When a company borrows money — through debentures, term loans, or bonds — it pays interest. But here's the twist that makes Cost of Debt (Kd) unique: interest is a tax-deductible expense. So the government effectively subsidises part of the borrowing cost. That's why we always calculate Kd on an after-tax basis.
Think of it this way: Rajesh & Co. Pvt. Ltd. pays ₹12 lakh in interest annually. At a 30% tax rate, this saves ₹3.6 lakh in tax. The real cost to the company is only ₹8.4 lakh. That's the after-tax cost of debt — and that's what goes into your WACC calculation.
There are two flavours the ICAI tests you on. For irredeemable (perpetual) debt — debt that never needs to be repaid — the formula is straightforward: Kd = I(1 − t) / NP, where I = annual interest, t = tax rate, and NP = net proceeds (issue price minus flotation/underwriting costs). For redeemable debt — far more common in exams — use the approximation formula: Kd = [I(1−t) + (RV − NP)/n] / [(RV + NP)/2], where RV = redemption value and n = years to maturity. The numerator captures the after-tax interest plus the annual capital gain/loss on the debt; the denominator is the average capital invested by the lender.
Net Proceeds (NP) is the amount the company actually receives after deducting issue expenses. If debentures are issued at a discount or involve underwriting fees, NP < face value. This is a favourite exam trick — don't confuse face value with NP. Also note: if debt is issued at a premium, NP > face value, which lowers Kd. If redeemed at a premium, RV > face value, which raises Kd. Both adjustments flow through the formula automatically. This topic is asked frequently as a 5–8 mark numerical in Paper 6.