About the after-tax cost of debt
The after-tax cost of debt is what borrowing actually costs you once any tax deduction on the interest is applied. In US corporate finance, it is the most important input into the WACC (Weighted Average Cost of Capital) calculation: a 6% bond at a 21% corporate tax rate has an after-tax cost of 4.74%. The same logic applies to personal borrowing whenever the interest is tax-deductible: home mortgage interest in the US, Section 24(b) home loan interest in India, business interest globally, and investment interest in countries with negative-gearing rules like Australia.
Why this matters: comparing a 6% mortgage to a 7% expected market return looks like a wash. But if the mortgage rate is effectively 4.08% after deduction, the opportunity cost of paying it down is much higher than the headline suggests. Most prepay-vs-invest decisions hinge on this single number.
How the math works
- Pre-tax rate: the nominal interest you pay (e.g. 6% mortgage rate).
- Marginal tax rate: the rate on your last dollar of income (US 2026: 10/12/22/24/32/35/37%). The deduction reduces taxable income, so each dollar of interest deducted saves you that fraction of a dollar in tax.
- Deductible fraction: what share of your interest is actually deductible after caps. US mortgage on acquisition debt > $750K is partly capped; student loan capped at $2,500 deduction; UK owner-occupier is 0%; India self-occupied capped at Rs 2 lakh.
- Tax shield: dollar value of the deduction = annual interest x deductible fraction x marginal rate. This is real cash flow back in your pocket via lower tax.
- Equivalent hurdle: the lowest after-tax return your invested cash needs to beat for paying down debt to be the wrong choice. Same number as the after-tax rate itself.
Country deductibility rules at a glance
| Country | Mortgage (owner-occupier) | Student loan | Investment / buy-to-let | Business interest |
|---|---|---|---|---|
| US | Yes, on acquisition debt up to $750K (TCJA cap). Must itemize. | Yes, up to $2,500 above-the-line (phase-out $85-100K single, $170-200K MFJ). | Yes, against rental income; passive loss rules apply. | Yes, subject to Section 163(j) 30% adjusted EBITDA cap (post-2022 rule). |
| UK | No deduction since 2017 phase-out completed in 2020. | No. | Basic-rate 20% tax credit only (Section 24 Finance Act 2015). | Yes, ordinary trading expense. |
| Canada | No, owner-occupier interest is not deductible. | Yes, non-refundable 15% federal credit (CRA Line 31900). | Yes, when borrowed to earn income. Smith Maneuver converts owner-occupier to investment loan over time. | Yes, ordinary CRA T2125 expense. |
| India | Old regime: Section 24(b) up to Rs 2 lakh (self-occupied), unlimited (let-out). New regime: only for let-out. | Section 80E: unlimited interest for 8 years. | Let-out: unlimited interest under Section 24(b) old regime. | Yes, full deduction as business expense. |
| Australia | No for owner-occupier. | No (HECS/HELP is not interest-bearing in the deductible sense). | Yes, negative gearing against any income at marginal rate. | Yes, ordinary business expense. |
Same 6% debt, five countries, 30% bracket
How the same nominal interest rate translates into very different after-tax costs depending on country rules. Assumes deductibility applies in full where allowed.
| Country | Pre-tax rate | Deductible? | After-tax rate | Tax shield on $400K |
|---|
Sensitivity: marginal-rate scenarios
How the after-tax cost moves as your bracket changes (US mortgage, fully deductible).
| Bracket | Pre-tax rate | After-tax rate | Tax shield (annual) | Effective savings |
|---|
Worked example: 6% US mortgage on a $400K balance at 32% bracket
- Annual interest paid (year 1): $400,000 x 6% = $24,000.
- Deductible portion: 100% (loan is under the $750K TCJA cap and you itemize).
- Tax shield: $24,000 x 32% = $7,680 per year in reduced federal tax.
- After-tax effective rate: 6% x (1 - 0.32) = 4.08%.
- Equivalent investment hurdle: 4.08% after-tax. If your taxable account expected return is above 4.08% post-tax, holding the mortgage and investing the cash is positive expected value.
- Practical caveat: if you take the standard deduction ($30,000 MFJ 2026), the mortgage interest deduction has no incremental value. Recompute with deductible fraction = 0.
When the after-tax framing breaks down
- You take the standard deduction. Only the interest above the standard deduction threshold provides marginal value. For most US borrowers with mortgages under $300K and the $30,000 MFJ standard deduction in 2026, the practical deduction is zero.
- SALT cap interaction. The $10,000 state-and-local-tax cap means itemizing requires significant other deductions. High-tax states (CA, NY, NJ) often lose deductibility despite paying high property tax.
- Bracket changes during the loan. Retirement, job loss, or a windfall changes your marginal rate. The after-tax cost moves accordingly.
- AMT (US) or surcharge bands (India, UK). Quirky regime transitions can spike or zero the effective rate.
- Passive activity rules (US). Rental losses are limited to $25K and phase out by AGI, capping the after-tax benefit of negatively geared US rentals.
The five-country table at 30% marginal bracket
| Country | Mortgage 6% after-tax | Investment 6% after-tax | Personal 6% after-tax |
|---|---|---|---|
| US | 4.20% | 4.20% | 6.00% (not deductible) |
| UK | 6.00% (no deduction) | 4.80% (BTL basic-rate credit) | 6.00% (not deductible) |
| Canada | 6.00% (no deduction) | 4.20% (Smith Maneuver) | 6.00% (not deductible) |
| India | ~5.00% (Rs 2L cap reduces effective benefit) | 4.20% (let-out unlimited) | 6.00% (not deductible) |
| Australia | 6.00% (no deduction) | 4.20% (negative gearing) | 6.00% (not deductible) |
WACC: cost of debt in corporate finance
For a public company, the WACC formula is: WACC = (E/V) x Re + (D/V) x Rd x (1 - Tc), where Rd is the pre-tax cost of debt and Tc is the corporate tax rate. The (1 - Tc) term is the same after-tax adjustment shown here. US C-corp rate is 21% federal, so a 6% corporate bond has an after-tax cost of 4.74%. UK is 25% (above GBP 250K profit), so the same 6% bond effectively costs 4.50%. India corporate rate ranges 22-30% depending on regime.
For personal finance, this same wedge is what makes mortgage debt cheap relative to risk-free returns, and is the reason most planners avoid prepaying low-rate mortgages.
The formula explained
This calculator applies the standard after-tax cost-of-debt formula with country-specific deductibility logic:
1. interest_annual = balance x pre_tax_rate
2. deductible = min(interest_annual, country_cap)
3. shield = deductible x marginal_rate
4. after_tax_rate = pre_tax_rate - (shield / balance)
5. opportunity_cost = max(0, expected_market - after_tax_rate)
The formula reduces to the textbook form Rd x (1 - Tc) when interest is 100% deductible and there is no dollar cap.
To verify, plug in (6.0, 32, US, mortgage, 400000, 7.0): expect after-tax rate 4.08%, tax shield $7,680, equivalent hurdle 4.08%.
Frequently asked questions
What is the after-tax cost of debt formula?
After-tax cost = pre-tax rate x (1 - marginal tax rate x deductible fraction). When interest is 100% deductible the formula simplifies to pre-tax rate x (1 - marginal rate). A 6% rate at 32% bracket: 6% x (1 - 0.32) = 4.08%. The deductible fraction handles partial-deduction cases like the US $750K mortgage cap or India's Rs 2 lakh Section 24(b) limit.
Does the TCJA $750K mortgage cap still apply in 2026?
Yes. The Tax Cuts and Jobs Act lowered the home acquisition debt cap from $1 million to $750,000 (single + MFJ; $375,000 MFS) for mortgages originated after December 15, 2017. The provision is currently scheduled to revert to $1 million at the end of 2025 unless Congress extends, but as of the 2026 tax year planning baseline, the $750K cap is treated as continuing. The SALT cap of $10,000 means itemizing is harder, so the practical value of the deduction is lower for many filers.
How does the student loan interest deduction phase out?
The US student loan interest deduction is capped at $2,500 per year and phases out by MAGI: between $85,000 and $100,000 single, $170,000 and $200,000 MFJ for 2026. Above the top of the band the deduction is zero. The deduction is above-the-line (no itemizing required), making it accessible to almost everyone within the income band.
What is the Section 163(j) 30% adjusted EBITDA cap?
For US businesses, Section 163(j) limits net interest expense to 30% of adjusted taxable income (ATI). From 2022, depreciation and amortization no longer add back to ATI, tightening the cap meaningfully. Small businesses with average gross receipts under $30 million are generally exempt. Excess interest carries forward indefinitely. Real estate and farming businesses can elect out by using ADS depreciation.
How does Section 24(b) work in India?
Under the old tax regime, Section 24(b) allows a deduction of up to Rs 2 lakh per financial year on interest paid on a home loan for a self-occupied property. For let-out property the deduction is unlimited (subject to set-off rules capping rental loss at Rs 2 lakh against other heads). Section 80EEA adds another Rs 1.5 lakh for first-time buyers on properties under Rs 45 lakh. The new regime (default from FY 2023-24) does not allow Section 24(b) for self-occupied homes, only for let-out properties.
What is negative gearing in Australia?
When the costs of holding an investment property (mortgage interest + maintenance + depreciation) exceed the rental income, the net loss can be offset against your other taxable income at your marginal rate. A 6% interest rate at a 37% marginal bracket has an effective after-tax cost of roughly 3.78% if the property is fully negatively geared. The 50% CGT discount on sale (held more than 12 months) often makes this strategy pay off.
Does the UK allow mortgage interest deduction for landlords?
Not as a P&L expense. The Section 24 Finance (No. 2) Act 2015 phased out the deduction between 2017 and 2020, replacing it with a basic-rate (20%) tax credit. A higher-rate (40%) landlord no longer benefits at their full marginal rate, only at 20%, which can push otherwise profitable buy-to-lets into negative-cash-flow territory.
Should I prepay debt or invest the cash?
Compare your after-tax cost of debt to the after-tax expected return of your investment alternatives. If your mortgage is effectively 4.08% and you expect 5-7% real return on a low-cost stock index, math favors investing on expected value, with the caveat that returns are volatile. Tax-deferred space (US 401(k), Roth IRA, India NPS) usually wins first because of the additional tax shield. Risk-free debt paydown is the right call when you cannot stomach equity volatility or when discipline is a concern.