The $600-a-Month Question
About three years into our mortgage, my wife and I found ourselves with approximately $600 per month of discretionary cash that we had not previously had — a combination of a salary increase, her returning to full-time work after a period part-time, and finally paying off the last of our car loans. The question of what to do with that $600 sounds like a good problem to have, and it is. But it was also genuinely difficult.
The two obvious options: apply the extra $600 to our mortgage principal each month, or invest it in index funds through our taxable brokerage account. We had already maxed out our 401(k) and Roth IRA contributions for the year, so this was money beyond the tax-advantaged space.
I am the kind of person who cannot make a significant financial decision without building the model first. So I built the model. And then I built it again when I did not like the answer the first time. And eventually I came to a conclusion that the math alone did not fully determine.
The Mortgage Payoff Math
Our mortgage: $340,000 remaining balance, 3.25% fixed rate, 22 years left on a 30-year loan. Monthly payment: $1,478.
If I apply an extra $600 per month to principal:
- The loan pays off in approximately 13 years instead of 22 — saving 9 years of payments
- Total interest saved: approximately $67,000
- The guaranteed return on this strategy: 3.25% — the interest rate I stop paying
That $67,000 in saved interest is real money. But the "return" on mortgage prepayment is capped at the mortgage rate — 3.25% in our case. And crucially, that return is after-tax in a complicated way: mortgage interest is only deductible if you itemize, and we take the standard deduction, so our effective cost of mortgage debt is exactly 3.25%.
The Investment Math
Now the comparison: what if I invest that $600 per month instead of applying it to the mortgage?
$600 per month for 22 years (the remaining mortgage term) at 7% annual return:
- Total contributions: $158,400
- Terminal balance: approximately $404,000
- Compound interest earned: approximately $245,600
The investment scenario wins — dramatically. The compound interest calculator makes this clear with almost brutal efficiency: investing at an expected 7% return versus prepaying debt at 3.25% produces a gap of hundreds of thousands of dollars over 22 years.
But there is a catch the calculator does not capture, which I will get to.
What Happens When You Run a Shorter Timeframe
The 22-year comparison is misleading in one important way: if I prepay the mortgage aggressively, I free up the entire $2,078 monthly payment (original $1,478 + $600 extra) in year 13, 9 years early. That freed cash can then be invested for the remaining 9 years of the original mortgage term.
So the fairer comparison is:
Option A (invest $600/month for 22 years): Terminal balance at year 22: approximately $404,000. Plus the house is paid off.
Option B (prepay mortgage, then invest everything): Mortgage paid off at year 13. Then invest $2,078/month for 9 years at 7%: terminal balance at year 22: approximately $336,000. Plus the house is paid off.
Option A still wins by approximately $68,000 at the 22-year mark. The math continues to favor investing over prepayment, even accounting for the freed cash flow after early payoff.
The Variables the Calculator Cannot Measure
Here is where I spent most of my two years of deliberation — on the factors that the compound interest calculator genuinely cannot quantify.
The 7% assumption is not guaranteed. Stock market returns are volatile. A diversified index fund has historically returned approximately 7% real over long periods, but "historically" and "will" are different words. In any given 13-year window, the market could return 3%, 12%, or -2%. The 3.25% mortgage payoff return is guaranteed. The investment return is not.
The psychological value of a paid-off house is real. My wife grew up in a household where her parents' mortgage was a constant source of financial stress. For her, the security of owning our home outright has a value that does not appear in any compound interest calculation. I do not think this is irrational — the peace of mind from eliminating a large fixed monthly obligation is worth something, and people underweight it when they run purely financial comparisons.
Job security risk. If I lose my job, a paid-down mortgage means I need less monthly income to stay in the house. A large investment portfolio does not directly reduce my monthly obligations the same way. In a worst-case scenario, liquid investments can cover mortgage payments — but that requires selling investments, potentially at a bad time.
What We Actually Decided
After two years of running variations on this calculation — different return assumptions, different mortgage scenarios, different time horizons — we made a decision that the math alone did not determine.
We split the $600: $200 to mortgage prepayment, $400 to the taxable investment account.
Is this mathematically optimal? No. The pure investment strategy wins on expected value by a significant margin. But "optimal" and "right for us" are different questions. The $200 monthly prepayment gives my wife a concrete sense that we are making progress toward owning the house outright. The $400 monthly investment means we are not leaving the compound interest advantage entirely on the table.
I ran the split scenario through the calculator: $400/month invested for 22 years at 7% produces approximately $269,000 — still meaningfully more than the mortgage prepayment alternative, while honoring the psychological value of debt reduction for both of us. It is a compromise, but it is a compromise we can both commit to without either of us feeling like we lost the argument.
The Framework I Would Suggest
If you are facing this same decision, here is how I would approach it:
- If your mortgage rate is above 6%: Prepayment is much more competitive with investing, especially in a lower-return environment. The guaranteed return on debt elimination starts to approach realistic investment returns.
- If your mortgage rate is below 4%: The math overwhelmingly favors investing. You are borrowing cheap money and putting it to work at a higher expected return. The spread is hard to give up.
- Between 4%–6%: This is where personal factors matter most — risk tolerance, job security, psychological relationship with debt. Run both scenarios in the calculator, then make the decision that accounts for the variables the numbers miss.
At 3.25%, our mortgage is clearly in "invest the difference" territory mathematically. We chose a hybrid anyway, and I do not regret it. Use our compound interest calculator to model your specific mortgage rate, balance, and monthly surplus — the numbers will tell you the expected-value answer clearly, and then you can decide how much weight to give the factors the calculator cannot see.