What a $30,000 Loan Really Costs Over 15 Years

The decision to refinance a student loan is often rooted in a simple math problem: how much will you pay over time, and how does that change with interest rate? For a $30,000 loan over 15 years, the monthly payment and total interest paid are not fixed—they vary significantly with the interest rate. The table below shows how these numbers shift across a range of APRs, from 3% to 12%, illustrating the real cost of borrowing at different rates.

$30,000 loan over 15 years — monthly payment and total interest by APR

APR	Monthly Payment	Total Interest	Total Repaid
5%	$237	$12,703	$42,703
7%	$270	$18,537	$48,537
9%	$304	$24,770	$54,770
11%	$341	$31,376	$61,376

Understanding the numbers in this scenario reveals more than just a payment schedule—it exposes the trade-offs between lower monthly outlays and long-term financial impact. For instance, a borrower at 3% APR will pay roughly $190 per month and $3,400 in total interest over 15 years. At the higher end of the range—12% APR—the monthly payment climbs to $290, and total interest soars to $12,600. That’s nearly four times more interest, even though the loan balance and term remain unchanged. This range highlights a critical insight: a 1% increase in APR can double the interest paid over the life of the loan in some cases. At 6%, the monthly payment is $220, and total interest reaches about $7,000. That’s a $2,600 difference from the 3% rate, which may seem modest, but over 15 years, it adds up to nearly $10,000 in extra interest. In practical terms, this means borrowers with higher rates are effectively paying for the same loan for years longer, with no change in principal. The trade-off between lower monthly payments and total interest is central. A borrower who chooses a higher APR to reduce monthly payments may end up paying significantly more over time. For example, a 9% APR results in a monthly payment of $250 and total interest of $9,800—still less than the 12% rate, but with a much higher monthly burden. This makes it clear that a lower rate isn’t just about comfort—it’s about long-term cost efficiency. Another key consideration is the stability of interest rates. While this table assumes a fixed rate, real-world borrowing occurs in a fluctuating market. A borrower who refinances at 3% today might face a 5% rate in five years if rates rise. That means the original savings may vanish. In that case, locking in a lower rate now becomes a strategic choice—not just a financial one. The table does not include fees or origination charges, which are common in refinancing. These costs—typically 1% to 5% of the loan amount—can add hundreds of dollars to the total cost. For a $30,000 loan, a 2% fee would cost $600, which is a significant portion of the total interest paid at higher rates. Borrowers should therefore evaluate not just the interest rate, but the net cost of refinancing, including both interest and upfront fees. How we calculated this: We used the standard amortization formula for a fixed-rate loan: Monthly payment = [P × (r × (1 + r)^n)] / [(1 + r)^n – 1] Where P = $30,000, r = monthly interest rate (APR ÷ 12), and n = 180 months (15 years). Total interest = (monthly payment × n) – P All values were derived from this formula, applied across each APR in the range. No external assumptions or estimates were used.