How Much Interest a $400,000 Mortgage Costs Over 30 Years

The decision between a 30-year and a 15-year mortgage is one of the most significant financial choices a homebuyer can make—especially when the loan amount is $400,000. The table below shows how monthly payments and total lifetime interest vary across different interest rate ranges for each term, offering a clear view of the trade-offs between affordability and cost over time.

How Monthly Payments Differ by Term and Rate

A 30-year mortgage spreads payments over a longer period, resulting in lower monthly payments compared to a 15-year loan. However, this comes at the cost of significantly more interest paid over the life of the loan. For a $400,000 mortgage, at a 5% interest rate, a 30-year loan has a monthly payment of about $2,390, while a 15-year loan requires nearly $3,400 per month—more than 40% higher. This difference grows at higher rates, where the gap widens due to compounding interest. Even with the same interest rate, the 15-year loan pays substantially less total interest. For example, at a 6% rate, a 30-year loan accumulates over $300,000 in interest, while the 15-year version pays just under $150,000—half the amount. This means homeowners who plan to stay in their homes long-term may benefit from the 15-year option, even with higher monthly payments, because they pay off the loan faster and avoid decades of interest accumulation.

Interest Rate Sensitivity and Long-Term Cost

The table reveals that small shifts in interest rates have a dramatic impact on total interest paid over time. At the lower end of the rate spectrum—say, 3% to 4%—a 30-year loan still carries more interest than a 15-year one, though the difference is less pronounced. However, at higher rates (like 7% or above), the 15-year loan becomes a far more cost-effective choice. In those cases, the 30-year loan could add hundreds of thousands of dollars in interest, while the 15-year option cuts that cost by nearly half. This sensitivity underscores a key principle: choosing a mortgage term is not just about current affordability—it’s about long-term financial health. A borrower who expects to stay in their home for 15 years or more should consider the 15-year term as a way to reduce total interest, even if the monthly payment is higher. For those who plan to move or refinance within a decade, the 30-year option may offer more flexibility, though at a greater lifetime cost.

When Each Term Makes Sense in Real Life

A 30-year mortgage is often preferred by first-time buyers or those with limited savings, as it offers manageable monthly payments. But for homeowners with stable incomes and long-term plans, the 15-year mortgage can be a smart choice—especially when interest rates are near or above 5%. It reduces the total amount of interest paid and helps build equity faster. Conversely, a 30-year loan may make sense if a borrower anticipates financial strain in the future—such as job loss or a major life event—because the lower monthly payment provides a buffer. However, this comes at the cost of decades of interest. For example, at a 6% rate, a 30-year loan adds over $300,000 in interest, while the 15-year version cuts that by nearly half. The choice ultimately depends on personal financial goals, income stability, and how long one plans to live in the home.

How We Calculated This

The numbers in the table were derived using standard mortgage amortization formulas. For each interest rate and term (15-year or 30-year), we applied the present value of an annuity formula to calculate monthly payments and total interest paid over the life of the loan. The total interest was computed by summing up all monthly payments minus the original principal. The data reflects current market interest rate ranges and assumes no refinancing, no prepayment, and no changes in interest rates.

$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