Long-term investment growth
Compound Interest Calculator
Estimate how your starting balance, monthly contributions, return and time horizon may combine to build wealth through compounding.
✓ Private✓ No registration✓ Browser-based✓ Educational estimates
Model your investment growth
Use realistic assumptions and compare a cautious, central and optimistic scenario.
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Your calculation in plain English
What this compound interest calculator does
This calculator estimates how money may grow when investment returns are added to the balance and then begin earning returns of their own. It combines an initial investment, regular monthly contributions, an assumed annual return and an investment period. An optional inflation assumption shows an estimate of future purchasing power in today’s money.
The tool is useful for retirement planning, education savings, long-term investing and testing the effect of starting earlier or contributing more. It is not a forecast of market performance. Real investments rise and fall, whereas a calculator uses a smooth average return to make scenarios easier to compare.
How compound growth is calculated
The starting investment grows each month at an equivalent monthly rate. The monthly contribution is then added, and the process repeats throughout the selected period. Total contributions are separated from investment growth so you can see how much of the final balance came from your own money and how much came from returns.
The inflation-adjusted result divides the future balance by the cumulative effect of the inflation rate. This matters because a large future number may buy less than the same number today. Use the real-value estimate when comparing a distant goal with present-day living costs.
Use a range, not one answer.
Try a lower return, your central assumption and a higher return. If your goal only works in the optimistic case, consider increasing contributions or allowing more time.
How to interpret your results
The projected portfolio is the estimated balance at the end of the period. Total contributions include the starting investment and all monthly deposits. Investment growth is the difference between the projected balance and those contributions. When growth represents a large percentage of the result, time and compounding are doing more of the work.
The inflation-adjusted value is especially important for long horizons. It translates the future balance into an estimate of today’s purchasing power. It does not predict the exact future cost of living, but it prevents nominal growth from appearing more valuable than it may be in real terms.
Common mistakes to avoid
- Using an unrealistic return: recent strong performance may not continue.
- Ignoring fees and taxes: both can reduce the return you keep.
- Forgetting inflation: nominal wealth and real purchasing power are different.
- Assuming contributions never change: income and saving capacity often rise over time.
- Waiting for a perfect entry point: delay can reduce the years available for compounding.
Frequently asked questions
What is compound interest?
Compound interest means returns are earned on the original money and on returns accumulated in earlier periods.
How accurate is this calculator?
The mathematics is accurate for the assumptions entered, but actual investment returns will vary and are not guaranteed.
What annual return should I use?
Use a cautious long-term assumption and test several scenarios rather than relying on one figure.
Are monthly contributions added at the beginning or end of the month?
This calculator models contributions at the end of each month, a common ordinary-annuity convention.
Does the result include inflation?
The main result is nominal. A separate real-value estimate adjusts for the inflation assumption entered.
Are fees and taxes included?
No. Reduce the return assumption if you want to approximate ongoing fees or tax drag.
Why can growth exceed my contributions?
With enough time, returns are earned on an increasingly large balance, including previous gains.
Is a higher compounding frequency always better?
For a given effective annual return, the difference is already reflected in that return. Focus on the return you can reasonably expect after costs.
Continue your financial journey
Financial disclaimer: This calculator and guide are provided for educational information only. They do not constitute financial, investment, tax or legal advice. Actual outcomes will vary.