Simple vs Compound Interest: One Example on a Real Debt
Simple interest is charged only on the amount you borrowed. Compound interest is also charged on interest already added, so a debt grows faster, and the gap gets bigger every month.
What is the difference between simple and compound interest?
Simple interest is interest charged only on the amount you first borrowed.
Compound interest is interest charged on the amount you borrowed plus the interest already added to it.
Australia’s government money site, Moneysmart, defines compound interest as interest on the starting amount and on the interest built up, on money borrowed or invested. The US SEC’s Investor.gov gives the same idea: interest on the starting amount and on interest already added.
In short: simple interest stays flat. Compound interest snowballs.
(New to interest? Start with what is interest.)
How does simple interest work?
The amount you borrowed never changes, so the interest is the same every year.
The formula in words: interest equals the amount borrowed, times the yearly rate, times the number of years.
The UK’s MoneyHelper gives a simple loan example. Borrow £1,000 at 10% for a year and you pay back £1,100. That is the amount borrowed plus one year of interest.
How does compound interest work on a debt?
MoneyHelper says compound interest means adding interest to a loan where interest has already been charged. So the next lot of interest is worked out on a bigger amount.
So each month:
- Interest is worked out on what you owe.
- That interest is added to what you owe.
- Next month, interest is worked out on the new, bigger total.
The formula in words: what you owe equals the amount borrowed, times (one plus the monthly rate) raised to the power of the number of months.
The monthly rate is the yearly rate divided by 12. (More on this in monthly rate vs yearly rate.)
What does this look like with real numbers?
Say you owe $4,200 at 19.9% a year. To keep it simple, you make no payments and add no new spending.
Simple interest. 4,200 × 0.199 = about $836 a year.
- After 1 year: $836 of interest.
- After 3 years: 4,200 × 0.199 × 3 = about $2,507.
Compound interest (added monthly). The monthly rate is 19.9 ÷ 12 = 1.658%.
- Month 1: 4,200 × 0.01658 = about $70. You now owe $4,270.
- Month 2: interest on $4,270 is about $71.
- Month 3: about $72. You now owe about $4,412.
Each month the interest is a little bigger, because it is charged on the last month’s interest too.
| After | Simple interest | Compound interest | Extra from compounding |
|---|---|---|---|
| 1 year | $836 | $916 | $80 |
| 3 years | $2,507 | $3,393 | $886 |
After three years, compounding adds $886 on top of simple interest. That is money paid for interest on interest.
Real debts are not quite like this. You make payments, and your lender may add interest more or less often. But the idea is the same: interest that is not paid off starts to earn interest of its own.
Why does compound interest matter if you owe money?
Because the longer a balance sits, the faster it grows.
At first, the gap looks small. After one year in the example it is $80. After three years it is $886. Time is what makes compounding expensive.
This is also why small payments hurt. If your payment barely covers the interest, the balance stays high, and you keep paying interest on a big number. (See why the minimum payment keeps you in debt.)
How do you make compound interest work less against you?
You cannot change how your lender works out interest. You can change how long the balance sits there and how big it is.
- Pay more than the interest each month. Then the balance shrinks, and so does next month’s interest.
- Pay sooner. Less time means fewer rounds of interest on interest.
- Clear the highest rate first. Compounding costs most where the rate is highest.
Do this today (10 minutes)
- Find each debt’s balance and yearly rate on your latest statement.
- Work out this month’s interest: balance × yearly rate ÷ 12.
- Check your agreement for how often interest is added. Note it next to each debt.
Nada does this math for you. Add your debts and see how much interest each one costs you every month. Get Nada
FAQ
Is compound interest always worse than simple interest for a borrower? At the same rate, yes. Compound interest charges interest on interest, so the total is always higher once more than one period has passed. The gap is small at first and grows over time. In the example, it is $80 after one year and $886 after three.
How often is interest compounded? It depends on the lender. It can be daily, monthly or yearly. The more often interest is added to your balance, the faster it grows. Your credit agreement or statement should explain how your interest is worked out. If it is not clear, ask your lender.
Does paying off a debt early stop compound interest? Yes. Interest is worked out on what you still owe. Once the balance is zero, there is nothing left to charge interest on. Any extra payment also lowers the balance, so next month’s interest is smaller. That is why paying more than the minimum saves money.
Why does my balance go up when I do not spend anything? Because interest is added to what you owe. With compound interest, that new interest is then charged interest too. If your payment is smaller than the interest added each month, the balance rises even with no new spending. Check your statement to compare the two.
Sources
Read in October 2026.
- Moneysmart (ASIC): Compound interest (glossary)
- MoneyHelper: Interest rates explained
- Investor.gov (US SEC): Compound interest
Written by the Nada team. Reviewed October 2026.
This is general information, not personal advice.