Loan Interest Calculator

Loan interest is determined by the principal amount borrowed, the annual percentage rate (APR), the compounding frequency, and the total duration of the term. For standard amortizing loans, the monthly payment (M) is calculated using the formula: M = P [ r(1 + r)^n ] / [ (1 + r)^n - 1 ], where P represents the principal loan amount, r is the periodic interest rate (annual rate divided by the number of payment periods per year), and n is the total number of payments across the entire term. The total interest paid is subsequently found by multiplying your scheduled payment by the total number of periods, and then subtracting the initial principal amount.

Imagine you take out a personal loan of $15,000 with a fixed annual interest rate of 7.5% over a term of 3 years (36 monthly payments). First, convert the annual interest rate to a monthly rate: 7.5% divided by 12 gives 0.00625 per month. Next, apply the amortization formula to find your monthly payment: M = 15000 * [ 0.00625(1 + 0.00625)^36 ] / [ (1 + 0.00625)^36 - 1 ], which results in approximately $466.82 per month. Multiply this monthly payment by 36 months to find the total lifetime payment of $16,805.52. Finally, subtract your original $15,000 principal to reveal a total interest cost of $1,805.52.

To reduce the amount of interest you pay over the life of your loan, consider making bi-weekly payments instead of monthly payments, which results in one extra full payment each year. Additionally, always look for loans with no prepayment penalties so you can make extra principal-only contributions whenever your cash flow allows, shrinking the base upon which future interest is calculated.