Interest is the price of money. When the bank pays you for keeping money in a savings account, that’s interest. When you pay extra on a loan or a credit card balance, that’s also interest. Once you understand how it works, both sides start making a lot more sense. This guide explains what interest is, how it’s calculated, and why a small percentage can become a big number over time.
Quick answer: what is interest?
Interest is a fee paid for borrowing money — or a payment received for letting someone else borrow yours. Banks pay interest on savings accounts and CDs because they’re effectively borrowing your money. Lenders charge interest on loans and credit cards because they’re lending money to you.
In short: when money sits with someone else, interest is what changes hands while it’s there.
For Teachers
What Is Interest? (Teacher Lesson) — lesson plan, video, discussion questions, and a printable worksheet for classroom use.
How interest is paid in everyday accounts
You’ll see interest most often in three places:
- Savings accounts and CDs — the bank pays you interest for keeping money on deposit.
- Loans — you pay interest to a lender for the use of borrowed money (auto loans, mortgages, student loans, personal loans).
- Credit cards — you pay interest if you don’t pay the full statement balance by the due date.
In every case, the math is built around the same idea: a percentage of the amount of money sitting there is paid over a period of time.
The basic interest formula
Simple interest is calculated like this:
- Interest = principal × rate × time
Where principal is the amount of money involved, rate is the interest rate (usually expressed as a yearly percentage), and time is the number of years the money is held.
Example: if you put $1,000 in an account paying 4% simple interest for one year, you earn $1,000 × 0.04 × 1 = $40. After two years at the same rate, you’d earn $80 in total.
Most real accounts don’t use simple interest, though. They use compound interest.
Simple interest vs. compound interest
This is the most important interest concept to understand.
Simple interest
Simple interest is calculated only on the original principal. The interest doesn’t earn its own interest. You see this on some short-term loans, car loans, and student loans — the interest is figured on the starting balance, payment after payment.
Compound interest
Compound interest is calculated on the principal plus the interest already earned. So next month, you earn interest on a slightly larger balance, and the month after that on an even larger one. Over time, this snowballs.
Most savings accounts, CDs, and credit cards use compound interest. So do almost all investment accounts.
The difference, in numbers
Say you deposit $1,000 at 5% per year. Here’s what you’d have after 10 years:
| Type | After 10 years | Total interest |
|---|---|---|
| Simple interest (5% on $1,000) | $1,500 | $500 |
| Compound interest (5%, compounded yearly) | ~$1,629 | ~$629 |
| Compound interest, 30 years | ~$4,322 | ~$3,322 |
Same starting amount, same rate — very different results, especially as years go by. That’s the power of compounding when you’re saving. It’s also why credit card debt grows so fast when you don’t pay it off.
APR vs. APY: rates with two different jobs
Banks and lenders quote interest in two different ways depending on which side you’re on.
APR — what loans charge
APR stands for annual percentage rate. It’s the yearly cost of borrowing, including most fees, expressed as a percentage. You’ll see APR on credit cards, mortgages, auto loans, personal loans, and student loans. APR usually doesn’t include compounding effects, which is why your real cost can be a little higher on cards that compound interest daily.
To go deeper into how this works on credit cards, see APR Explained: How Credit Card Interest Works.
APY — what savings accounts pay
APY stands for annual percentage yield. It’s the yearly return on a savings account, CD, or money market account, including compounding. That’s why APY is the better number to compare savings accounts — it tells you what you’ll actually earn in a year.
Two accounts with the same interest rate but different compounding frequencies will have slightly different APYs. The one that compounds more often pays a tiny bit more.
How interest grows your savings
Compound interest is the reason small, steady savings can become real money over time. Two things drive how much it grows:
- The rate. A 4% APY savings account grows your money roughly twice as fast as a 2% account.
- The time. The longer the money stays, the more compounding does for you.
Quick example: $5,000 left untouched in an account paying 4% APY would be worth roughly:
- ~$5,200 after 1 year
- ~$6,083 after 5 years
- ~$7,401 after 10 years
- ~$10,956 after 20 years
If you also added even a small amount each month, those numbers grow much faster. That’s the case for keeping savings somewhere with a competitive rate — see How to Earn More on Your Savings.
How interest costs you money on debt
Compound interest works just as fast against you when you’re the borrower — especially with credit cards.
Credit cards usually compound interest daily. So if you carry a $5,000 balance at 22% APR and only pay the minimum, almost half of what you’re sending the bank each month is just paying interest, not paying down the debt. That’s why a $5,000 balance can take more than 15 years to pay off if you only make minimum payments.
For a deeper breakdown, see Only Paying the Minimum on Your Credit Card? and Credit Cards and Carrying a Balance.
Fixed vs. variable interest rates
Whether you’re saving or borrowing, interest rates come in two flavors:
- Fixed rate — the rate doesn’t change. Common on mortgages, auto loans, personal loans, and CDs.
- Variable rate — the rate can move up or down based on a published benchmark (often a Federal Reserve rate). Common on credit cards, HELOCs, some student loans, and most savings accounts.
On the saving side, variable rates are why a savings account paying 4.5% APY today might pay 3.0% next year — or 5.0%. On the borrowing side, variable rates are why a credit card’s APR can quietly climb when broader rates rise.
The Rule of 72 (a useful shortcut)
There’s a quick mental-math trick called the Rule of 72. Divide 72 by an annual interest rate to get the rough number of years it takes for an amount to double at that rate.
- At 4% — 72 ÷ 4 = ~18 years to double.
- At 6% — 72 ÷ 6 = ~12 years to double.
- At 9% — 72 ÷ 9 = ~8 years to double.
This is a rough estimate, but it’s close enough to make rate differences feel real. Three percent more interest can cut your doubling time roughly in half.
Where you’ll see interest in everyday life
| Where | Who pays whom | Type |
|---|---|---|
| Savings account / money market | Bank pays you | Compound (APY) |
| CD (certificate of deposit) | Bank pays you | Compound (APY) |
| Mortgage | You pay the lender | Compound (APR) |
| Auto loan | You pay the lender | Simple or compound (APR) |
| Student loan | You pay the lender | Compound (APR) |
| Credit card balance | You pay the lender | Daily compound (APR) |
| Personal loan | You pay the lender | Compound (APR) |
| Treasury bonds and savings bonds | Government pays you | Varies |
Common mistakes
- Comparing a savings account’s APR to another account’s APY — always compare APY to APY.
- Assuming a low advertised rate is the rate you’ll actually get on a loan; your credit profile matters.
- Paying only the minimum on a credit card and not realizing how much of the payment is interest.
- Leaving an emergency fund in a big-bank account paying near-zero interest when high-yield options pay much more.
- Forgetting that interest earned on savings is taxable income (the bank sends a 1099-INT each year).
- Treating “low interest” loans as free money — even 6% on a long loan adds up to thousands.
What to do next
- Find the APY on your savings account. If you don’t know it, look on a recent statement or in the app.
- Compare your rate to current top high-yield savings APYs. If yours is much lower, consider switching — see How to Earn More on Your Savings.
- Find the APR on every loan or credit card you have. List them, highest to lowest.
- Pay down the highest-rate debt first — that’s where compound interest costs you the most.
- If you have an emergency fund or savings goal, set up an automatic transfer so the balance grows month by month and compounding has something to work with.
- Once a year, recheck rates. Both savings APYs and loan APRs move with broader interest rates.
Interest works the same way whether you’re earning it or paying it — small percentages, applied repeatedly, become big numbers. Once you can read the rates, you can make better choices on both sides of the balance sheet.
Further Reading
- What Is a Savings Account?
- How to Earn More on Your Savings
- APR Explained: How Credit Card Interest Works
- Credit Cards and Carrying a Balance
- Only Paying the Minimum on Your Credit Card?
- Money Basics
This article is for general educational purposes only and does not constitute financial advice. Interest rates and account terms change — check current rates with the bank or lender before making decisions.