Cost of Debt

The cost of debt is a forward-looking, marginal concept: it answers what the firm would pay to raise one more unit of borrowing today. For an issuer with liquid public bonds, the natural estimate is the yield to maturity on that debt: the single discount rate that equates the present value of all remaining coupon and principal payments to the current market price. If a bond trades below par, its yield exceeds its coupon and the firm's true borrowing cost has risen; if it trades above par, the reverse holds.

When debt is thinly traded or entirely private, analysts fall back on matrix pricing, taking yields on bonds of comparable rating, maturity and sector, or on the debt-rating approach: a benchmark risk-free yield of matching maturity plus a credit spread appropriate to the issuer's rating. Both are estimates of the same market-clearing yield, not of the accounting interest actually recorded.

Interest is generally deductible for corporate tax purposes, so each unit of interest paid reduces the tax bill. The economic cost to shareholders is therefore the pre-tax yield reduced by the tax shield: multiply the market yield by one minus the marginal tax rate. A firm borrowing at a six percent yield under a twenty-five percent marginal rate carries an after-tax cost of four and a half percent.

The correct rate is the marginal rate applicable to the next unit of income, not the effective rate implied by dividing tax expense by pre-tax income, which is distorted by one-off items, deferred taxes and foreign earnings mix. The shield also disappears where it cannot be used: a firm with large loss carryforwards, or one whose deduction is capped by earnings-based interest limitation rules, effectively borrows at close to the pre-tax yield. Preferred dividends, by contrast, are typically not deductible at all.

Reported interest expense divided by average debt gives an average historical rate, which drifts away from the market yield whenever rates or spreads move. The debt footnote is the more useful disclosure: it lists instruments, coupons, maturities and, often, fair values, allowing an analyst to see the maturity wall and the yield at which upcoming refinancings will clear.

Capital structure decisions and valuation both hinge on the marginal figure. In a WACC used to discount future cash flows, the relevant input is what the firm would pay on new debt of similar seniority and duration, in the currency of the cash flows being discounted. Floating-rate borrowings reprice automatically and already reflect current conditions; long fixed-rate debt does not, which is precisely why book interest costs can understate economic borrowing costs for years after a rate cycle turns.

At Level I the expected competencies are concrete. Compute a bond's yield to maturity from price, coupon and maturity, then convert it to an after-tax cost by applying one minus the marginal tax rate. Build a synthetic yield from a government benchmark plus a rating-based spread. Recognise when a quoted yield is the wrong input: for callable or putable debt, yield to worst or an option-adjusted spread better reflects the expected cost, and for deeply distressed bonds the promised yield overstates the expected return to lenders and therefore the firm's true funding cost.

Candidates should also be able to justify why cost of debt sits below cost of equity for the same issuer (lenders rank ahead of shareholders and receive contractual payments) and how a change in leverage or rating feeds back into the spread and thus into WACC.