The short answer: paying extra on your mortgage is a guaranteed return equal to your interest rate. Investing the same money might beat that, or it might not. The higher your rate, the stronger the case for paying down the loan. The lower your rate, the stronger the case for investing. This article is based on Episode 2 of The Wealth Track, my personal finance podcast, and you can watch it below.
A quick note before the numbers: I'm not a financial advisor. Everything here is an illustration with round assumptions, not personal advice.
Where most homeowners actually are
If you're sitting at the kitchen table wondering about this, you're in large company. In the 2020-2024 period, 39.4% of owner-occupied homes were owned free and clear, up from 34.4% in 2010-2014 (U.S. Census Bureau, American Community Survey 5-year estimates).
On the other side, 59.7% of owned homes had a monthly mortgage payment in 2024 (U.S. Census Bureau, ACS 1-year estimates). These are different surveys covering different time windows, so don't add or subtract them. The picture is still clear: many owners are reaching the finish line, and most still have a payment.
An extra mortgage payment is an investment
It's a slightly odd way to say it, but every extra dollar you send to your loan stops charging you interest. If your mortgage rate is 6.5%, that dollar effectively earns 6.5%.
With a fixed rate, the bank can't change that number on you. So paying down debt is a guaranteed return equal to your interest rate. One caveat: it's guaranteed in the sense of interest you avoid, not cash that shows up in an account.
The example: $500 extra a month on a $300,000 loan
Say you borrow $300,000 on a 30-year fixed mortgage at 6.5%. Your principal and interest payment is about $1,896 a month. Over 30 years you'd pay roughly $383,000 in interest, which is more than the loan itself.
Now add $500 a month to that payment. The loan is gone in about 17.5 years instead of 30, and you save roughly $180,000 in interest.
Notice what you didn't need: no stock picking, no market timing. Just a payment you control.
The other side: invest the $500 instead
Keep everything identical: same loan, same rate, same $1,896 payment. The only difference is what happens to the extra $500.
- Person A sends it to the mortgage.
- Person B invests it in a broad stock index fund.
I have to make an assumption, so I'll say it plainly: Person B's investments grow 7% a year, compounded monthly. That is not a promise. Markets go up and down, sometimes for years. It's a round number to run the math with.
Check in at 17.5 years, the moment Person A's mortgage is gone:
| Person A (extra to mortgage) | Person B (invests at 7%) | |
|---|---|---|
| Mortgage balance | $0 | about $194,000 |
| Investments from the extra $500 | $0 | about $205,000 |
| Net position | $0 | about +$11,000 |
Person B comes out ahead by only about $11,000 on a $300,000 loan. That's not nothing, but it's small. The reason is that 7% versus 6.5% is barely a gap. Person B is taking stock market risk to maybe earn half a point more than Person A gets for sure.
And that comparison is before taxes on investment gains, fund fees, or any mortgage interest deduction. Fees in particular can eat into that half point, which is why it's worth knowing what investment fees really cost.
One more detail I left out on purpose: once Person A's loan is gone, about $2,400 a month is freed up (the $1,896 payment plus the extra $500) and can be invested. I stopped the clock at that moment, so it isn't counted above.
The point isn't that A wins or B wins. When your mortgage rate is high, the two choices can land surprisingly close together, and one of them carries no market risk.
What if the market returns 4% instead?
Change one thing. Person B earns 4% a year instead of 7%.
After 17.5 years, B's investments are worth about $152,000. B still owes about $194,000. B is now behind by roughly $42,000.
| B's annual return | Investments after 17.5 years | Mortgage still owed | Net vs. Person A |
|---|---|---|---|
| 7% | about $205,000 | about $194,000 | about +$11,000 |
| 4% | about $152,000 | about $194,000 | about -$42,000 |
Same decision, very different result. That's the risk, and paying down the mortgage doesn't have it.
It also works in reverse. If your mortgage rate were 3%, the math tilts the other way, because investing would have a much lower hurdle to beat.
A rule of thumb based on your rate
Here's the rough rule I use:
- Higher rate: the stronger the case for paying down the mortgage.
- Lower rate: the stronger the case for investing.
There is no exact line. Where you land depends on how much risk you can stomach, and that's the part a spreadsheet misses.
What the spreadsheet misses
Liquidity. Money in your house is hard to get out. You can't sell a bathroom to cover a car repair. Money in savings or investments, you can reach.
Taxes. Retirement accounts can have tax benefits, and an employer match is an instant return. If you're leaving a match on the table, that comes first. Mortgage interest may be deductible for some people and not for many others, so check your own situation.
Peace of mind. Some people sleep better with no mortgage. That's a real benefit, and it doesn't show up in a return calculation. Others would feel uneasy with a big chunk of their wealth locked inside a house. Both feelings are valid.
There's a middle path
You can split it: some to the mortgage, some to investing. That isn't a cop-out. It can be a way to reduce regret whichever way things go.
Do these before either option
Before sending extra money to the mortgage or the market, handle the basics:
- Build an emergency fund. If you're not sure how much, I wrote about how much you really need.
- Capture your employer match. It's an instant return.
- Pay off high-interest debt, like credit cards.
These usually beat both options.
What to do this week
1. Know your rate. Pull up your latest mortgage statement and write down your interest rate and remaining balance. That one number frames the whole decision.
2. Cover the basics first. Emergency fund, employer match, high-interest debt. Then decide about the mortgage.
3. Run both scenarios. Use a calculator and try a pessimistic return and an optimistic one. If a choice only wins in the optimistic case, that tells you something. Then pick on purpose and write down why. Revisit it once a year, because rates and life both change.
The best choice is the one you can stick with.
Tracking it in NetTrack
If you want to see this with your own numbers, that's why I built NetTrack. It brings your accounts, investments, property, and debts together into one net worth number, so you can watch your mortgage balance and your investments move side by side. You can start on the free plan.