The price of borrowing money
When you borrow money — from a bank, a credit card, a car dealership, or even a friend who charges interest — you agree to pay it back plus an extra amount. That extra amount is interest.
Interest is the price the lender charges for letting you use their money. It compensates them for the risk that you might not pay it back, and for the fact that they can't use that money while you have it.
Understanding how interest works — and how quickly it can turn a small loan into a large one — is one of the most practical financial skills available.
The basic mechanics
When you borrow $1,000 at 10% annual interest, you owe:
- The principal: the $1,000 you borrowed
- The interest: $100 for the first year (10% of $1,000)
If you pay it back in one year, you pay $1,100 total. Simple enough.
But most loans aren't paid back in one lump sum. They're paid back in monthly installments over months or years. During that time, interest accumulates on whatever balance remains.
This is where it gets important to understand — and where many people are surprised by the actual total cost of borrowing.
What APR tells you
APR stands for Annual Percentage Rate. It represents the yearly cost of a loan, including both the interest rate and any fees, expressed as a percentage.
When comparing loans, look at the APR rather than just the interest rate. A loan with a 6% interest rate but high origination fees might have an APR of 7.5%, making it more expensive in total than a loan advertised at 6.9% with no fees.
Some common APRs to know:
- Federal student loans: around 5–8% depending on the year and loan type
- Car loans: 5–12% for typical borrowers, varies by credit score
- Personal loans: 8–20% typically
- Credit cards: 15–30% for most cards
The higher the APR, the more expensive borrowing becomes — and the more urgently you want to pay it down.
A concrete example that makes it real
Here's an example that illustrates why interest rate and repayment timeline together determine total cost:
A teenager takes out a $5,000 personal loan at 12% APR to buy a used car.
- If they repay in 1 year: total paid is roughly $5,340 — interest cost of $340
- If they repay in 3 years: total paid is roughly $5,983 — interest cost of $983
- If they repay in 5 years: total paid is roughly $6,668 — interest cost of $1,668
The loan amount is identical. The interest rate is identical. The only variable is how long they take to repay. But extending the repayment from 1 year to 5 years nearly quintuples the total interest paid.
This is why financial advisors generally recommend paying off debt as quickly as circumstances allow.
Credit card interest: the expensive kind
Credit card interest deserves its own explanation because it operates differently from installment loans.
Credit cards charge interest only on balances you carry — amounts not paid in full by the due date. If you pay your full balance every month, you pay no interest at all. The card is essentially free to use.
But if you pay only the minimum payment, interest is charged on the remaining balance, typically at 18–30% APR. At 24% APR:
- A $500 balance with minimum payments takes roughly 3 years to repay and costs about $700 total
- A $2,000 balance with minimum payments can take over 8 years and cost more than $3,200
The gap between what was borrowed and what is repaid is significant, and it grows with the balance and the rate.
How to use this knowledge
For any borrowing decision:
- Know the APR
- Know the repayment period
- Calculate (or look up using a free loan calculator) the total amount you'll actually pay
Compare that total cost to what you're getting. A $5,000 car that ends up costing $6,668 might still be worth it if the car is necessary and reliable. A $1,200 gaming system that costs $1,800 on a payment plan with high APR is a harder case to make.
The goal isn't to never borrow. It's to borrow with clear eyes about what borrowing actually costs.
Finly teaches teenagers about debt, interest, credit, and every real-world money concept before they need it — completely free. Start at learnfinly.com and borrow only when you actually understand the terms.
