Pay Off Your Mortgage Early or Invest? The FIRE Math
"Should I pay off my mortgage early or invest?" gets asked constantly, and the usual answers are slogans. One camp says debt is risk and a paid-off house is freedom. The other says you're giving up stock market returns to save 3%.
Both camps are right about different people. The answer depends on your mortgage rate, where the money would otherwise go, and when you plan to stop working. This post runs the actual numbers for two common situations, then covers the part most comparisons skip: what a mortgage does to your FIRE number.
The one rule that settles most of it
Every extra dollar you put toward the mortgage earns exactly your mortgage rate, guaranteed.
Prepay a 7% loan and you've locked in a 7% return with no market risk. For most households it's also effectively tax-free: with a 2026 standard deduction of $32,200 for married couples ($16,100 single), most people don't itemize, so their mortgage interest saves them nothing on taxes. Every dollar of interest you avoid is a dollar of real savings.
The comparison is not "mortgage rate vs. the stock market's best decade." It's "a guaranteed X% vs. an uncertain return that has to beat X% after taxes and after you account for the risk you're taking."
That's why the answer flipped for a lot of people. Plenty of homeowners locked in near 3% in 2020 and 2021. New buyers in late September 2026 faced a 30-year average of 7.03%, according to Freddie Mac's weekly survey. Same question, very different math.
Worked example 1: a $300,000 loan at 7%
Take a new $300,000, 30-year fixed mortgage at 7.0%.
- Principal and interest: $1,996 a month
- Total interest over 30 years if you just pay on schedule: $418,527
Now add $1,000 a month in extra principal.
- The loan is paid off in 12 years and 7 months
- Total interest drops to $151,924, a saving of $266,603
That interest figure is the number people quote, and it's misleading on its own. The real question is what you'd have if you'd invested the $1,000 instead. So compare the two paths over the full 30 years:
- Payoff path: pay $1,000 extra until the loan is gone, then invest the full $2,996 a month (old payment plus the extra) for the remaining years.
- Invest path: pay the mortgage on schedule and invest $1,000 a month for 30 years.
| If investments earn | Payoff path at year 30 | Invest path at year 30 | Winner |
|---|---|---|---|
| 4% a year | $903,021 | $694,049 | Payoff, by $209,000 |
| 7% a year | $1,218,427 | $1,219,971 | A tie |
At a 7% mortgage rate, investing only breaks even if your investments return 7% a year, every year, and that's before tax. In a taxable brokerage account, a 7% return becomes something lower after taxes on dividends and gains, while the mortgage savings stay whole. Unless the money would go into a tax-advantaged account, prepaying a 7% mortgage is very hard to beat on expected value, and it beats it easily on risk.
Worked example 2: the same loan at 3%
Same $300,000, 30-year loan, but at 3.0%.
- Principal and interest: $1,265 a month
- Total interest on schedule: $155,332
- With $1,000 a month extra: paid off in 13 years and 6 months, interest $64,819, a saving of $90,514
| If investments earn | Payoff path at year 30 | Invest path at year 30 | Winner |
|---|---|---|---|
| 4% a year | $633,697 | $694,049 | Invest, by $60,000 |
| 7% a year | $839,944 | $1,219,971 | Invest, by $380,000 |
At 3%, the guaranteed return is low enough that even conservative investments can match it in many years, and they stay liquid. Prepaying a 3% loan means accepting a low return in exchange for peace of mind. That can still be a fine trade, but it is a trade.
All examples assume a level return every year and ignore taxes on the investments, which slightly flatters the invest path.
The FIRE number trap: don't add 25 times your mortgage payment
Most FIRE math starts with annual spending × 25, which is the 4% rule turned into a target. If you're still paying a mortgage when you retire, it's tempting to throw the payment into spending and multiply it by 25 along with everything else.
That overstates what you need, often by a lot.
The 25x multiplier assumes an expense that lasts forever and rises with inflation. A fixed-rate mortgage payment does neither. It stays the same in dollars and stops on a known date.
Back to the 7% loan. Say you retire 10 years in, with 20 years of payments left:
- Annual principal and interest: $23,951
- 25 × $23,951 = $598,772 added to your FIRE number
- Remaining loan balance: $257,437
- Cash needed today to fund all 240 remaining payments from a safe account earning 4%: $329,368
The honest cost of carrying that mortgage into retirement is somewhere between $257,000 and $330,000, not $600,000. Add 25 times the payment and you could keep working years longer than you need to.
A cleaner way to plan:
- Run your FIRE number on spending without the mortgage's principal and interest. Keep property tax, insurance and maintenance in. Those last as long as you own the house and rise over time, so they belong in the 25x.
- Treat the mortgage as a separate, fixed liability. Either pay it off by the time you retire or hold a separate pot roughly equal to the remaining balance, kept in something low-risk.
The free FIYR FIRE calculator works the same way. It asks for what you'll spend in retirement and tells you to leave out costs that will end before you retire, such as a mortgage you'll have paid off. For the full formula, see the FIRE number explained in plain English.
What a paid-off house does in early retirement
Beyond the returns math, a paid-off house changes three things once you stop working.
You withdraw less. Without $24,000 a year in principal and interest, your portfolio has less to do. That's a lower withdrawal rate on the same savings, or the same withdrawal rate on less savings.
Bad markets hurt less. The danger in early retirement is selling investments during a crash to cover fixed bills. A smaller fixed bill means fewer shares sold at low prices.
Your taxable income can be lower. If your spending comes from traditional IRA withdrawals or Roth conversions, every dollar you don't need is a dollar of income you don't report. That gives you more room under tax brackets and under the ACA subsidy cutoff, which for 2026 coverage sits at $84,600 of income for a two-person household. We walk through that interaction in how to access retirement money before 59½.
The downside is liquidity. Extra payments are locked inside the house. Your monthly payment doesn't fall until the loan is fully paid off (unless your lender allows a recast), and you can't sell one bedroom to cover a job loss. Half-paid-off is the least flexible position: you've given up the cash, but you still have the bill.
Where the extra money should go first
Before prepaying any mortgage, most people should work through these:
- Get the full employer 401(k) match. That's an instant return no mortgage can match.
- Pay off high-interest debt. Credit cards at 20%+ come first. Our snowball vs. avalanche guide covers the order.
- Build an emergency fund. Money sent to the mortgage can't pay for a layoff.
- Fund tax-advantaged accounts (401(k), IRA, HSA), at least to the level your FIRE plan needs.
After that, the mortgage rate decides:
- Under about 4%: investing usually wins on expected value. Prepay only if being debt-free is worth the cost to you.
- About 4% to 6%: a real judgment call. Splitting extra money between the two is a reasonable answer, not a cop-out.
- Above about 6%: prepaying is a strong, low-risk choice, especially compared with holding bonds or cash, and especially if the alternative is a taxable account.
The middle path: the side fund
You don't have to pick a side today. Some people invest the extra money in a separate brokerage account earmarked for the mortgage. When the account equals the remaining loan balance, they can pay off the house in one go, or keep both if the investments have done well.
The side fund keeps your money liquid while you work and preserves the option to be mortgage-free by retirement. The tradeoff is discipline: the money has to stay earmarked, and in a down market the account may fall short of the balance for a while.
Tracking it in FIYR
In FIYR you can add your home as a real estate asset. If you mark it as financed, FIYR creates the matching mortgage liability and counts only your equity (home value minus the loan balance) in your net worth. Any liability with an interest rate, a term and an original amount gets an amortization schedule with an extra-payment what-if: drag the slider to add a monthly amount or extra payments per year and see the new payoff date and the interest saved.
That shows you the payoff side of the decision with your own loan. Your FIRE projection shows the investing side. If you're still getting the liability side of your net worth in order, start with how to track liabilities accurately.
Frequently asked questions
Is it better to pay off a mortgage or invest? It depends mostly on the rate. Prepaying earns your mortgage rate with no risk. At 3%, investing has historically come out ahead over long periods. At 7%, investments have to earn more than 7% after tax just to match prepaying, which is a high bar.
Should I pay off my mortgage before I retire early? It's not required, but plan for it. A paid-off house lowers your withdrawals and your taxable income. If you keep the mortgage, set aside roughly the remaining balance in low-risk savings instead of adding 25 times the payment to your FIRE number.
Does mortgage interest still reduce my taxes? Only if you itemize. With the 2026 standard deduction at $32,200 for married couples and $16,100 for single filers, most households don't, so their mortgage interest brings no tax benefit.
How much does an extra $1,000 a month save on a $300,000 mortgage? On a 30-year loan at 7%, about $266,600 in interest, with payoff in 12 years and 7 months. At 3%, about $90,500, with payoff in 13 years and 6 months.