15-year vs 30-year mortgage: which actually saves you more

It is the most consequential checkbox in the entire mortgage process, and most buyers tick it without a second thought: 15-year or 30-year? The 30-year fixed is the default in the US for a reason — it keeps the monthly payment low. But that lower payment hides a much larger lifetime cost. Here is the actual math, and a framework for deciding which term is right for you.

The same loan, two terms

Let's take a concrete example: a $400,000 loan. To keep it fair, we'll give the 15-year a slightly lower rate, because lenders almost always price shorter terms 0.5–0.75 points cheaper — they take on less duration risk, so they charge less for it.

30-year at 6.5%

• Monthly payment (P&I): ~$2,528
• Total interest paid: ~$510,000
• Total paid (principal + interest): ~$910,000

15-year at 5.875%

• Monthly payment (P&I): ~$3,344
• Total interest paid: ~$202,000
• Total paid (principal + interest): ~$602,000

The 15-year payment is about $816 higher per month. In exchange, you pay roughly $308,000 less in interest over the life of the loan, and you own the home free and clear in half the time.

That is the trade in one sentence: the 15-year costs more each month and far less in total. You can run these numbers for your own price and rate with our mortgage calculator.

Why the interest gap is so enormous

Two forces compound here. First, you are borrowing the money for half as long, so interest has half the time to accumulate. Second — and this is the part people miss — a 15-year loan pays down principal much faster from the very first payment.

In month one of the 30-year loan above, about $2,167 of your $2,528 payment goes to interest and only $361 to principal. On the 15-year, a much larger slice hits principal immediately, so every following month accrues interest on a smaller balance. The effect snowballs. This is amortization, and it is why the interest savings are wildly disproportionate to the term difference.

The case for the 30-year

The lower payment is not just a convenience — it is genuine financial flexibility, and flexibility has real value:

• Cash-flow cushion. That $816/month difference is money you keep on hand for emergencies, irregular income, or simply not living at the edge of your budget.
• You can invest the difference. If you reliably invest the $816 each month at a 7% average return, over 30 years that stream compounds to well over $900,000. Whether that beats the guaranteed ~$308,000 interest savings of the 15-year depends entirely on actual market returns and your discipline. Model it with our compound interest calculator.
• You can pay it off early voluntarily. A 30-year with extra principal payments gives you 15-year-like results when you can afford it and 30-year safety when you can't. You keep the option; the 15-year takes it away.

The case for the 15-year

• Guaranteed, risk-free savings. The ~$308,000 is certain. The "invest the difference" argument depends on market returns that may not materialize and discipline most people don't sustain.
• A lower rate. You're borrowing at a cheaper price the whole way.
• Forced discipline. The higher required payment makes you build equity whether or not you feel like investing that month. For many people, "automatic" beats "optional."
• Debt-free years sooner. Owning your home outright in 15 years changes what's possible — retirement, a career change, or simply a much lower cost of living.

The honest decision framework

Don't choose based on which total cost is lower — of course the 15-year wins on paper. Choose based on whether the higher payment is comfortable, not merely possible:

1. Can you afford the 15-year payment with room to spare — still funding retirement, still holding an emergency fund, still able to absorb a job loss? If yes, the 15-year's guaranteed savings are hard to beat.
2. Would the higher payment leave you stretched — no emergency fund, skipping retirement contributions to make it work? Then take the 30-year. A 15-year mortgage you can't sustain is far worse than a 30-year you can. The flexibility isn't a consolation prize; it's the safer financial position.
3. Somewhere in between? Take the 30-year and make extra principal payments in the months you can. You capture much of the savings while keeping the escape hatch.

A note on "investing the difference"

This argument is mathematically real but behaviorally fragile. The 15-year forces the savings; the 30-year only enables them. Studies of actual behavior consistently show most people who take the 30-year intending to invest the difference end up spending some of it. If you are honest with yourself that the money will get spent, the 15-year's forced discipline may be worth more to you than the theoretical edge of investing.

There's also a tax wrinkle in the US: mortgage interest is only deductible if you itemize, and since the 2017 standard-deduction increase, most homeowners no longer do. Don't assume your "effective" mortgage rate is meaningfully below the headline rate unless your tax return confirms it.

The bottom line

The 30-year minimizes your required payment and maximizes flexibility. The 15-year minimizes your lifetime cost and forces you to build wealth. Neither is universally "smarter" — the right answer depends on how comfortably your budget absorbs the higher payment and how honestly you'll invest the difference if you don't.

Run your own numbers before you decide. Plug your price, down payment, and rate into the mortgage calculator, then use the compound interest calculator to see what investing the payment difference would actually grow to. The decision is too expensive to make on a default.