TL;DR
Prepay your mortgage when your after-tax mortgage rate exceeds your expected after-tax investment return. For most people who locked in a 3% rate during 2020–2021, investing wins by a mile. For someone who took a mortgage at 7%+ in 2023–2025, the answer is much closer.
After-Tax Mortgage Rate
The mortgage interest deduction is only useful if you itemise. After the 2017 Tax Cuts and Jobs Act, the standard deduction is $14,600 (single) and $29,200 (MFJ) in 2024, with the SALT cap limiting state and local taxes (including property tax) to $10,000. Most middle-class homeowners no longer itemise.
If you don't itemise: your effective mortgage rate equals the stated rate. A 7% mortgage costs you 7% after tax. Period.
If you do itemise (high property tax states, big charitable giving, MFJ filers with $29,200+ in deductions before mortgage interest):
- 7% mortgage at the 24% federal bracket = effective ~5.3% after tax (very rough; only the marginal interest above the standard deduction actually saves tax)
The honest take: post-TCJA, fewer than 10% of homeowners get any tax benefit from mortgage interest. Run the math; don't assume.
Expected After-Tax Investment Returns
- 401(k) match — instant 50–100% return on the matched portion. Always max this first, no exceptions.
- Roth IRA / Traditional IRA — long-run real return ~5–6%, nominal ~7–8%. Tax-free at withdrawal (Roth) or tax-deferred (Trad).
- HSA — triple tax-advantaged. ~7% nominal, fully tax-free if used for qualified medical expenses.
- Taxable brokerage (S&P 500 index) — ~7% nominal long-run, taxed at LTCG (15–20% federal + state).
Decision Matrix
| Mortgage rate | Account type for surplus | Action |
|---|---|---|
| 3.0% (2020–21 lock) | Any | Invest. Don't even think about it. |
| 5.0% | 401(k) up to match | Match first, then invest |
| 6.5% | Roth IRA | Invest (still beats 6.5% on risk-adjusted basis) |
| 7.5% | Taxable brokerage | Lean prepay; ~7% guaranteed return |
| 8.0%+ | Any | Prepay first, then invest |
Worked Example: $400K Mortgage at 7%
You have $1,000/month extra. Two paths:
Path A — Prepay $1,000/month extra principal:
- Saves ~$180,000 in interest over the life of the loan
- Pays loan off ~10 years early
- Guaranteed 7% return
Path B — Invest $1,000/month in S&P 500:
- Assuming 7% nominal return, ~$300K after 30 years (taxable account, post-LTCG)
- Higher expected wealth but with sequence-of-returns risk
Path C (the one most planners recommend) — Match accounts first, prepay surplus:
- 401(k) up to full employer match
- Max HSA ($4,300 / $8,550 in 2025–26)
- Max Roth IRA ($7,000)
- Then split remaining surplus 50/50 between extra mortgage principal and taxable brokerage
Prepayment Penalties (Mostly Gone)
Thanks to Dodd-Frank (2010), qualified mortgages on owner-occupied homes can no longer have meaningful prepayment penalties beyond the first three years, and most lenders charge none at all. Investment property loans, jumbo loans, and some non-QM products may differ — check your note.
What About Refinancing?
If your current mortgage is above 6.5% and current rates dip below 5.5%, refinancing can save more than years of extra principal payments. Run a break-even on closing costs (typically $3,000–$8,000) — if you'll stay in the home longer than the break-even, refi.
Behavioural Tax
The mortgage payoff brings real psychological calm. Many financially literate people prepay even when the math slightly favours investing — and end up retiring earlier and happier. If a paid-off house is a goal that motivates you to stay disciplined, weight that into your decision.
Run your full picture (mortgage, 401(k), Roth, HSA, brokerage, kids' 529) through the AI Financial Planner →