductible. When a company pays £100 in interest, it saves £25 in taxes (assuming a 25% tax rate), so the true cost of debt is £75, or 75% of the stated rate.

This is why the WACC formula includes the term (1 - Tc) applied only to debt:

After-Tax Cost of Debt = Rd × (1 - Tc)

Do not apply the tax adjustment to the cost of equity. Dividends and retained earnings are paid from after-tax cash flows, so the cost of equity implicitly includes the tax effect.

Impact on WACC

A higher tax rate lowers WACC because the tax shield on debt is larger. This is why highly profitable companies with high marginal tax rates tend to use more debt—the tax benefit is substantial.

Worked Example: SaaS Company WACC Calculation

Let's calculate WACC for a mid-market SaaS company, CloudTech Inc., using realistic assumptions.

Inputs

Step 1: Calculate Cost of Equity

First, unlever the peer's beta:

Unlevered β = 1.30 / [1 + (1 - 0.25) × 0.3]
             = 1.30 / 1.225
             = 1.061

Re-lever for the target capital structure:

Levered β = 1.061 × [1 + (1 - 0.25) × 0.67]
          = 1.061 × 1.5025
          = 1.594

Calculate the cost of equity using CAPM:

Re = 4.5% + 1.594 × 5.5%
   = 4.5% + 8.77%
   = 13.27%

Step 2: Calculate After-Tax Cost of Debt

Rd (after-tax) = 6.0% × (1 - 0.25)
               = 6.0% × 0.75
               = 4.5%

Step 3: Calculate Weights

E / V = £3,600 / £3,750 = 96.0%
D / V = £150 / £3,750 = 4.0%

Step 4: Calculate WACC

WACC = (0.96 × 13.27%) + (0.04 × 4.5%)
     = 12.74% + 0.18%
     = 12.92%

CloudTech's WACC is 12.92%. This is the discount rate to use in a DCF valuation. If the company's unlevered free cash flows grow at 8% annually, the valuation will be based on a 12.92% discount rate minus 8% growth = 4.92% perpetual growth..

Typical WACC by Industry

Always benchmark your calculated WACC against comparable companies. If your estimate is far outside the industry range, re-examine your inputs.

Common Mistakes to Avoid

1. Using Book Values Instead of Market Values

Mistake: Calculating D and E from the balance sheet.

Why it breaks: Book value is historical and irrelevant to current investor expectations. If a company's market capitalization is £5 billion but book equity is £2 billion, using £2 billion will overweight debt and overstate WACC.

Fix: Always use market values. Equity = shares outstanding × current price. Debt = market value of debt obligations, or book value if recently issued at market rates.

2. Confusing Levered and Unlevered Beta

Mistake: Using a peer's levered beta without adjusting for differences in capital structure.

Why it breaks: If the peer has 20% debt and your target company has 50% debt, the levered betas are different even if the underlying business risk is the same. Your cost of equity will be wrong.

Fix: Always unlever the peer's published beta, then re-lever for your target capital structure. Use the formula:

Unlevered β = Levered β / [1 + (1 - Tax Rate) × (D/E)]

3. Using a Stale Risk-Free Rate

Mistake: Using a 2024 Treasury yield in a 2026 valuation.

Why it breaks: Interest rates change over time. Using yesterday's rate introduces bias. A 2% error in the risk-free rate can easily lead to a 0.5%–1% error in WACC, which compounds into significant valuation errors.

Fix: Always use the current (valuation date) yield on the 10-year government bond. Update it weekly if you are running multiple valuations.

4. Forgetting the Tax Adjustment on Debt

Mistake: Using the pre-tax cost of debt in the WACC formula.

Why it breaks: Interest is tax-deductible. If you ignore the tax benefit, WACC will be overstated and the company undervalued.

Fix: Always apply (1 - Tax Rate) only to the cost of debt:

WACC = (E/V) × Re + (D/V) × Rd × (1 - Tc)
           ↑ no tax adjustment
                        ↑ tax adjustment here

5. Mismatching the Equity Risk Premium

Mistake: Using a 3% ERP because "that's what worked in my last model," without checking current market conditions.

Why it breaks: The equity risk premium reflects market expectations and can shift (though slowly). Using an outdated ERP will lead to a systematically wrong cost of equity.

Fix: Document your ERP assumption and use consistent values across all companies in your analysis. For US companies, 5.5% is a widely-accepted middle ground. Revisit this assumption annually.

Sensitivity and Scenario Analysis

WACC is an estimate, and all estimates are sensitive to assumptions. Build a sensitivity table to understand how changes in key inputs affect valuation.

In Excel, create a two-way table with:

• One axis: Risk-free rate (±0.5%)
• Other axis: Equity risk premium (±0.5%)

This quickly shows which assumptions matter most. If WACC changes by 1% when the risk-free rate moves 0.5%, that input is highly sensitive and warrants careful research.

For advanced analysis, use a three-scenario model:

Run your DCF valuation using all three WACC figures. The resulting valuation range reflects the sensitivity of value to cost of capital assumptions.