15-Year vs 30-Year Mortgage: Complete Cost-Benefit Analysis

The choice between a 15-year and 30-year mortgage is one of the most consequential financial decisions a homebuyer makes. The 30-year mortgage offers lower monthly payments but costs dramatically more in total interest. The 15-year mortgage requires higher payments but builds equity faster and saves tens of thousands in interest. This analysis compares both options mathematically and discusses when each makes sense.

The baseline comparison

For a $400,000 mortgage, comparing 15-year and 30-year terms at typical rate spreads (15-year rates are usually 0.25-0.5% lower):

The 15-year mortgage saves $302,500 in interest but costs $847 more per month. Over the life of the loan, the 15-year is dramatically cheaper, but the higher monthly payment may be unaffordable.

The opportunity cost analysis

The real comparison is not just interest saved but what you could earn by investing the payment difference. If you take the 30-year mortgage and invest the $847 monthly difference at 8% return, after 30 years you would have approximately $1,250,000. Meanwhile, after 15 years when the 15-year mortgage is paid off, you could invest the full $3,375 monthly for the remaining 15 years, accumulating approximately $1,170,000 by year 30.

Mathematically, if your investment return exceeds your mortgage rate, the 30-year plus investing comes out ahead. At 8% investment return vs 6.5% mortgage rate, the 30-year plus investing wins by about $80,000 over 30 years. However, this assumes you actually invest the difference consistently for 30 years, which most people do not do.

When the 15-year makes sense

• You can comfortably afford the higher payment (below 28% of gross income)
• You are risk-averse and prefer guaranteed savings over potential investment returns
• You are approaching retirement and want to be debt-free
• You value the psychological benefit of being mortgage-free
• You expect investment returns below your mortgage rate

When the 30-year makes sense

• The higher 15-year payment would strain your budget
• You want to maximize investment contributions (especially tax-advantaged)
• You expect to move within 10-15 years
• You want flexibility to make extra payments when possible but not be obligated
• You expect investment returns above your mortgage rate

The hybrid approach

Many financial advisors recommend a hybrid: take the 30-year mortgage for lower required payments, but make extra payments as if it were a 15-year. This gives you the flexibility to reduce payments if needed (job loss, emergency) while achieving 15-year payoff if everything goes well. Use the sevi.fun Mortgage Calculator and Loan Calculator to model your specific scenario.