It's the most debated question in personal finance: "Should I pay off my mortgage or invest?" Your parents say get rid of debt. Your spreadsheet says the market returns more. Both are right — depending on the situation. Let's run the numbers on a $200,000 mortgage at 7% over 30 years, with $500/month extra to either prepay the loan or invest.
The Setup
Mortgage: $200,000 at 7%, 30-year fixed
Normal payment: $1,331/month
Extra cash available: $500/month
Market return assumption: 8% (S&P 500 historical average)
Option A: Pay $500 Extra Toward Mortgage
Adding $500/month to your mortgage payment means $1,831 total. The loan gets paid off in 18 years instead of 30, saving $135,000 in interest. After year 18, the full $1,831/month that was going to the mortgage can now be invested.
| Amount | |
|---|---|
| Loan paid off in | 18 years (vs 30) |
| Total interest paid | $155,000 (vs $290,000) |
| Interest saved | $135,000 |
| Years 19-30: $1,831/month invested at 8% | Grows to $365,000 |
| Net worth at year 30 | $365,000 + paid-off house |
Option B: Invest $500/Month at 8%
Keep the full 30-year mortgage. Invest the extra $500/month in a total stock market index fund.
| Amount | |
|---|---|
| Total invested over 30 years | $180,000 ($500 × 360 months) |
| Investment value at year 30 (8%) | $745,000 |
| Mortgage remaining | $0 (paid off on schedule) |
| Total interest paid to bank | $290,000 |
| Net worth at year 30 | $745,000 + paid-off house |
Investing wins by $380,000 on paper. But there's a massive psychological dimension: Option A gives you a debt-free house in 18 years. Option B keeps you in debt for the full 30. That's 30 years of monthly mortgage payments, 30 years of knowing the bank can foreclose. The spreadsheet doesn't measure peace of mind.
After Taxes: The Gap Narrows
If this is a taxable brokerage account, capital gains tax takes a bite:
| Option A (Pay Off + Invest Later) | Option B (Invest Throughout) | |
|---|---|---|
| Pre-tax investment value | $365,000 | $745,000 |
| Gains | $145,000 (12 years of contributions) | $565,000 (gains) |
| Tax at 15% | −$21,750 | −$84,750 |
| After-tax | $343,250 | $660,250 |
After-tax difference: still about $317,000 in favor of investing. But if you're in a higher bracket or tax rates rise, the gap shrinks further.
When Paying Off the Mortgage Wins
✅ Pay off when:
- Your mortgage rate is above 6% — a guaranteed 7% return is hard to beat on a risk-adjusted basis
- You're within 10 years of retirement — reducing fixed expenses before retirement income drops is smart
- You value certainty over optimization — a paid-off house can't be foreclosed on
- You'd stress-spend the invested money — many people invest the extra but then raid the account for cars and vacations
✅ Invest when:
- Your mortgage rate is below 5% — you're likely to earn more in the market
- You have 15+ years until retirement — time smooths out market volatility
- You have other debt at higher rates — pay off credit cards (20%+) before prepaying a 7% mortgage
- You're not maxing tax-advantaged accounts — a 401(k) match or Roth IRA is far better than mortgage prepayment
The Hybrid Approach
You don't have to pick one. Split the $500: $250 to mortgage, $250 to investing. The mortgage gets paid off in ~24 years (saving $68K in interest), and the investment grows to $372K. Not the mathematical best — but the emotional best for many people.
Key Takeaways
- $200K mortgage at 7%: prepaying $500/mo saves $135K interest, investing $500/mo builds $745K — $380K gap
- After tax, the gap drops to $317K — still substantial, but not as stark
- Mortgage rates above 6%: prepay makes mathematical sense. Below 5%: invest. 5-6%: depends on tax situation and risk tolerance
- Peace of mind has real value — a paid-off house eliminates the single largest monthly expense forever
- Hybrid: split the difference. Half to principal, half to investing. Best of both worlds