You have an extra $300 a month. The mortgage statement says you owe for another 27 years. The market keeps compounding somewhere without you. Every personal-finance forum has a thousand-comment war about what to do next — because this is the rare money question where the spreadsheet and the psyche genuinely pull in different directions.
Here's the honest framework, with the math for both sides.
Every extra dollar toward your mortgage earns a guaranteed, risk-free return equal to your mortgage rate. Prepay a 6.5% loan and you've locked in 6.5% — no volatility, no crash, no sequence risk. It's the safest "investment" most households can access.
Every extra dollar invested in a diversified index fund earns an expected return — historically about 10% nominal (roughly 7% after inflation) for stocks over long horizons, but with brutal interim swings and no guarantees.
| Your mortgage rate | The math says |
|---|---|
| Under ~4% | Invest. A cheap fixed-rate mortgage is a gift — inflation quietly pays part of it for you. |
| 4–6% | Genuine toss-up. Taxes, timeline, and temperament decide (see below). |
| Over ~6.5% | Prepaying is a strong guaranteed return few portfolios reliably beat after tax. Doing some of both is very defensible. |
Take a $350,000 loan at 6.5% over 30 years, with $300/month extra:
On expected value, investing usually edges ahead when rates are mid-range. But "expected" is doing heavy lifting in that sentence: a bad first decade of returns can leave the investor behind the prepayer for twenty years. The prepayer's result has no error bars.
Investment gains get taxed (unless sheltered in a retirement account); mortgage prepayment's "return" is tax-free by nature. But if you invest inside a tax-advantaged account with an employer match, the match is an instant 50–100% return that beats any mortgage rate — never skip a match to prepay a mortgage.
Money invested can be sold in a bad week if life demands it. Money in home equity is locked behind a refinance or sale — you can't eat a paid-off bedroom. This is why prepaying only makes sense after your emergency fund is full.
A fixed mortgage payment gets easier every year in real terms — you repay with cheaper dollars. High inflation quietly transfers wealth from the lender to you, which strengthens the case for keeping a low-rate loan and investing instead.
The behavioural truth: the mathematically optimal plan you abandon in a crash is worth less than the "suboptimal" plan you actually follow. If a paid-off house would let you sleep, take career risks, or stop panic-checking your portfolio, that peace of mind has real, compounding value. Nobody ever regretted owning their home outright — plenty of people regret selling stocks at the bottom.
Run your own numbers: see exactly how many years and how much interest an extra monthly payment saves on your loan — then compare it with what the same money does compounding in the market.
Try the Mortgage Calculator →There is no universal answer — there's your rate, your tax situation, and your temperament. Above ~6.5%, prepaying is a guaranteed return worth taking seriously. Below ~4%, the market is the better bet for money you won't need soon. In between, splitting the difference is not indecision — it's diversification between a certain gain and a probable bigger one. Whichever way you lean, make the choice deliberately, with your own numbers in front of you.