← Blog · May 22, 2026 · mortgage, investing
Pay Extra Mortgage Principal or Invest the Difference?
You have an extra $500 a month. Your mortgage is at 6.75%. The S&P 500 has averaged about 10% nominal since 1928. The math says invest. Your brain says pay the mortgage. Which is right?
Honestly, it depends on which year you ask the question.
The cold math
Compare apples to apples. Paying off a 6.75% mortgage is a guaranteed 6.75% return, tax-free (you do not pay taxes on money you do not pay in interest). Investing in an index fund returns whatever the market does, with all the normal caveats — long-term average roughly 7% real (after inflation), with some years up 30%, others down 35%.
If you adjust for tax: the stock market returns are taxable when you sell. Long-term capital gains are 15-20% federal, plus state. So a nominal 10% market return is closer to 7.5-8% after-tax. The 6.75% mortgage payoff starts to look competitive.
Now adjust for the mortgage interest deduction (if you itemize — most do not post-TCJA). If your marginal rate is 32%, your effective mortgage rate is 6.75% × (1 - 0.32) = 4.6%. Now the market wins handily.
Run your numbers
Use our Mortgage Calculator to see the amortization curve, and Compound Interest for the investment side. Compare the future value of $500/month paid into your loan versus $500/month into an index fund over the same horizon.
For a typical scenario: $300k balance at 6.75%, $500/month extra:
- Pay off mortgage: shave 8 years off, save about $120,000 in interest.
- Invest at 7% nominal: $500/month for 22 years grows to about $375,000.
Investing wins by $255k in this case, but with significant volatility risk along the way. If you started in 2008 you would have been underwater on the investment for years before recovering.
The emotional argument
Personal finance is not always personal MATH. Three real considerations:
- Sleep at night. A paid-off house means rent of about $0 forever. That removes a category of fear that no stock balance does. Some people value this enough to leave $100k on the table.
- Discipline. The mortgage payoff is automatic — the bank takes your money. Investing requires showing up monthly and not panic-selling in downturns. If you would not actually invest the difference, the comparison is moot.
- Cash flow. A paid-off mortgage frees up $2,000-3,000/month for life. That is a different financial reality than "has $400k in an account."
The hybrid approach (what most people should do)
Max your retirement match first. Then max your IRA. Then split: half goes to taxable investments, half goes to extra mortgage principal. You get tax-advantaged growth, market participation, AND mortgage progress. The decision becomes less binary.
For the high-confidence math person at low mortgage rates (under 5%): invest. For the "sleep matters" person at higher rates: pay off. For everyone in between: half each, and stop optimizing.
Run your scenario
- Mortgage Calculator — see what extra principal does to your timeline
- Biweekly Mortgage — the lazy way to pay extra
- Compound Interest — what the same money does in stocks
- ROI Calculator — compare actual returns honestly
- Refinance Calculator — sometimes the real win is a lower rate