The mechanics
A company with £100m of EBIT and 25% corporate tax rate pays £25m in tax, leaving £75m of after-tax earnings. If that same company carries £200m of debt at 6%, it pays £12m of interest annually. Taxable income falls to £88m, and tax falls to £22m — a £3m annual tax saving. This £3m is the annual tax shield. Discounted as a perpetuity at the cost of debt (6%), the present value of the tax shield is £12m x 25% / 6% = £50m — adding £50m to firm value purely through the capital structure decision. This is the core insight of Modigliani-Miller with taxes: levered firm value = unlevered firm value + PV(tax shield).
APV — adjusted present value
The tax shield is formally incorporated through the Adjusted Present Value (APV) methodology, which separates the value of operating cash flows from the value of financing effects. APV = NPV of unlevered cash flows + PV of tax shields (+ PV of other financing effects, such as subsidised debt or distress costs). APV is particularly useful for LBO and project finance analysis where the debt schedule changes significantly over time — making WACC (which assumes constant leverage) less appropriate. In an APV, you discount operating cash flows at the unlevered cost of equity (removing the benefit of leverage), then add back the tax shield value explicitly.
The limits of the tax shield
The tax shield argument implies maximising debt maximises firm value — but real-world constraints intervene. Financial distress costs rise with leverage: as debt increases, the probability of default rises, and the expected costs of distress (legal fees, management distraction, supplier and customer losses of confidence) offset the tax shield benefit. The optimal capital structure lies where the marginal benefit of the tax shield equals the marginal cost of financial distress — the "trade-off theory" of capital structure. Additionally, the 2017 US Tax Cuts and Jobs Act introduced a 30% of EBITDA cap on interest deductibility, partially blunting the US tax shield advantage for highly leveraged firms.
“The tax shield is not a free lunch — it is a government subsidy for debt. Understanding where that subsidy ends and distress costs begin is the essence of capital structure theory.”
What this means for you
The tax shield is fundamental to understanding why private equity firms use high leverage in LBOs: they are partly buying a company and partly arbitraging the tax system. A company acquired with 60% debt generates significant tax shield value that flows to equity holders. For corporate finance practitioners, APV and the tax shield concept are essential tools for valuing leveraged transactions, project finance deals, and any situation where the financing mix changes materially over the projection period. For equity investors, high leverage companies trade at a premium to the unlevered equivalent — but only if operating cash flows are stable enough to service the debt without triggering distress costs that exceed the tax benefit.