The conventional wisdom says a 15-year mortgage saves you a fortune in interest. That's true in isolation — but it ignores what you could do with the money you save each month on the lower 30-year payment. When you account for opportunity cost, the answer isn't as obvious as most articles suggest.
Let's use a concrete example: a $400,000 home with 20% down ($80,000), leaving a $320,000 mortgage. Married filing jointly, $150,000 household income, in Ohio.
| 15-Year at 6.0% | 30-Year at 6.75% | |
|---|---|---|
| Monthly P&I | $2,700 | $2,076 |
| Total interest paid | $166,000 | $427,000 |
| Monthly difference | $624/month | |
On the surface, the 15-year loan saves you $261,000 in interest. That's the number that gets people excited. But there's a hidden cost.
With the 30-year mortgage, you pay $624 less per month. If you invested that $624 every month at a 7% annual return (a reasonable long-term stock market assumption), after 15 years you'd have approximately $195,000 in investments. After 30 years, you'd have substantially more.
The question isn't just "how much interest do I save?" — it's "does the interest savings exceed what I'd earn by investing the difference?"
Key insight: If your expected investment return exceeds your mortgage rate, the 30-year mortgage can actually leave you wealthier despite paying more interest. The math depends on your specific rates, time horizon, and tax situation.
The 15-year mortgage is the better financial choice when you wouldn't actually invest the savings — and most people wouldn't. If that extra $624/month would go toward lifestyle spending rather than an index fund, the forced savings of the 15-year mortgage is a guaranteed win. You can't spend money that goes toward principal paydown.
The 15-year also wins if you're risk-averse. Paying off your mortgage is a guaranteed return equal to your interest rate. Investment returns are not guaranteed. If sleeping well at night matters more than theoretical optimization, the 15-year is the right call.
The 30-year wins when you are disciplined enough to invest the monthly savings consistently, when your investment return exceeds your mortgage rate, and when you value the flexibility of a lower required payment. Life happens — job loss, medical expenses, a child's tuition — and having a lower mandatory payment provides a financial cushion the 15-year doesn't offer.
There's also a tax consideration. Mortgage interest is deductible (up to $750,000 in debt), which effectively reduces your mortgage rate. A 6.75% rate at a 22% marginal tax bracket is really a 5.27% after-tax cost — making it even easier for investment returns to exceed.
Most people don't stay in a home for the full loan term. If you're planning to sell in 7-10 years, the comparison changes significantly. In the early years, a 15-year mortgage builds equity much faster — after 7 years, you'd have about $175,000 in equity on the 15-year versus $55,000 on the 30-year (excluding appreciation). That equity difference can be worth more than the investment returns on the monthly savings.
Enter your home price, rates, and time horizon. See the true cost difference including taxes, equity, and opportunity cost.
Compare mortgages free →If you can comfortably afford the higher 15-year payment and you know you'll stay in the home long-term, the 15-year mortgage is a reliable wealth-building tool. If you need the flexibility of a lower payment, want to maximize investment returns, or might move within 10 years, the 30-year with disciplined investing of the difference can come out ahead. Don't let anyone tell you the answer is obvious — it depends on your numbers.