How Interest Rates Work
Interest is the price of money. When you borrow, it is what you pay the lender; when you save, it is what the bank pays you. The rate is usually quoted per year, but how it actually behaves depends on where it is applied. Here is how it works in the places you meet it most.
The two flavours: simple and compound
Simple interest is charged only on the original amount. Compound interest is charged on the original amount plus interest already added, so it grows faster over time. Most savings and long-term debt compound; some short-term loans use simple interest.
How interest works on savings
Your balance earns interest, and that interest then earns interest of its own. Put 1,000 in an account at 4% compounded monthly and after a year you have about 1,041, a little more than the 40 that simple interest would pay, because the interest itself started earning. The more often it compounds (daily beats monthly beats yearly) and the longer you leave it, the more you end up with. See it in action with the Compound Interest Calculator.
How interest works on a loan
On a normal loan you pay a fixed amount each month. Early payments are mostly interest because the balance is large; later ones are mostly principal. A shorter term means higher payments but far less total interest. Try the Loan Calculator.
Fixed vs variable rates
A fixed rate stays the same for the life of the loan or savings product, so your payment or return is predictable. A variable rate moves up and down with a benchmark, so it can start low and rise, or fall when rates drop. Fixed gives certainty; variable gives you whatever the market does next. Which is better depends on where rates are heading and how much predictability you value.
Why do interest rates change?
Most everyday rates ultimately track a benchmark rate set by a country's central bank. When the central bank raises its rate to cool inflation, borrowing gets more expensive and savings pay more; when it cuts to support the economy, the opposite happens. That is why mortgage and card rates move together across a whole market even though no single lender controls them.
How interest works on a credit card
Cards are the most expensive because the rate is high and interest is charged on the remaining balance every month. If you only pay the minimum, most of it goes to interest and the balance barely moves. The Credit Card Payoff Calculator shows how much faster a higher payment clears it.
How interest works on a mortgage and car loan
Both work like an amortising loan: a big balance repaid over years, with interest front-loaded. Even a small difference in rate moves the total by thousands. Use the Mortgage Calculator or Car Loan Calculator to compare.
APR vs APY: read the right number
APR is usually the plain yearly rate, and on borrowing it also folds in compulsory fees; the US Consumer Financial Protection Bureau describes the APR as a broader measure of borrowing cost than the rate alone. APY includes the effect of compounding, so it is a little higher. Savings accounts often advertise APY (it looks bigger); loans often quote APR (it looks smaller). Always compare like with like.
How to calculate daily interest from an APR
Most loans and mortgages charge interest on the daily balance. Convert the annual rate to a daily one, then multiply by the balance:
- Daily rate = APR ÷ 365
- Daily interest = balance × daily rate
Worked example
A $200,000 mortgage at 6% APR: daily rate = 0.06 ÷ 365 = 0.0001644. Daily interest = 200,000 × 0.0001644 = $32.88 a day, which is about $1,000 in a 30-day month. As you pay the balance down, the daily interest falls with it.
"Interest per annum" just means the simple yearly rate before compounding; a bank quoting 6% per annum on a savings balance pays roughly that over a year, a little more if it compounds monthly.
Put it to work: the APY calculator turns a stated rate into the real yearly yield, the future value calculator projects a balance forward, and the debt consolidation calculator compares loan rates side by side.
Frequently asked questions
What is the difference between APR and APY?
APR is the plain yearly rate, and on loans it also includes fees; APY includes compounding, so it is a little higher. Savings usually quote APY, loans quote APR.
Why do interest rates go up and down?
Most everyday rates track a benchmark set by a country's central bank. When it raises the rate to fight inflation, borrowing costs more and savings pay more; when it cuts, the reverse happens.
Why is credit card interest so expensive?
The rate is high and interest is charged on the remaining balance every month, so paying only the minimum barely reduces what you owe.
Does a shorter loan term cost less?
Less in total interest, though the monthly payment is higher. A shorter term clears the balance faster, so less interest builds up.
How do I calculate daily interest from an APR?
Divide the APR by 365 to get the daily rate, then multiply by the balance. At 6% APR, $200,000 accrues 200,000 × (0.06/365) = about $32.88 a day.
How do banks calculate mortgage interest?
Most charge interest on the outstanding daily balance: daily rate (APR ÷ 365) times the balance, summed over the month. As the principal falls, so does the interest portion of each payment.