Compound Interest Calculator
What Is Compound Interest?
Compound interest is interest earned on your original money and on the interest it has already earned. Instead of growing in a straight line, your balance grows faster each period, because every new round of interest is calculated on a bigger total. Albert Einstein is often credited with calling it the eighth wonder of the world, though there is no solid proof he said it. Whoever said it, the idea is real: given enough time, compounding is one of the most powerful forces in personal finance.
Our free Compound Interest Calculator above shows you exactly how it works. Enter an initial deposit, a monthly contribution, an interest rate, the number of years and how often interest compounds, and you will see your future value, your total contributions and the interest you earned.
How to Use This Calculator
- Choose your currency.
- Initial deposit: the amount you start with. It can be zero if you only plan to add monthly contributions.
- Monthly contribution: the amount you add at the end of each month.
- Annual interest rate: the yearly return you expect, for example 7.
- Years to grow: how long the money stays invested.
- Compounding: annually, quarterly, monthly or daily.
- Press Calculate to see your results.
The Compound Interest Formula
For a single lump sum with no extra contributions, the formula is:
A = P × (1 + r ÷ n)n × t
- A is the final amount.
- P is the starting principal.
- r is the annual interest rate as a decimal (7% = 0.07).
- n is the number of times interest compounds per year.
- t is the number of years.
When you also add money every month, each contribution grows for a different length of time. This calculator handles that by converting your rate to an equivalent monthly rate, then adding your balance growth and your contribution month by month. Contributions are assumed to be made at the end of each month.
Worked Example
Imagine you start with $5,000, add $200 every month, earn 7% per year compounded monthly, and leave the money for 10 years.
- Total contributions: $5,000 + ($200 × 120 months) = $29,000
- Future value: about $44,665.27
- Interest earned: about $15,665.27
More than a third of your final balance comes from interest, not from money you put in. Your initial $5,000 alone, with no monthly contributions, would grow to about $10,048 in the same ten years.
Time Is the Biggest Factor
The longer your money compounds, the more dramatic the effect. Using the same starting deposit, monthly contribution and 7% rate, look at what happens as the years increase:
| Years | Total Contributed | Interest Earned | Final Balance |
|---|---|---|---|
| 10 | $29,000 | $15,665 | $44,665 |
| 20 | $53,000 | $71,379 | $124,379 |
| 30 | $77,000 | $207,577 | $284,577 |
After 30 years, interest is far larger than everything you contributed. Starting ten years earlier does not just add ten years of contributions; it gives every dollar more time to multiply.
Does Compounding Frequency Matter?
Yes, but less than most people expect. Here is the same example (a 10-year plan at 7%) with different compounding frequencies:
| Compounding | Final Balance |
|---|---|
| Annually | $44,046.10 |
| Monthly | $44,665.27 |
| Daily | $44,722.76 |
Going from annual to monthly compounding adds a little over $600, but going from monthly to daily adds only about $57. Your interest rate, your contribution amount and especially your time horizon matter far more.
The Rule of 72
A quick mental shortcut: divide 72 by your annual interest rate to estimate how many years it takes your money to double. At 7%, 72 ÷ 7 is roughly 10.3 years. At 4%, it is about 18 years. The rule is an approximation, but it is handy for rough planning.
Ways to Make Compounding Work Harder
- Start early. Even small amounts benefit hugely from extra years.
- Contribute regularly. Automating a monthly transfer builds the habit and uses every month of growth.
- Increase contributions over time. Raising your monthly amount with each pay rise speeds things up.
- Keep fees low. A 1% yearly fee can quietly take a large share of your long-term growth.
- Reinvest earnings. Withdrawing interest stops the compounding effect.
Compound Interest Works Against You on Debt
The same force that grows savings also grows debt. Credit card balances and some loans compound against you, which is why paying down high-interest debt is often a smart first step. To see how a loan accumulates interest, try our Loan EMI Calculator.
Frequently Asked Questions
What interest rate should I use?
Use the rate offered by your savings account, bond or investment. A savings account may pay a few percent, while long-term stock market returns have historically been higher but vary a lot from year to year and are never guaranteed. It can help to run the calculator with a conservative, a moderate and an optimistic rate.
Is the result guaranteed?
No. The calculator assumes a constant rate every year. Real investments rise and fall, and savings rates can change. Treat the result as an estimate to guide your planning, not a promise.
Does the calculator include taxes or inflation?
No. Taxes, fees and inflation will reduce your real returns, so your actual purchasing power in the future will usually be lower than the number shown.
What is the difference between simple and compound interest?
Simple interest is calculated only on the original principal. Compound interest is calculated on the principal plus all earlier interest, so it grows faster over time.
Should I use a lump sum or monthly contributions?
If you have a lump sum available, investing it earlier gives it more time to grow. If not, regular monthly contributions are an excellent way to build wealth gradually. The calculator lets you combine both.
Planning a Big Purchase?
If you are saving for a home, see what your monthly payment could look like with our Mortgage Calculator.
Disclaimer: Results from this calculator are estimates for informational purposes only and are not financial advice. Actual returns vary and are not guaranteed. Please consult a qualified financial professional before making investment decisions.