WACC sits at the centre of most finance assignments. It is the discount rate used for NPV, the hurdle rate for IRR, and the benchmark used to judge whether a company is creating or destroying value. It is also where a lot of marks disappear quietly, because the formula itself is easy — almost everyone gets that right — and the marks are hiding in the inputs that feed it.
This guide walks you through the WACC formula, shows you how to derive the cost of equity using CAPM and the cost of debt the correct way (after tax), explains why market values are preferred over book values for the weights, and runs the whole calculation end to end on one company. By the end you will know not just how to produce a WACC figure, but how to defend each input to a marker — which is the part of the question that actually scores.
What WACC Actually Represents
WACC — the Weighted Average Cost of Capital — is the blended cost of all the capital a company uses to fund itself. A company is typically financed by a mix of equity (shareholders' funds) and debt (loans, bonds). Each has a cost: equity holders expect a return reflecting the risk they take, and debt holders charge interest. WACC takes both costs, weights them by how much of each type of capital the company uses, and produces one combined rate.
Why your assignment cares: WACC is the minimum return a company needs to earn on its investments to satisfy everyone funding it. If a project's return exceeds WACC, value is created. If it does not, value is destroyed. That is why WACC is the standard discount rate for investment appraisal — see our NPV and IRR guide for how it feeds into project decisions.
One assumption to flag: WACC is most appropriate for projects with risk similar to the company's existing business. For a project significantly more or less risky than the firm as a whole, a risk-adjusted rate is more appropriate. Strong assignments mention this limitation when applying WACC to a specific project.
The WACC Formula
In its standard form for a company financed by equity and debt only:
Where:
E = market value of equity
D = market value of debt
V = total capital (E + D)
Re = cost of equity
Rd = cost of debt (pre-tax)
T = corporate tax rate
Three things to internalise before you start substituting numbers:
- The weights must sum to 1. E/V plus D/V equals 100% by construction. If they do not, you have made an error somewhere.
- The cost of debt is multiplied by (1 − T). Interest is tax-deductible, so the real cost of debt to the company is the after-tax figure. Skipping the (1 − T) overstates WACC every time.
- Market values, not book values. Use the market value of equity (share price × shares outstanding) and the market value of debt where possible. Book values are accepted only when market values are unavailable.
Step 1 — Calculate the Cost of Equity (CAPM)
The Capital Asset Pricing Model (CAPM) is the standard way to estimate the cost of equity in assignments. It states that the return required by equity investors equals the risk-free rate plus a premium for taking on the systematic risk of holding the company's shares.
Where:
Rf = risk-free rate (usually a government bond yield)
β = beta (the share's sensitivity to market movements)
Rm = expected market return
(Rm − Rf) = equity risk premium
In assignment terms: Rf usually comes from a 10-year government bond yield, beta is given in the brief or sourced from a financial database, and the equity risk premium is either provided or taken from a standard published estimate. State your source for each input — that is what makes the calculation defensible.
Step 2 — Calculate the Cost of Debt (After Tax)
The cost of debt is the rate of return debt holders require. In an assignment, you will usually be given the pre-tax cost of debt directly — as the interest rate on the company's bonds or its average borrowing rate. From there, the only step is to convert it to an after-tax figure by multiplying by (1 − T).
For example, if the pre-tax cost of debt is 6% and the corporate tax rate is 25%, the after-tax cost is 6% × 0.75 = 4.5%. This adjustment exists because interest payments reduce taxable profit; the tax saving is a real cash benefit to the company.
A Worked Example — Atlas Plc
Let's calculate WACC end to end for a fictional listed company, Atlas Plc. The figures below are constructed for demonstration — the method is identical for any real company data you are given.
Market value of debt: £60 million
Beta: 1.20
Risk-free rate (10-yr gilt): 4.0%
Equity risk premium: 5.0%
Pre-tax cost of debt: 6.0%
Corporate tax rate: 25%
Step A — Market Values and Weights
D = £60m
V = E + D = £260m
E/V = 200 ÷ 260 = 0.769 (76.9%)
D/V = 60 ÷ 260 = 0.231 (23.1%)
Weights sum to 100%, as they must.
Step B — Cost of Equity via CAPM
= 4.0% + 1.20 × 5.0%
= 4.0% + 6.0%
= 10.0%
Step C — After-Tax Cost of Debt
= 6.0% × 0.75
= 4.5%
Step D — Combine into WACC
= (0.769 × 10.0%) + (0.231 × 4.5%)
= 7.69% + 1.04%
= 8.73%
Interpretation: Atlas Plc has a weighted average cost of capital of approximately 8.73%. Any investment expected to return more than this creates shareholder value; any return below this destroys it. This is the rate that would be used as the discount rate for evaluating Atlas's typical investment projects.
Try It Yourself — Interactive WACC Calculator
Now run your own numbers. Enter your company's figures below and the calculator works through exactly the same steps as the Atlas Plc example above — deriving the cost of equity via CAPM if you need it, applying the after-tax adjustment to debt, weighting each component, and showing the full working you can paste into your assignment. It also flags the common input mistakes as you go. Prefer a clean, full-page version you can bookmark? Use our WACC Calculator tool.
📊 Your Inputs
Use market values for equity & debt where possible — hover the ? on each field for guidance.
Stuck deriving the inputs, or running out of time?
Our finance specialists build the full WACC calculation — CAPM, cost of debt, weighting, interpretation — written to your university's marking criteria and your deadline.
2:2 vs First: Interpreting the Result
Most students stop at the WACC figure. The marks beyond a pass come from what you do with it — and from showing critical awareness of the assumptions feeding it.
"The company's WACC is 8.73%. This is the rate the company should use as its cost of capital. Any project with a return above 8.73% should be accepted."
"Atlas Plc's WACC of 8.73% provides a defensible discount rate for valuation and project appraisal, but is sensitive to the inputs chosen — particularly the beta of 1.20 and the equity risk premium of 5%, both of which can vary by source and period. The figure also assumes the current capital structure is the target structure, which may not hold if Atlas plans to alter its gearing. For projects materially riskier than the firm's core business, a risk-adjusted rate would be more appropriate."
The first-class version is not technically harder — it is showing the marker that the student understands WACC is an estimate built on debatable assumptions, not a precise number handed down from the textbook. That habit of qualification is what every finance rubric rewards.
Five Mistakes That Cost Students Marks
Forgetting the (1 − T) on the cost of debt. Using the pre-tax rate directly overstates WACC and breaks the link to how debt actually costs the firm.Fix: Always multiply the pre-tax cost of debt by (1 − T). State the tax rate you used.
Mixing book and market values. Using the book value of equity but the market value of debt (or vice versa) makes the weights internally inconsistent.Fix: Use market values for both E and D. If you have to use book values for debt because market values are unavailable, say so explicitly.
Reporting CAPM inputs with no sources. A risk-free rate, beta, and equity risk premium with no stated source read as invented.Fix: Cite each input — gilt yield from the central bank, beta from a database such as Bloomberg or FAME, equity risk premium from a recognised published estimate.
Treating the WACC figure as precise. WACC is built on estimates — beta, premium, market values, future tax rates — all of which can move.Fix: Add a sensitivity comment showing how WACC changes if beta or the equity risk premium is, say, 1 point higher or lower.
Using WACC for a project with very different risk to the company. WACC reflects the average risk of the firm — applying it to a much riskier or safer project distorts the appraisal.Fix: Acknowledge this limitation. For materially different risk profiles, mention the case for a project-specific discount rate.
Frequently Asked Questions
What does WACC stand for and what does it measure?
Why is the cost of debt adjusted for tax in WACC?
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