Compound Interest Calculator
See how a starting balance and monthly savings can grow over time.
What is compound interest?
Compound interest means you earn interest on your original money and also on the interest it has already earned. Each period, the balance that earns interest gets a little bigger, so growth speeds up over time. Albert Einstein is often credited with calling it the eighth wonder of the world, but there is no proof he said it. The maths, however, is real.
The compound interest formula
Balance = P x (1 + r)^n + D x ((1 + r)^n - 1) / r
P is your starting balance, D is the monthly deposit, r is the yearly return divided by 12 and n is the number of months. Our calculator assumes monthly compounding and deposits at the end of each month.
How time changes the result
Time is the biggest ingredient. Here is 10,000 growing at 7% a year with no further deposits:
| Years | Balance |
|---|---|
| 10 | 20,097 |
| 20 | 40,387 |
| 30 | 81,165 |
The second decade adds about 20,000 and the third adds about 41,000, even though you did nothing extra. That is compounding at work.
The power of regular deposits
Adding money each month makes a large difference. Saving 200 a month at 7% from zero gives roughly:
| Years | You put in | Balance |
|---|---|---|
| 10 | 24,000 | 34,617 |
| 20 | 48,000 | 104,185 |
| 30 | 72,000 | 243,994 |
After 30 years, you have paid in 72,000 but the balance is about 244,000. More than two thirds of it is growth. Starting earlier matters more than saving a little extra later.
How the return rate changes the outcome
Saving 200 a month for 20 years (48,000 paid in) ends at about 82,207 at a 5% return, 104,185 at 7% and 133,577 at 9%. A few points of return can be worth tens of thousands, which is why fees matter: a fund that charges 1% a year takes a real bite out of long-term growth.
Rule of 72: a quick shortcut
To estimate how long money takes to double, divide 72 by the yearly return. At 7% it takes about 10 years. Try it with our Rule of 72 calculator.
Compound interest also works against you
The same effect makes debt grow quickly. Credit card balances compound against you, so a balance you do not pay down can snowball. See how long a card takes to clear with the credit card payoff calculator.
Realistic expectations
- Returns are not guaranteed. Stock markets can fall for years. A steady 7% is an assumption, not a promise.
- Inflation reduces buying power. Check the effect with the inflation calculator.
- Tax and fees reduce growth. Use tax-advantaged accounts where available in your country.
- Simple interest is different. It pays only on the original amount. Compare with the simple interest calculator.
Next steps
If you have a target in mind, the savings goal calculator shows the monthly amount needed to reach it, and the retirement savings calculator estimates a long-term pot. This page is for information only and is not financial advice.
Frequently asked questions
What is compound interest?
It is interest earned on both your original money and the interest already added.
Is a 7% return realistic?
It is a common long-term assumption for stocks, but returns change every year and can be negative.
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Last updated October 2026. For information only, not financial advice.