When developers talk about buying a home, the conversation always hits the same fork: 15-year or 30-year mortgage? The 30-year's pitch is a lower monthly payment. The 15-year's pitch is dramatically less interest. Both are true — but the gap is much bigger than most people expect, and it deserves real math instead of a gut feeling.
Run the numbers
Take a $250,000 loan at 6.5%:
- 30 years: about $1,580/month and roughly $318,900 in total interest
- 15 years: about $2,178/month and roughly $142,000 in total interest
The 15-year payment is ~$600/month higher, but it eliminates ~$177,000 in interest — more than half the interest bill. That's not pocket change; that's years of tuition, a serious head start on retirement, or a lot of freedom.
Why the gap is so wide
Amortized loans front-load interest. In year one of a 30-year mortgage at 6.5%, the vast majority of each payment is interest. Over three decades, interest keeps accruing on a large balance. A 15-year loan attacks both levers at once: fewer years of accrual and, typically, a lower rate.
The counterargument: opportunity cost
A 15-year term isn't automatically correct. The extra $600/month could go into index funds instead. If your expected investment returns comfortably beat your mortgage rate, the 30-year can win on paper — but only if you actually invest the difference. Most people don't, which is why the 15-year is effectively a forced-savings machine with a guaranteed 6.5% return.
A simple framework
- Can you handle the 15-year payment without stretching? If yes, the interest savings usually win.
- Would the difference actually be invested? If it would evaporate into spending, the 15-year wins by default.
- What's the rate gap? 15-year rates run lower; a 0.5-1% difference makes the shorter term even more compelling.
Do the math on your own loan
Every loan is different — your rate, amount, and timeline change the answer. Whenever I'm comparing terms, I keep a free loan calculator open in a tab; it shows the monthly payment, total interest, and a full amortization schedule, so I can see exactly when the interest share of each payment drops below half. Five minutes with real numbers beats a year of opinions.
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