You have an extra $500 a month and a 30-year mortgage at 6.23%. Should you throw the cash at the loan or buy index funds? This is one of the most-asked personal-finance questions of 2026, and the answer is rarely either-or. With Freddie Mac's 30-year fixed averaging 6.23% the week of April 23, 2026, and the S&P 500 returning roughly 10% nominally over the long run, the spread between the two options is real money — but the spread alone does not tell the whole story.
This guide walks the math, shows where prepaying actually wins, and explains the hybrid approach most homeowners settle on. None of this is personalized advice; the goal is to make the trade-offs concrete so you can decide on your own numbers.
The decision in one minute
The classic rule of thumb is to compare your after-tax mortgage rate to the expected after-tax return on your investments. If a diversified investment portfolio is reasonably likely to outpace your mortgage rate over your remaining loan term, the math favors investing. If the spread is thin or negative, prepaying becomes more attractive because the return is guaranteed.
In 2026, with mortgage rates in the low-6s and long-run stock returns near 10% nominally (about 7% after inflation), most homeowners with a long horizon are looking at a positive expected spread — but it comes with risk, taxes, and behavioral wrinkles that change the answer at the margins.
The math: how much each dollar earns
When you make an extra principal payment, you earn a guaranteed return equal to your loan's interest rate. A $1,000 prepayment on a 6.23% mortgage saves you roughly $1,000 × 6.23% = $62.30 of interest in year one, and that savings compounds because next year's interest is calculated on a smaller balance.
When you invest that same $1,000 in a diversified stock-and-bond portfolio, your return is expected but not guaranteed. The S&P 500's nominal long-run average is roughly 10% per year and about 6–7% after inflation, according to Macrotrends' 1928–2026 dataset. Bonds historically earn less. A typical 80/20 stock-bond mix has averaged somewhere in the 8–9% nominal neighborhood over multi-decade periods.
The headline spread — expected investment return minus mortgage rate — is usually positive at today's rates, but you do not get the headline. You get the headline minus taxes, minus fees, minus sequence-of-returns risk, and minus whatever your behavior costs you.
Taxes change the after-tax rate
Most US homeowners take the standard deduction ($15,000 single, $30,000 married filing jointly for 2025 returns per IRS Topic No. 551). If you don't itemize, your mortgage interest is already not reducing your federal tax bill, so the rate you "really" pay is the full coupon rate. Investment gains in a taxable brokerage are typically taxed at the long-term capital gains rate (0%, 15%, or 20% depending on income, per IRS Topic No. 409).
If your investments are inside a tax-advantaged account — a 401(k), IRA, Roth IRA, or HSA — the after-tax math becomes much more favorable to investing because growth is tax-deferred or tax-free.
A worked example: $50,000 to deploy over 15 years
Imagine you have a fresh 30-year mortgage of $400,000 at 6.23%. Your principal-and-interest payment is about $2,460/month. You also have $50,000 sitting in a savings account and want to make it work harder. You consider three paths over the next 15 years.
| Path | What you do | End-of-year-15 outcome (illustrative) |
|---|---|---|
| A. Lump-sum prepayment | Drop $50,000 on principal day one; keep the same monthly payment | Mortgage paid off ~5.5 years earlier; ~$132,000 in lifetime interest avoided |
| B. Invest lump sum | Put $50,000 in a diversified index fund at ~7% real return | Portfolio grows to roughly $138,000 in real (inflation-adjusted) dollars |
| C. Hybrid | Split: $25,000 prepay, $25,000 invest | ~$66,000 lifetime interest avoided + ~$69,000 portfolio |
The investing path comes out ahead in expected dollars at a 7% real return. Path A is the least risky and the most psychologically satisfying. Path C captures most of the upside while keeping a meaningful guaranteed return.
Show the prepay math
On a $400,000 / 30-year / 6.23% loan, the standard amortization schedule has you paying $483,725 in interest over the full term (just under $884,000 total). A one-time $50,000 principal drop in month one trims roughly $132,000 of that interest and shortens the payoff by about five and a half years — you can verify any specific scenario in our mortgage calculator.
Show the invest math
$50,000 compounding at 7% real for 15 years equals $50,000 × 1.07^15 ≈ $137,952 — that is in today's dollars. At a less optimistic 5% real, it grows to about $103,946. At a 10% nominal return (no inflation adjustment), it grows to roughly $208,862. You can re-run any return scenario in our investment return calculator or the compound interest calculator.
When investing wins (and by how much)
Investing tends to beat prepaying when several conditions line up: your mortgage rate is meaningfully below the long-run expected return on your portfolio; you have a long time horizon; the money is going into a tax-advantaged account; you have not yet captured your full employer 401(k) match (a 50% or 100% instant return); and you have the discipline to keep buying when the market falls 30%.
The single highest-priority dollar in most household budgets is the one that earns the employer 401(k) match. According to a 2024 Vanguard "How America Saves" report, the average company match is around 4.6% of pay; that is a guaranteed 50–100% same-year return that no 6.23% mortgage can match. Skipping the match to prepay a mortgage is almost never optimal.
When paying off early wins
Prepaying climbs the priority list when your mortgage rate is high relative to plausible portfolio returns, when you are within ~10 years of retirement, when your tax-advantaged accounts are already maxed, when you would otherwise invest in a low-yield savings account or money-market fund (the FDIC's national savings average is just 0.38% as of April 2026), or when carrying debt into retirement would force you to draw more from a portfolio at a bad time.
- Behavioral fit: if a paid-off house lets you sleep at night and stay invested in the rest of your portfolio, that is worth real money.
- Sequence-of-returns risk: if you retire into a bear market, a smaller mortgage means smaller required withdrawals from a falling portfolio.
- Loan type: homeowners with adjustable-rate, FHA, or non-conforming loans sometimes have less attractive economics than a 30-year fixed at 6.23% — we cover the rate-side trade-offs in our ARM vs fixed-rate guide.
The hybrid strategy most people use
In practice, most personal-finance plans don't pick one extreme. A common ordering of dollars looks like this:
- Capture the full 401(k) match. Free money first.
- Build a 3–6 month emergency fund in a high-yield savings account — well-known online banks were offering up to about 5.00% APY as of late April 2026 (per Fortune's daily HYSA roundup).
- Pay off high-interest debt (credit cards, anything north of ~8–10%). A 6.23% mortgage does not belong in this bucket; a 24% credit card does.
- Max tax-advantaged retirement — the 2026 employee 401(k) deferral limit is $24,500 and the IRA limit is $7,500 (both per IRS Notice 2025-67, October 2025).
- Invest in taxable brokerage for medium-term goals.
- Make extra principal payments on the mortgage with whatever remains, especially in the last 10–15 years before retirement.
If you want to prepay without changing your monthly cash flow, look at biweekly payments — effectively making one extra full payment a year. We walk through the savings in our biweekly mortgage payments guide.
Five factors most calculators ignore
The clean spreadsheet answer rarely survives contact with real life. Five things tilt the decision in ways that pure spreadsheets miss.
1. Liquidity
A dollar in your brokerage can be sold and used for a medical bill or a job loss within days. A dollar in your mortgage principal can only come back out via a HELOC, cash-out refinance, or sale. If your emergency reserves are thin, prepaying turns liquid cash into illiquid equity, which is the wrong direction.
2. Behavior
The biggest predictor of whether the "invest" path wins is whether you stay invested through the next 30% drawdown. If you sold during 2008 or 2020, the math has to come down quite a bit before investing pays off in your hands. A guaranteed 6.23% prepayment return is also one you cannot sabotage.
3. Inflation
A fixed-rate mortgage is one of the few financial products that gets cheaper every year inflation runs above your loan's effective rate. If inflation averages 3%, a 6.23% mortgage carries a real cost closer to 3.2%. Holding a long mortgage in a high-inflation regime is not the same as holding it in a deflationary one.
4. Risk premium
Stocks have averaged ~7% real over a century, but with deep multi-year drawdowns. A guaranteed 6.23% on the mortgage and an expected 7% on stocks are not the same kind of "return" — one is risk-free at the margin, the other is risky. How much premium you need to take that extra risk is personal.
5. Age and runway
Time horizon dominates. At 30 with 35 years to retirement, you can ride out three bad decades and still come out ahead investing. At 60 with five years left and a mortgage you would like gone, the calculus narrows quickly.
How to actually make the decision
A practical framework, in three steps:
- Run your numbers in our mortgage calculator with and without an extra principal payment to see exactly how much interest you would save and how many months of payments disappear.
- Project the alternative in our investment return calculator at three return scenarios (conservative 4% real, base 7% real, optimistic 9% real) so you understand the range, not just the average.
- Pick the split that fits your sleep. There is no prize for getting the spreadsheet exactly right. A 50/50 split between extra principal and a tax-advantaged account is hard to beat for most middle-income households.
Frequently Asked Questions
Is it worth paying off a 3% mortgage early?
Generally no. A 3% mortgage is below long-run inflation and well below expected stock or bond returns, so the math strongly favors investing. The exception is if peace of mind from a paid-off house is worth more to you than the expected investment surplus.
Should I pay off a 7%+ mortgage instead of investing?
The case becomes much closer. A 7% guaranteed return is competitive with a 7% expected real stock return without the risk. Most analysts still recommend investing while paying the mortgage on schedule, but a hybrid (extra principal + tax-advantaged investing) is reasonable.
Does the mortgage interest deduction change the answer?
Only if you itemize. Most filers take the standard deduction ($15,000 single / $30,000 MFJ for tax year 2025 per IRS Topic No. 551). For itemizers, the after-tax mortgage rate is roughly the coupon rate × (1 − marginal tax rate); a 6.23% loan effectively becomes around 4.7% at a 24% bracket, which tilts further toward investing.
What about a recast or refinance instead of prepaying?
If you want lower required payments without giving up the principal payoff, a recast can re-amortize your loan after a lump-sum principal drop for a small fee. A refinance at a lower rate can save more if rates have dropped meaningfully and you'll keep the loan long enough to break even on closing costs.
Should I prepay before maxing my 401(k) or IRA?
Almost never — especially before capturing the full employer match, which is a one-time, immediate 50–100% return on the matched portion. Tax-advantaged retirement contributions also shield growth from taxes for decades.
Is a paid-off house an "investment"?
Not in the conventional sense. Your home appreciates with regional housing markets, but it does not pay dividends or rent. The "return" on prepaying is the interest you avoid, which is real but does not compound the way invested assets do.
What's the simplest rule of thumb?
If your after-tax mortgage rate is materially below the expected after-tax return on a diversified portfolio and you have a long horizon, lean toward investing. If the spread is small or negative, lean toward prepaying. When in doubt, split the difference.
This article is for general informational purposes only and is not financial, tax, or investment advice. Figures reflect conditions as of April 2026 and may change. Consult a qualified financial professional before making decisions about your money.