Compound Interest Calculator
See how much your money earns with compound interest. Enter the starting amount, the monthly contribution and the rate, and get the final value, the growth chart and the year-by-year table in seconds. It is free and shows exactly how much came from interest.
Wealth growth
Year-by-year projection
| Year | Interest this year | Total invested | Total interest | Wealth |
|---|
How the Compound Interest Calculator works
The calculator projects your money’s growth month by month, adding interest on interest: each period, the return is applied to the balance already accumulated, not just to the starting amount. That effect is what makes wealth grow faster the longer the horizon.
The math is based on the compound interest formula with contributions: FV = PV·(1+i)ⁿ + PMT·[((1+i)ⁿ − 1) / i], where PV is the starting amount, PMT the monthly contribution, i the monthly rate and n the number of months. You also choose whether the contribution lands at the start or end of each month, which slightly changes the final result.
What you need to enter
- Starting amount: how much you already have to begin. It can be zero.
- Monthly contribution: how much you plan to invest each month.
- Interest rate: annual or monthly, net of taxes and fees.
- Period: how long the money stays invested, in years or months.
- Contribution timing: at the start or end of the month.
How to read the results
After calculating, you see a panel with:
- Final value: everything you will have at the end of the period.
- Total invested: the starting amount plus all contributions.
- Total interest: how much the money earned on its own.
- Interest share: the slice of the final value that came purely from interest.
Over long horizons, it is common for interest to exceed the amount contributed. That is where compound interest shows its strength: time starts working for you.
How much a monthly contribution grows with compound interest
To get a sense of scale, here is how much someone accumulates contributing $500 per month at 10% per year, starting from $1,000, over different periods:
| Period | Total invested | Final value | Interest only |
|---|---|---|---|
| 5 years | ~$31,000 | ~$40,000 | ~$9,000 |
| 10 years | ~$61,000 | ~$102,500 | ~$41,500 |
| 20 years | ~$121,000 | ~$372,000 | ~$251,000 |
| 30 years | ~$181,000 | ~$1,040,000 | ~$859,000 |
Notice how the interest slice grows over time: at 30 years, more than 80% of the wealth came from returns, not from your pocket. That is the engine behind goals like the first million.
Simple or compound interest: the difference
With simple interest, the return is always calculated on the starting amount, so growth is linear. With compound interest, it applies to the updated balance, which already includes prior interest, and growth becomes a curve. Here is $10,000 at 10% per year, with no new contributions:
| Period | Simple interest | Compound interest |
|---|---|---|
| 10 years | $20,000 | ~$25,900 |
| 20 years | $30,000 | ~$67,300 |
| 30 years | $40,000 | ~$174,500 |
The gap starts small and becomes enormous: at 30 years, compound interest delivers more than four times what simple interest would.
How to make the most of compound interest
The most powerful factor is time. Starting early, even with little, usually beats starting late with large contributions. That is why the best day to start investing is always as early as possible.
Then, be consistent: regular contributions keep the snowball rolling. Reinvest the returns instead of withdrawing them, because reinvesting is exactly what creates the compounding effect. And aim for a return that fits your profile: each extra point on the rate, held for many years, makes a big difference in the final result.
Important considerations
The simulation is a projection, not a guarantee. First, returns vary in the real world: the calculator assumes a constant rate, but actual returns swing over time. Use a realistic rate for your type of investment.
Also consider inflation: the final value is nominal, and its purchasing power will be lower down the road. And remember taxes: always enter the net return, after taxes and fees, so the result reflects what actually reaches your pocket.
For specific goals, combine this simulation with the retirement calculator and the emergency fund one.
Frequently asked questions
Compound interest is interest that applies to the starting amount and also to the interest already accumulated. Instead of always earning on the same base, your money starts earning on an ever-larger balance. This "interest on interest" effect is what makes wealth grow faster over the long term.
With simple interest, the return is always calculated on the starting amount, so it grows in a straight line. With compound interest, the return is calculated on the updated balance, which includes prior interest, so growth is exponential. The longer the term, the wider the gap between the two: that is why long-term investments tend to use compound interest.
The formula is FV = PV·(1+i)^n + PMT·[((1+i)^n − 1)/i], where PV is the starting amount, PMT the monthly contribution, i the monthly rate and n the number of months. The first term projects the starting amount; the second adds the growth of each contribution. This calculator does the math for you and also shows the year-by-year progression.
You choose. The calculator has a "% p.a. / % p.m." toggle. When you enter an annual rate, it converts to the equivalent monthly rate effectively, with i = (1 + annual_rate)^(1/12) − 1, not by simply dividing by 12. That keeps the result consistent regardless of the option. Always use the net return, after taxes and fees.
Contributing $500 per month at 10% per year (effective rate), starting from $1,000, you reach about $102,500 in 10 years. Of that total, $61,000 came from your pocket and roughly $41,500 came purely from interest. Over 20 years, the same contribution passes $370,000, showing how time amplifies compounding.
It does, and consistency usually matters more than the amount. Small, regular contributions, kept for many years, benefit fully from compounding. What makes the difference is starting early and not interrupting the contributions: time is the ingredient that turns modest amounts into meaningful wealth.
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