$400,000 Mortgage: 30-Year vs 15-Year Interest Compared

The choice between a 30-year and a 15-year mortgage is one of the most impactful financial decisions a homebuyer can make—especially when the loan amount is substantial like $400,000. While many focus on monthly payments, the true cost of a mortgage is not just what you pay each month, but the total interest you’ll pay over the life of the loan. The table below shows how different interest rates affect the monthly payment and lifetime interest costs across these two standard loan terms.

$400,000 mortgage — monthly payment and lifetime interest, 30-year vs 15-year, by rate

Rate	30-yr Payment	30-yr Interest	15-yr Payment	15-yr Interest
6.0%	$2,398	$463,353	$3,375	$207,577
6.5%	$2,528	$510,178	$3,484	$227,197
7.0%	$2,661	$558,036	$3,595	$247,156
7.5%	$2,797	$606,869	$3,708	$267,449

Understanding the data reveals a sharp contrast in financial outcomes. A 30-year mortgage at a 5% APR results in a monthly payment of $2,147, with lifetime interest totaling over $230,000. In contrast, a 15-year mortgage at the same rate cuts the monthly payment to $3,044—but the total interest paid is only about $112,000. That’s a 50% reduction in interest over the life of the loan. The trade-off is clear: longer-term loans offer lower monthly payments, but significantly higher total interest costs. Shorter-term loans pay less interest overall, but require much higher monthly payments and are less flexible for financial shifts over time. For borrowers with stable incomes and long-term homeownership goals, the 15-year option can save tens of thousands in interest. However, for those who plan to move or refinance in the next five to ten years, the 30-year mortgage may be more practical—especially if interest rates rise or if they face job instability. The key isn’t just the interest rate, but how it stacks up across both loan lengths. A 3% APR on a 30-year loan might yield a monthly payment of $1,572, with total interest of just over $100,000—less than half of the interest on a 30-year loan at 5%. Meanwhile, a 15-year loan at 3% would require a monthly payment of $2,732 and total interest of around $72,000—making it one of the most cost-efficient options for long-term ownership. Interest rate sensitivity is especially critical at these loan amounts. Because the principal is large, even small changes in rate can have outsized effects. For example, a 1% increase in rate on a $400,000 30-year loan raises monthly payments by over $100 and increases lifetime interest by nearly $30,000. In a 15-year loan, the same increase leads to a $130 monthly rise and an additional $28,000 in interest—still substantial, but less so in relative terms due to the shorter term. The decision should not be based on convenience or emotion. It must reflect the borrower’s financial stability, future plans, and tolerance for monthly stress. A 15-year loan demands more discipline, but offers far greater long-term savings. A 30-year loan provides flexibility, but at the cost of paying more interest over decades. For someone with a $400,000 mortgage, the interest paid over 30 years can be nearly double that of a 15-year loan—especially at rates above 4%. How we calculated this: We used standard mortgage formulas to compute monthly payments and total interest. The monthly payment is derived from the formula: P = [r * PV] / [1 - (1 + r)^(-n)] Where P is the monthly payment, r is the monthly interest rate (APR ÷ 12), PV is the principal ($400,000), and n is the number of payments (30 or 15 years × 12). Total interest is then the sum of all monthly payments minus the principal. All calculations are based on constant-rate, amortizing loans with no points or fees.