The after-tax cost of debt is the rate a borrower actually bears on its borrowings once the tax deduction for interest is taken into account, and the formula is the pre-tax borrowing rate multiplied by one minus the marginal tax rate. It is a one-line calculation whose difficulty lies entirely in its two inputs, because neither the borrowing rate nor the tax rate is simply read off a document.
After-tax cost of debt = Rd x (1 - t)
Rd the pre-tax cost of debt, the rate the borrower pays today
t the marginal tax rate that applies to the interest deductionWhere this rate is used, and why the equity leg of a funding mix carries no equivalent factor, is set out in cost of capital. What follows is how each of the two inputs is set.
Setting Rd, the Pre-Tax Rate
Three routes are in common use, and they answer slightly different questions.
The first is the market route. Where the borrower has traded debt, the yield an investor would earn holding that debt to maturity is the rate the market currently demands, which is a better reading of today’s cost than a coupon fixed when the paper was issued.
The second is the books route, and it is the one a private company without traded debt can run from its own statements. Divide interest expense for the period by the average balance of interest-bearing debt over that period. It is easy to compute and it is backward-looking by construction, so a borrower whose rates reset or whose credit has changed will get a stale answer.
The third is the synthetic route. Start from a government yield of matching maturity and add a spread appropriate to the borrower’s credit. It is an estimate, and it should be labelled as one.
Whichever route is used, the rate should carry the full cost of the borrowing rather than the stated coupon alone. Discount, premium and issue costs are spread across the life of the instrument and lift or lower the rate the borrower really pays, which is the mechanism effective interest rate works through. A loan with a stated rate of 7% and meaningful origination fees does not cost 7%.
Setting t, the Tax Rate
The rate in the formula is the marginal rate, meaning the rate that applies to the next dollar of income the interest deduction removes. It is not the effective rate reported in the financial statements, which is total tax expense over pre-tax income and is pushed around by credits, permanent differences and prior-year items that have nothing to do with the marginal value of one more deduction. The distinction, and how the reported rate is built, is covered in effective tax rate.
For a C corporation the federal piece is flat. The tax imposed is 21 percent of taxable income (26 U.S.C. 11(b)), so there is no bracket to locate and the marginal federal rate is the statutory rate.
State tax stacks on top of it, but not by simple addition, because state and local income taxes are themselves allowed as a deduction in computing federal taxable income (26 U.S.C. 164(a)(3)). Each dollar of state tax cuts the federal base, so where the state gives no deduction back for federal tax, the combined marginal rate is the federal rate plus the state rate reduced by the federal rate.
Combined t = t(fed) + t(state) x (1 - t(fed))
with a 6% state rate:
= 0.21 + 0.06 x (1 - 0.21)
= 0.21 + 0.0474 = 0.2574
Rd after tax, on a 7.5% pre-tax rate:
= 7.5% x (1 - 0.2574) = 5.57%
adding the two rates straight instead:
Combined t = 0.21 + 0.06 = 0.27
Rd after tax = 5.48%Adding the two rates straight overstates the shield, because it ignores the federal deduction the state tax itself creates. The gap is a fraction of a percentage point on the rate, and what grows with a leveraged balance sheet is the dollars it costs rather than the gap itself.
Pass-Through Entities Carry No Federal Rate at the Entity Level
This is where the formula gets applied to a business it does not fit. An S corporation is not subject to the taxes imposed by that chapter except as subchapter S itself otherwise provides (26 U.S.C. 1363(a)), and a partnership is not subject to that income tax either, which is the split federal income tax liability works through.
So at the entity level the federal rate is zero, and the entity’s after-tax cost of debt equals its pre-tax cost of debt for federal purposes. The interest deduction has not vanished, it has moved: it flows through to the owners and shields income at whatever rate each of them faces. That means a pass-through’s shield is worth different amounts to different owners, and none of those amounts belongs to the entity. If the analysis is being run at owner level, the owner’s own marginal rate is the honest input, and it should be stated as an assumption rather than presented as a company rate. The gross-up above is a corporate calculation and does not carry across to an individual, whose state and local deduction runs under its own limits, set out in SALT deduction.
When the Shield Is Worth Less Than the Formula Assumes
The multiplication assumes the deduction is available in full, in the current period. Business interest can be capped with the disallowed part carried forward, and a borrower with no taxable income has nothing for the deduction to shelter this year, so under either the rate the borrower really bears sits above the one the formula returns. Both limits are worked through in tax shield.
Common Questions
No, as long as the marginal rate sits between zero and one. The factor one minus t is then a number between zero and one, so multiplying by it can only hold the rate flat or reduce it. The two rates are equal when t is zero, which is the ordinary condition of a partnership or an S corporation at entity level, and the gap widens as the marginal rate rises.
Interest-bearing debt, measured consistently with whatever interest expense or market yield is being used. Trade payables carry no stated interest and belong in working capital rather than in a funding rate, and how creditor funding is read as a proportion of the balance sheet is a separate question held by debt to capital.
Accountably places trained offshore accountants and tax preparers inside US CPA and EA firms, and a firm can put a real block of its own recurring work through a trial task before anything depends on it.
