How Much Interest a $300,000 Mortgage Costs Over 30 Years — What It Really Means

The decision between a 30-year and a 15-year mortgage isn’t just about duration—it’s a fundamental trade-off in how much you pay over time, how much interest you’ll accumulate, and how your financial obligations shape your long-term stability. For a $300,000 mortgage, the difference in monthly payments and lifetime interest can be substantial, especially when interest rates vary. The table below shows how these figures shift across common APR ranges for each loan term.

$300,000 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%	$1,799	$347,515	$2,532	$155,683
6.5%	$1,896	$382,633	$2,613	$170,398
7.0%	$1,996	$418,527	$2,696	$185,367
7.5%	$2,098	$455,152	$2,781	$200,587

When comparing a 30-year and a 15-year mortgage, the primary distinction lies in the balance between affordability and total cost. A 30-year mortgage offers lower monthly payments, making it easier to manage for those with tighter budgets or variable income. However, over the full term, the total interest paid is significantly higher—often more than double that of a 15-year loan. For instance, at a 6% APR, a 30-year mortgage might result in over $200,000 in interest, while the same loan at 15 years could cost less than $100,000. This difference grows with higher APRs, where the long-term cost of borrowing escalates dramatically. The 15-year option, while demanding higher monthly payments, drastically reduces the total interest burden. This makes it especially attractive for borrowers with stable incomes, strong credit, and a long-term commitment to homeownership. At a 5% APR, a 15-year loan might save over $150,000 in interest compared to a 30-year loan—equivalent to nearly 15% of the original loan amount. These savings compound over time, especially when interest rates are at or near historic lows. Interest rates are the key variable here. The table shows that even small changes in APR can alter the lifetime cost by tens of thousands of dollars. For example, a 1% increase in APR from 5% to 6% can add over $50,000 in interest for a 30-year loan—more than the difference in payments between 15-year and 30-year terms at lower rates. This means that borrowers with longer terms are more sensitive to rate fluctuations, while 15-year borrowers benefit from fixed, lower interest costs over time. When evaluating which term makes sense, context matters. A 30-year mortgage may be ideal for new homeowners with limited savings or those expecting future income increases. A 15-year mortgage is better suited for individuals who can afford higher payments, have a stable income, and are confident in their long-term financial outlook. In either case, the APR range directly influences the total cost of ownership, and choosing one term over another should be based on personal financial goals, not just interest rate levels. It’s also important to consider that mortgage interest is typically tax-deductible. While this benefit applies equally to both terms, it doesn’t change the underlying cost of interest paid—only how much of that cost is offset by tax savings. Therefore, the total interest paid remains the most critical metric for long-term affordability. How we calculated this: We used standard mortgage amortization formulas to compute monthly payments and total interest paid over the life of each loan. The calculations were based on a $300,000 principal, with interest rates ranging from 3% to 7% in 0.5% increments. For each APR and term, we applied the standard formula for monthly payment: M = P [ i(1+i)^n ] / [ (1+i)^n – 1 ] where M is the monthly payment, P is the principal, i is the monthly interest rate (APR/12), and n is the number of payments (term in years × 12). Total interest was then derived by multiplying the monthly payment by the number of months and subtracting the principal. All figures are based on fixed-rate, level-payment mortgages with no prepayment penalties or refinancing.