Understand exactly how loan interest is calculated — with worked examples for simple interest, compound interest, and monthly amortized payments like mortgages and car loans.
Loan interest is one of the most important numbers in personal finance, yet most borrowers have no idea how it is actually calculated. Understanding the math behind your mortgage, car loan, or student loan lets you spot bad deals, compare offers fairly, and plan for early repayment.
This article walks through three interest models from the simplest to the most realistic: simple interest, compound interest, and monthly amortization. Each one comes with a step-by-step worked example using numbers you can verify on paper or in a calculator.
Interest is the price of borrowed money. When you take out a loan, the lender charges a percentage of the outstanding balance as a fee for letting you use their capital. That percentage, applied over a period of time, is the interest rate.
Three inputs control every interest calculation: the principal (the amount borrowed), the rate (expressed as a percent per year), and the term (how long the loan lasts). How those three interact depends on which interest model applies.
Simple interest is the easiest model. Interest is charged only on the original principal, never on previously accrued interest.
Formula: Interest = Principal × Rate × Time
You borrow $5,000 at a simple annual rate of 6% for 3 years.
Simple interest is rare in consumer loans but common in short-term personal loans between individuals and in some auto finance contracts.
Compound interest charges interest on the principal AND on any interest already accrued. This is how savings accounts grow and how credit card balances balloon.
Formula: A = P × (1 + r / n)^(n × t)
Where A is the future amount, P is principal, r is the annual rate as a decimal, n is the number of compounding periods per year, and t is the number of years.
You owe $5,000 on a credit card at 18% annual interest compounded monthly for 3 years, with no payments.
Compare that to simple interest on the same numbers — $2,700. Compounding added $845 over three years on a $5,000 balance. This is why minimum credit card payments are such a trap.
Mortgages, car loans, and most personal loans use monthly amortization. You pay the same amount every month. Early payments are mostly interest; later payments are mostly principal. By the final payment the loan is exactly paid off.
Monthly payment formula: M = P × [ r(1 + r)^n ] / [ (1 + r)^n − 1 ]
Where M is the monthly payment, P is principal, r is the MONTHLY interest rate (annual rate divided by 12), and n is the total number of monthly payments.
You take a $200,000 mortgage at 6% annual interest for 30 years.
That last number is the one that changes how most people think about mortgages. On a 30-year loan at 6%, you pay more in interest than you originally borrowed.
An amortization schedule shows, for each monthly payment, how much goes to interest and how much goes to principal. Early in the loan the interest portion dominates because the outstanding balance is high. As the balance shrinks each month, less interest accrues, so more of your payment chips away at principal.
On the $200,000 mortgage example, the very first payment of $1,199.10 splits as $1,000 interest (200000 × 0.005) and $199.10 principal. The second payment applies against a slightly lower balance: the interest portion is $999.00 and the principal portion is $200.10. By month 360 the split is roughly $6 interest and $1,193 principal.
The APR (Annual Percentage Rate) on a loan is typically a bit higher than the stated interest rate. APR folds in mandatory fees — origination charges, points, private mortgage insurance — so two loans with the same rate but different fees can have different APRs.
When comparing offers, compare APRs, not just headline rates. A loan with a 5.9% rate and $4,000 in fees is more expensive than a loan with a 6.1% rate and zero fees, even though the rate looks lower.
Because amortized loans front-load interest, any extra dollar you put toward principal early in the loan has an outsized effect. On the $200,000 mortgage example, paying an extra $100 every month cuts about 4.5 years off the term and saves roughly $46,000 in interest.
The reason is simple: every extra dollar of principal removes all the interest that dollar would have generated over the remaining months of the loan. The earlier the prepayment, the more interest it eliminates.
Simple interest is rare, compound interest is how balances grow when unpaid, and monthly amortization is how most real-world loans work. Plugging your own numbers into the right formula reveals the true cost of borrowing — and almost always makes a stronger case for paying loans down faster than the contract requires.
For fast scenario modeling, the free Loan Calculator on the Toolific Hub computes your monthly payment, total interest, and total repaid from a principal, rate, and term.