The 30-year fixed mortgage is the most popular home loan in America, and for a reason you\'ll recognize immediately: it has the lowest monthly payment. But the 15-year fixed is the one financial advisors quietly recommend. On the same $320,000 loan, the 15-year saves roughly $250,000 in lifetime interest — enough to fund a second home, a college education, or retirement a decade early. So why don\'t more people choose it? Because the math gets complicated fast, and the trade-off isn\'t just about interest.
The side-by-side numbers
Assume a $320,000 loan. In today\'s market, 15-year mortgages typically carry a rate about 0.5% below the 30-year — call it 6.25% vs 6.75%.
- 30-year at 6.75%: $2,075/month, $427,000 total interest, $747,000 total cost
- 15-year at 6.25%: $2,744/month, $174,000 total interest, $494,000 total cost
The 15-year costs $669 more per month but saves $253,000 over the life of the loan. Put another way, the 30-year costs you $1,130 per month in extra interest you wouldn\'t pay on the 15-year.
Why the 15-year rate is lower
Lenders price loans based on risk. A 15-year loan is paid off in half the time, which means half the exposure to interest rate changes, inflation, and borrower default. Investors in mortgage-backed securities reward that lower risk with a lower yield, which flows through as a lower rate to the borrower. The spread between 15 and 30 year rates averages 0.5%–0.75%.
What about the mortgage interest deduction?
The knee-jerk argument against paying off a mortgage faster is "but I lose the tax deduction." Let\'s check that math. For most U.S. households, the standard deduction in 2024 is $14,600 (single) or $29,200 (married filing jointly). You only benefit from the mortgage interest deduction if your itemized deductions — mortgage interest plus state and local taxes (capped at $10,000), charitable contributions, etc. — exceed the standard deduction.
On a $320,000 loan at 6.75%, first-year mortgage interest is about $21,500. Add $10,000 in SALT and you\'re at $31,500 — barely above the MFJ standard deduction. So the mortgage interest deduction "buys" you only a few thousand dollars of extra write-off above what you\'d get anyway. At a 24% tax bracket, that\'s maybe $500–$1,000 a year. Paying $11,000 more in interest to save $750 in taxes is a losing trade.
Opportunity cost: what could you do with the $669?
This is where the decision gets interesting. The $669/month you\'d save by choosing the 30-year isn\'t wasted — you could invest it. Historically, the S&P 500 has returned about 10% annualized. If you rigorously invest the difference every month for 30 years:
- $669/month at 10% for 30 years grows to roughly $1.5 million
- Meanwhile, the extra interest on the 30-year cost you $253,000
- Net advantage: about $1.25 million
That\'s the case for the 30-year in its strongest form. Two caveats: (1) 10% is the long-term average, not a guarantee, and real returns have been lower after inflation; (2) it assumes you actually invest the difference, not spend it. Most people don\'t.
The psychological argument
Behavioral finance research consistently finds that people who take out shorter-term loans end up wealthier than those who take the longer loan and promise themselves they\'ll invest the difference. The 15-year loan forces disciplined savings by making it non-optional — your paycheck has to cover a bigger payment. The 30-year loan only works if you actually invest the spread.
Ask yourself honestly: if I take the 30-year and set up an automatic transfer of $669/month into a low-cost index fund, will I still be doing that in year seven when the kitchen needs a remodel, the car dies, and the kids need braces? For most people, the answer is no.
Who should pick each loan
Pick the 15-year if:
- You\'re buying a home you intend to stay in for 10+ years
- The higher payment is comfortable — not more than 25% of your gross income
- You don\'t have high-interest debt you\'d pay down instead
- You want mortgage-free by retirement
Pick the 30-year if:
- You\'re a disciplined investor who will genuinely invest the difference
- You have variable income and want payment flexibility
- Your career is in a growth phase and your income will outpace the mortgage
- You\'re likely to move or refinance within 7 years
The middle path: 30-year with aggressive prepayment
A popular compromise is taking the 30-year loan for flexibility but voluntarily paying it like a 20-year. You get the safety net of a lower required payment, but by adding an extra principal payment each month you cut years off the schedule. On the $320,000 example, adding $400/month to principal turns the 30-year into a 19-year payoff and saves about $165,000 in interest.
Run your own numbers
Before you sign, run both options through our mortgage calculator. Compare the total interest, the total cost, and honestly ask whether you\'d invest the monthly difference if you took the longer loan.