Two companies with identical businesses and identical operating profits can have very different risk profiles for their shareholders, simply because of how they are financed. Capital structure — the split between borrowed money (debt) and owner money (equity) — is one of the most consequential decisions a management team makes, and one of the most important things for an investor to understand.
What is capital structure?
Capital structure describes the combination of debt and equity a company uses to fund its assets and operations. Equity is money invested by shareholders — they own a piece of the business and share in its profits, but their investment is permanent and pays no fixed interest. Debt is borrowed money — from banks, bond investors, or other lenders — that must be repaid with interest on a fixed schedule regardless of how the business performs.
The key ratio summarising a company's capital structure is the debt-to-equity ratio (D/E): total debt divided by total shareholders equity. A D/E of 1.0 means the company uses equal amounts of debt and equity. A D/E of 3.0 means for every £1 of equity, it has borrowed £3.
How debt amplifies risk: financial leverage
The mechanism by which debt increases shareholder risk is called financial leverage. Consider two companies, each owning £100 million of assets generating £10 million of operating profit annually. Company A has no debt — all £100 million is funded by equity. Company B has £80 million of debt at 5% interest (£4 million per year) and only £20 million of equity.
Both companies earn £10 million of operating profit. But Company A delivers £10 million to its equity holders (£100m invested), a 10% return. Company B delivers £10 million operating profit minus £4 million interest = £6 million to its equity holders (only £20m invested), a 30% return on equity. This is the upside of leverage — it dramatically boosts returns on equity in good times.
Now consider what happens if operating profit falls 50% to £5 million. Company A still delivers £5 million to equity holders — painful, but manageable. Company B earns £5 million but must still pay £4 million of interest, leaving only £1 million for equity holders — a 95% collapse in equity returns. And if profits fall another 20% to £4 million, Company B cannot even cover its interest payments — it faces financial distress.
Business risk vs financial risk
It is important to separate business risk — the inherent uncertainty in the company's operating profits — from financial risk — the additional risk imposed by having debt. A stable utility with highly predictable cash flows can safely carry a lot of debt because its business risk is low; the probability of profits falling so far that debt cannot be serviced is minimal. A cyclical manufacturer whose revenues can halve in a recession should carry much less debt; adding financial risk on top of already high business risk is dangerous.
This is why different industries have very different typical capital structures. Airlines and utility companies regularly run high leverage; pharmaceutical companies and software businesses often hold net cash, partly because their intangible assets (patents, code) cannot be pledged as collateral to lenders in the same way physical assets can.
Debt in a capital structure is like a lever under a boulder — it lets you move more weight than you could unaided, and in good conditions that is exactly what you want. But if the ground shifts beneath the lever, the amplification works in reverse: the boulder can crush you faster than if you had been pushing it by hand.
The tax shield: why debt is not always bad
One reason companies use debt is the interest tax shield. Interest payments on debt are tax-deductible, whereas dividends paid to equity holders are not. This means a company paying 5% interest on debt is effectively borrowing at a lower after-tax cost — at a 25% corporation tax rate, the effective after-tax cost of 5% debt is only 3.75%. This tax advantage of debt is one of the central arguments in the Modigliani-Miller theory of capital structure, which suggests companies should, in theory, use as much debt as possible — a conclusion that clearly breaks down once bankruptcy risk is added to the equation.