How Loan Interest Works
When you borrow money, you pay back the original amount (principal) plus interest — a fee charged by the lender for the use of their money. For most consumer loans, interest is calculated on the remaining balance each month, which means the total interest you pay depends heavily on how quickly you reduce that balance.
Understanding how loan interest is calculated gives you the tools to compare loan offers, decide whether to pay extra, and understand why a longer loan term means more total interest even if monthly payments are lower.
Monthly Payment Formula
M = P × [r(1+r)^n] / [(1+r)^n - 1]
Where:
M = monthly payment
P = principal (amount borrowed)
r = monthly interest rate = APR ÷ 12
n = total payments = years × 12
Example: $15,000 auto loan at 7% APR for 4 years:
r = 0.07 ÷ 12 = 0.005833
n = 48
M = 15000 × [0.005833 × (1.005833)^48]
/ [(1.005833)^48 - 1]
M ≈ $358.90/month
Total repayment: $17,227 | Interest: $2,227What Is Amortization?
Amortization is the process of spreading loan payments over time so each payment covers both interest and some principal. In an amortizing loan, every payment is the same amount — but early payments are mostly interest and later payments are mostly principal. This is because interest is charged on the remaining balance, which starts high and shrinks with each payment.
By the midpoint of a typical mortgage, less than half the original loan may be paid off. This is why paying even a small extra amount each month — applied to principal — has a disproportionate impact on total interest paid.
APR vs. Interest Rate
The interest rate is the base cost of borrowing, expressed annually. APR (Annual Percentage Rate) includes the interest rate plus any lender fees (origination fees, closing costs) expressed as an annualized rate. APR is always equal to or higher than the interest rate. When comparing loan offers, compare APRs — not just interest rates.
For mortgages, the gap between APR and interest rate can be significant (0.1–0.5%) due to closing costs. For personal loans, the gap is often smaller. The Truth in Lending Act (TILA) requires US lenders to disclose APR prominently.
Quick Tips
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A shorter loan term means higher monthly payments but far less total interest.
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Paying one extra monthly payment per year on a 30-year mortgage can cut the term by 4–5 years.
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Always compare APR across lenders — not just the monthly payment or interest rate.
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Refinancing makes sense when you can get a rate at least 0.5–1% lower and plan to stay long enough to recoup closing costs.
Frequently Asked Questions
Why is my first mortgage payment mostly interest?
Because interest is charged on the full remaining balance. On a $280,000 loan at 7%, month 1 interest = $280,000 × (0.07/12) = $1,633. Only the remainder of the payment reduces principal. As the balance falls, the interest portion of each payment shrinks.
What happens if I pay extra each month?
Extra payments applied to principal reduce the balance faster, which means less interest accrues each subsequent month. Over time this shortens the loan and reduces total interest paid substantially. Even an extra $100/month on a 30-year mortgage can save tens of thousands in interest.
Is a lower interest rate always better than a shorter term?
Not necessarily. A lower rate on a longer term can mean more total interest than a higher rate on a shorter term. Always compare total repayment (not just monthly payment) when evaluating loan options.
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