The 15-year mortgage saves you a quarter-million dollars in interest. But the 30-year might still be the smarter financial choice — if you do one specific thing with the money you save each month. Here are the exact numbers.
7 min read · startinvesting.ai
Using a realistic example: $400,000 home, 20% down payment ($80,000), $320,000 loan, 7% interest rate.
| 30-Year | 15-Year | |
|---|---|---|
| Monthly P&I Payment | $2,129 | $2,876 |
| Total Payments | $766,440 | $517,680 |
| Total Interest Paid | $446,440 | $197,680 |
| Interest Savings (15-yr) | — | $248,760 |
| Payoff | Year 30 | Year 15 |
$320,000 loan at 7% interest. Principal & interest only — does not include taxes, insurance, or PMI.
The 15-year mortgage saves $248,760 in total interest and you pay off the home 15 years earlier. That's genuinely significant. But the monthly payment is $747/month higher — and what you do with that $747 is the entire debate.
The counterargument to the 15-year mortgage: take the 30-year, and every month invest the $747 you're saving in a low-cost S&P 500 index fund instead.
$747/month invested at an 8% average annual return for 30 years grows to approximately $1,110,000.
Compare: the 15-year mortgage saves you $249,000 in interest. Investing the difference produces $1,110,000 in wealth. Even after paying the extra $249,000 in mortgage interest, you're ahead by roughly $860,000.
On paper, the math strongly favors the 30-year + invest strategy. But there's a massive asterisk.
The catch: you have to actually invest the difference
Studies consistently show that most people who take the 30-year intending to invest the difference… don't. The money gets absorbed into lifestyle spending. The 15-year mortgage is a forced savings mechanism — it's harder to spend money you never see.
There's another factor that strengthens the case for the 15-year: lenders typically offer 15-year mortgages at 0.5–0.75% lower rates than 30-year mortgages. At the time of writing, the national average for a 30-year is around 7%, while a 15-year might be 6.3%.
Using more realistic rates — 7% for 30-year, 6.3% for 15-year — the total interest savings from the 15-year widen further. The monthly payment comparison:
A popular middle ground: take the 30-year for payment flexibility, but make extra principal payments when finances allow — essentially self-imposing a faster payoff schedule without the obligation.
An extra $300/month in principal payments on a 30-year mortgage at 7% will pay off the loan in about 21 years and save roughly $140,000 in interest. Not as good as the 15-year, but with complete flexibility to stop extra payments if your income changes.
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Open Mortgage Calculator →Is a 15-year mortgage worth it?
A 15-year mortgage saves roughly $249,000 in total interest on a $320,000 loan at 7% compared to a 30-year. It's worth it if you can comfortably afford the higher monthly payment, you're risk-averse, and you won't reliably invest the monthly payment difference in the market.
How much more do you pay monthly on a 15-year vs 30-year mortgage?
On a $320,000 loan at 7%, a 15-year mortgage costs about $2,876/month vs $2,129/month for a 30-year — a difference of $747/month. Your payment is 35% higher, but you pay off the home in half the time.
What is the 'invest the difference' strategy?
Instead of choosing a 15-year mortgage, you take the 30-year and invest the monthly payment difference ($747/month) in an S&P 500 index fund. Over 30 years at 8% average annual return, that $747/month grows to approximately $1.1 million — far exceeding the $249,000 in interest savings from the 15-year. The catch: you have to actually invest the difference.
Should I get a 30-year mortgage and invest the difference?
Mathematically, 'invest the difference' usually wins over a 15-year mortgage — the stock market's historical returns outpace the interest savings. But this only works if you actually invest the money consistently for 30 years. Many people spend it instead. If you're disciplined and will invest, 30-year often wins on paper. If you're not sure, the 15-year is the forced-savings version.
Which mortgage term is better for first-time buyers?
Most first-time buyers choose the 30-year for the lower monthly payment and cash flow flexibility. It leaves more room for unexpected expenses, furnishing costs, and building an emergency fund. You can always make extra principal payments on a 30-year to pay it off faster — but you can't reduce payments on a 15-year if your income drops.
Does the 15-year mortgage always have a lower interest rate?
Usually, yes — 15-year mortgage rates are typically 0.5–0.75% lower than 30-year rates. This makes the 15-year even more attractive on a pure interest basis. In the example above using the same 7% rate, the actual savings would be even greater if you used realistic rate differentials.
For educational purposes only. Not financial advice. Mortgage calculations are estimates. Actual rates, payments, and savings vary by lender, credit score, and loan terms. Investment return projections assume 8% average annual return — actual returns vary. Consult a licensed mortgage professional before making home financing decisions.