How to use the compound interest calculator
- Enter your initial investment and choose a currency.
- Add an optional monthly contribution. Use 0 for a one-off deposit.
- Set the annual interest rate, period in years and compounding frequency.
- Read the future value and scroll the year-by-year table to watch growth accelerate.
Formula
Why compounding matters
Simple interest pays you only on what you put in. Compound interest pays you on what you put in plus everything it has already earned. The difference is tiny in year one and enormous by year thirty. That’s why starting early is often worth more than investing a larger amount later.
This calculator models both a starting lump sum and regular monthly contributions, so you can plan anything from a savings account to a long-term retirement or SIP investment.
How the calculation works
For a single deposit, the future value is:
A = P × (1 + r/n)^(n × t)
where P is the starting amount, r the annual rate as a decimal, n the number of compounding periods per year and t the number of years.
Monthly contributions are simulated month by month. Each deposit lands at the end of its month, and interest is credited at each compounding boundary on the full balance. The year-by-year table shows exactly what was added and what was earned in each year.
Worked example
Invest $10,000 at 7% compounded monthly for 10 years, adding $200 a month:
| Amount | |
|---|---|
| Initial investment | $10,000 |
| Contributions (120 × $200) | $24,000 |
| Interest earned | $20,713.58 |
| Future value | $54,713.58 |
Almost 38% of the final balance is interest you never deposited. Extend the same plan to 25 years and interest becomes the largest part of the total.
Nominal rate vs. effective annual rate
Banks often quote a nominal annual rate, but how often it compounds changes what you really earn. A 7% rate compounded monthly is an effective annual rate (EAR) of 7.229%; compounded daily it’s 7.250%. The calculator shows the EAR for whatever frequency you choose, so two offers can be compared on equal terms.
Three levers that drive growth
- Time. Doubling the period more than doubles the interest, because later years compound on a much larger base.
- Rate. Small differences add up. Over 30 years, 6% vs. 8% on the same contributions can mean a final balance roughly 45–50% larger.
- Consistency. Regular contributions smooth out timing and keep the snowball rolling.
Limitations to keep in mind
- Returns on stocks and funds are not fixed. Treat the rate as an average assumption and test a pessimistic case too.
- Inflation erodes purchasing power. Subtract expected inflation from the rate for a “real” figure.
- Taxes and fees can take a meaningful share of growth. A 1% annual fee costs far more than 1% of the final balance over long periods.
If you’re comparing saving against paying off debt, run your loan through the loan calculator. Paying off a 9% loan is a guaranteed 9% return.
Frequently asked questions
What is compound interest?
Compound interest is interest calculated on both your original deposit and the interest already added to it. Over time you earn "interest on interest", which makes growth accelerate the longer money is left invested.
How often should interest compound?
More frequent compounding (monthly or daily) gives a slightly higher result than annual compounding at the same nominal rate. The calculator shows the effective annual rate so you can compare offers fairly.
When are monthly contributions added?
Contributions are added at the end of each month and then earn interest from the next compounding period onward, which matches how most savings plans and SIPs work.
Is the result guaranteed?
No. A fixed rate is assumed for the whole period. Real investment returns vary year to year, and taxes, fees and inflation reduce what you actually keep.
What is the Rule of 72?
It is a quick estimate of how long money takes to double — divide 72 by the annual interest rate. At 8%, money doubles in roughly 9 years.