Mortgage Interest Deduction
Calculates deductible mortgage interest on primary and second home loans, subject to $750,000 acquisition debt limit ($1 million for pre-12/16/2017 loans). Interest must be paid on qualified residence debt and taxpayer must itemize to benefit.
How it works
1. Determine qualified residence debt
Qualified residence includes primary home and one second home. Debt must be secured by the property and used to buy, build, or substantially improve it (acquisition debt). Home equity debt no longer deductible after 2017 unless used for home improvements.
2. Apply debt limit based on origination date
Loans originated after 12/15/2017: $750,000 limit ($375k if MFS). Loans originated on or before 12/15/2017: $1,000,000 limit (grandfathered). Limit applies to combined principal of all qualifying loans.
Deductible Portion = min(Total Qualified Debt, Applicable Limit) / Total Qualified Debt
3. Calculate deductible interest
Multiply total interest paid by the deductible portion. If total debt is within the limit, all interest is deductible. If over the limit, prorate interest based on deductible debt ratio.
Deductible Interest = Total Interest Paid × Deductible Portion
4. Verify itemization benefit
Mortgage interest only reduces taxes if total itemized deductions (SALT + mortgage + charitable + medical) exceed the standard deduction. Otherwise, the standard deduction provides greater benefit.
Worked example
Married couple with primary residence mortgage and refinance
| Primary residence loan (originated 2022) | $600,000 |
|---|---|
| Interest paid in 2024 | $21,000 |
| Second home loan | None |
| Filing status | Married Filing Jointly |
| Other itemized deductions | $18,000 (SALT + charitable) |
| Standard deduction (2024 MFJ) | $29,200 |
**Step 1: Qualified Residence Debt**
- Primary residence loan: $600,000
- Secured by property: Yes
- Used for acquisition: Yes (purchase of home)
- Qualified debt: $600,000
**Step 2: Apply Debt Limit**
- Loan originated: 2022 (after 12/15/2017)
- Applicable limit: $750,000
- Total qualified debt: $600,000
- Deductible portion: min($600,000, $750,000) / $600,000 = 100%
**Step 3: Calculate Deductible Interest**
- Total interest paid: $21,000
- Deductible portion: 100%
- Deductible interest: $21,000 × 1.00 = $21,000
**Step 4: Verify Itemization Benefit**
- Mortgage interest: $21,000
- Other itemized deductions: $18,000
- Total itemized: $21,000 + $18,000 = $39,000
- Standard deduction (2024 MFJ): $29,200
- **Recommendation: Itemize** (saves additional $9,000)
- Tax benefit at 24% marginal rate: $9,000 × 0.24 = $2,160
Result: All $21,000 mortgage interest is deductible. Itemizing saves $2,160 more than taking the standard deduction.
Real-world context
Use cases
- Evaluating tax benefit of homeownership vs renting
- Determining whether to pay down mortgage or invest surplus cash
- Assessing value of mortgage points (prepaid interest) at closing
- Deciding whether to refinance (considering loss of grandfathered $1M limit)
Regulations
Mortgage interest deduction governed by IRC §163(h)(3). TCJA (2017) reduced limit from $1 million to $750,000 for new loans and eliminated home equity debt interest deduction (except for home improvements). Rules detailed in IRS Publication 936 (Home Mortgage Interest Deduction). Taxpayer receives Form 1098 from lender showing interest paid.
Strategic considerations
Grandfathered Debt
Loans originated before 12/16/2017 retain the $1 million debt limit. Refinancing these loans may lose this benefit unless the new loan amount does not exceed the old loan principal. This can significantly impact the tax benefit for high-value mortgages.
Deduction vs Investment Returns
The after-tax cost of mortgage interest is the interest rate times (1 - marginal tax rate). If investment returns exceed this after-tax cost, keeping the mortgage and investing may be financially optimal, though this ignores risk and personal preferences.
What drives the result
Directly increases itemized deductions on Schedule A, Line 8a
Debt over $750k limit ($1M for grandfathered) reduces deductible interest proportionally
Mortgage interest only provides benefit if total itemized exceeds standard deduction
New loan loses grandfathered $1M limit if old loan was pre-12/16/2017
Assumptions
- Debt limit is $750,000 for post-12/15/2017 loans, $1,000,000 for earlier loans
- Taxpayer itemizes deductions (mortgage interest has no value if taking standard deduction)
- Loan is secured by qualified residence (primary or second home)
- Debt proceeds used for acquisition or substantial improvement of home
- Points (prepaid interest) are deducted ratably over loan term unless paid at purchase
- The second home is not rented out (different rules apply to rental properties).
Limitations
- Does not model home equity debt used for non-home purposes (not deductible after 2017)
- Does not account for mortgage insurance premiums (separately deductible, subject to income phaseout)
- Assumes taxpayer has adequate documentation (Form 1098, records of home improvements)
- Does not model complex refinancing scenarios (cash-out refi, debt consolidation)
Related
- Standard vs Itemized Deduction — Mortgage interest only valuable if itemizing; standard deduction may provide greater benefit
- SALT Deduction (State and Local Taxes) — Combined with SALT, mortgage interest often drives itemization decision
- Deduction Bunching Strategy — Mortgage interest is steady annual deduction (not bunchable), but affects bunching strategy
- Charitable Contributions Deduction — Charitable gifts may be the "swing" deduction that makes itemizing worthwhile with mortgage interest
This page explains how Stratum models this calculation. It is educational material for financial professionals, not tax or legal advice, and tax law changes. Verify current figures against primary IRS sources before relying on them with a client.