Compare a 15-year and 30-year mortgage on the same loan. The 30-year payment is lower; the 15-year saves much more interest. The wildcard: what if you invest the difference?
15-year mortgage
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Total interest: —
30-year mortgage
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Total interest: —
Monthly payment difference
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If you took the 30-year and invested the difference for 30 years
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The real trade behind the term choice
Choosing between a 15-year and a 30-year fixed mortgage is not really a question about interest rates. It is a question about cash flow and discipline. The 15-year loan forces a larger payment and pays the house off in half the time, slashing total interest. The 30-year loan asks for far less each month and hands you the difference to do with as you please. This tool runs both loans on the same balance, then takes the popular counterargument seriously by investing the monthly gap and showing who comes out ahead after 30 years.
It is for the buyer who has already been approved and is staring at two amortization options, and for the refinancer deciding whether to shorten the term. Because 15-year rates usually run lower than 30-year rates, the tool lets you set each rate separately rather than assuming they match.
A $400,000 loan, both ways
Borrow $400,000 at 5.75 percent on a 15-year note and the principal-and-interest payment is $3,322 a month, with total interest of about $197,895 over the life of the loan. The same $400,000 at 6.5 percent over 30 years costs $2,528 a month but piles up roughly $510,178 in interest. The 30-year frees up $793 every month. The interest gap alone, more than $312,000, is the headline reason people reach for the shorter term.
| Measure | 15-year at 5.75% | 30-year at 6.5% |
|---|---|---|
| Monthly principal and interest | $3,322 | $2,528 |
| Total interest paid | $197,895 | $510,178 |
| Monthly cash freed up | none | $793 |
| Invested balance after 30 years | $1,058,976 | $720,598 |
Why investing the difference does not win here
The standard advice says take the cheaper 30-year payment, invest the $793 monthly gap, and let the market beat your mortgage rate. At a 7 percent return, investing that gap for 15 years and then letting it compound for another 15 grows to about $720,598. But that comparison forgets what the 15-year borrower does after year 15. Their house is paid off, freeing the entire $3,322 payment. If they invest that full amount through years 16 to 30 at the same 7 percent, they accumulate about $1,058,976. The 15-year-then-invest path wins by roughly $338,000.
The result flips if you assume a much higher investment return, a wider rate spread, or that the 30-year borrower also invests aggressively after their own payoff. The honest takeaway is that the math is close enough that behavior decides it. A 15-year loan is a forced savings plan; the 30-year only wins if you truly invest every freed dollar and never spend it.
The factors this comparison leaves out
A clean side-by-side of principal and interest hides a few real-world costs that can tilt your decision. The mortgage interest deduction, if you itemize, slightly lowers the after-tax cost of the 30-year's larger interest bill, narrowing the gap. Private mortgage insurance, which applies until you reach 20 percent equity, falls away faster on a 15-year because you build equity more quickly. And the opportunity cost cuts both ways: the larger 15-year payment is money you cannot put toward an emergency fund, a Roth IRA, or paying down higher-interest debt. If you are carrying credit card balances or have no cash cushion, the flexibility of the 30-year payment is often worth more than the interest savings of the 15-year, regardless of what the long-run wealth comparison says.
Two questions worth asking yourself
Can I just make extra payments on a 30-year instead?
You can, and it gives you flexibility the 15-year does not. Paying a 30-year like a 15-year shortens it without locking you into the higher required payment, so in a lean month you can drop back to the minimum. The catch is the rate: you usually pay the higher 30-year rate for that optionality, and most people lack the discipline to keep prepaying year after year.
Does the higher 15-year payment hurt my mortgage approval?
It can. Lenders qualify you on the payment, so the larger 15-year payment pushes up your debt-to-income ratio and may shrink how much house you can buy. If you are stretching to qualify, the 30-year is often the only realistic option regardless of the interest math.