What it means
Capital structure is the way a company funds itself, using a blend of debt (borrowed money) and equity (money from owners). Each source has a cost.
Debt usually costs less than equity because lenders are paid first and interest is generally tax deductible, while shareholders accept more risk and expect higher returns. The traditional view sits between two extremes.
At low levels of debt, replacing some expensive equity with cheaper debt lowers the weighted average cost of capital, usually shortened to WACC. As debt grows, however, lenders and shareholders see more risk of financial distress and demand higher returns, and eventually the rising costs outweigh the benefit of cheap debt.
This produces a U-shaped WACC curve with a lowest point at the optimal capital structure. At that point the cost of funding is at its minimum and the value of the firm, which is the present value of future cash flows discounted at WACC, is at its maximum.
Finance teams look for that balance when setting targets for debt levels. The approach contrasts with the Modigliani and Miller theory, which, under strict assumptions such as no taxes and no bankruptcy costs, says the mix does not affect value.
When taxes and distress costs are added, their model also points to a trade-off, so modern thinking has moved closer to the traditional view. In practice, the optimum differs by industry, because stable cash flows can support more debt than volatile ones.
Practical constraints matter too. Lenders set covenants, which are conditions in loan agreements, and rating agencies look at ratios such as debt to earnings, so a company cannot freely choose any level of debt.
The right mix also depends on the stage of the business, with young companies relying more on equity and mature, cash-generating ones using more debt. Because the optimum cannot be observed precisely, managers use judgement, comparisons with peers and scenario analysis.
The traditional view is best treated as a guide to the trade-off, not as a formula that delivers a single exact answer.
In practice
Real-world examples.
Example
A mature packaging manufacturer with steady cash flows replaces part of its equity with long-term loans. Its WACC falls from 10.5% to 9.5% as it adds debt. The CFO stops at the point where lenders begin to demand higher interest.
Example
A fast-growing software start-up with uncertain income keeps debt low and relies on equity. The founders note that high debt would raise the risk of distress in a downturn. They plan to add moderate debt only after cash flows become predictable.
Example
A property company is asked by its bank to keep debt below 60% of asset value. The finance team models WACC at several debt levels and finds the lowest cost near 50%. It sets 50% as its working target, leaving a safety margin below the covenant.
Formula
Calculation
WACC = (E / V x cost of equity) + (D / V x cost of debt x (1 - tax rate))
Suppose a company has equity of $600,000 and debt of $400,000, so total capital V = $1,000,000. The cost of equity is 12%, the cost of debt is 6% and the tax rate is 25%. Equity share = 0.60 x 12% = 7.2%. Debt share = 0.40 x 6% x (1 - 0.25) = 0.40 x 4.5% = 1.8%. WACC = 7.2% + 1.8% = 9.0%.Case study
Seen in the real world.
Stonebridge Hotels is a fictional hotel group used for illustration. Its finance director modelled WACC at debt levels from 0% to 70% of capital. The model showed the cost falling from 11% with no debt to a low of 8.8% at 40% debt, then rising to 10% at 70% because lenders priced in more risk.
The board agreed on a target range of 35% to 45% debt, with a lower limit during periods of weak bookings. In this illustrative story, the group refinanced two loans to reach the target and lowered its cost of capital by about one percentage point. The lesson is that the benefit of debt is real but only up to a point.
Watch out
Common mistakes.
- Assuming more debt is always cheaper. After a certain level, the extra risk raises the cost of both debt and equity.
- Using book values for the weights. Market values are normally preferred because they reflect what investors would require today.
- Treating the optimal structure as fixed. It changes with interest rates, tax rules, business risk and the company's stage.
Questions
People also ask.
What is the difference between the traditional approach and Modigliani-Miller?
The traditional view says an optimal mix exists, while the original Modigliani-Miller theory says the mix does not matter under perfect conditions.
Why is debt cheaper than equity?
Lenders have a prior claim on cash flows and often security over assets, and interest is generally tax deductible, whereas shareholders bear residual risk.
How do companies find their optimal structure?
They model WACC at different debt levels, compare peers, test scenarios and take account of lender covenants and ratings.
From the founder's library
Take it further with the book.
Build your financial confidence beyond this definition. Shihan's full-length guide, Accounting Fundamentals, takes the same plain-English approach and turns it into a complete, practical playbook for non-finance managers, business owners and students - with chapter-end quiz answers and presentation slides included.
25% off with code MMHQ25, applied at checkout. Priced in USD - checkout may show the equivalent in your local currency.
View the book and save 25%