Capital structure

A firm can fund itself by borrowing or by selling ownership, and the mix it chooses, its capital structure, shapes its risk, its cost, and its value.

Capital structure is the mix of debt and equity that a firm uses to finance its operations and growth. The proportions of borrowed money and owners' capital determine the firm's financial risk and its cost of capital, and the question of whether there is an optimal mix is one of the central debates in corporate finance.

Debt versus equity

The two basic sources of finance differ fundamentally. Debt is borrowed money that must be repaid with interest, ranks ahead of equity, and is usually cheaper, partly because interest is often tax-deductible and partly because lenders bear less risk. Equity is the owners' capital, which carries no obligation to repay but demands a higher return because shareholders bear the residual risk. The capital structure is the balance struck between these, between cheaper but obligating debt and costlier but more flexible equity.

The promise and peril of leverage

Because debt is cheaper, using more of it can lower a firm's overall cost of capital and magnify returns to shareholders, the appeal of leverage. But debt also brings fixed obligations that must be met regardless of how the business performs, raising the risk of financial distress and bankruptcy if results disappoint. More debt amplifies gains in good times and losses in bad, and beyond some point the rising risk of distress makes both debt and equity more expensive, offsetting the benefit. Capital structure is thus a trade-off between the cheapness of debt and the danger it brings.

The search for an optimum

Whether an optimal capital structure exists is a famous question. The Modigliani-Miller theorems showed that, under idealised assumptions with no taxes or distress costs, capital structure would not affect a firm's value at all, the mix would be irrelevant. Relaxing those assumptions, the tax advantage of debt and the costs of financial distress reintroduce a trade-off, implying an optimal balance that maximises value by exploiting debt's tax benefit without courting too much distress. In practice, locating this optimum is imprecise, and firms also weigh flexibility, signalling, and conditions.

Capital structure is the foundational financing decision, the choice of how much to borrow and how much to raise from owners, with consequences for risk, cost, and value. Its central tension, that debt is cheap but dangerous, frames a debate running from the elegant irrelevance of Modigliani and Miller to the practical search for a value-maximising balance, making the mix of debt and equity one of the enduring puzzles a firm must navigate.