Compound Interest Calculator
Enter a starting amount, how much you'll add each month, an expected annual return and a time horizon, and this calculator shows what your money could grow to — separating what you put in from the interest it earns. It works in any currency.
Why compounding builds on itself
Compound interest means you earn returns not just on your original money but on the returns already added. Each month's growth becomes part of the balance that grows next month, so the curve gets steeper over time. That is why the interest-earned figure above eventually dwarfs the amount you contributed — and why the length of time invested matters even more than the size of each contribution.
Worked example
Start with $1,000, add $200 a month, and assume a 7% annual return over 10 years: your money grows to about $36,600. Of that, you contributed $25,000 and compound interest added roughly $11,600. Stretch the horizon to 20 or 30 years and the interest portion grows dramatically — that is the reward for time in the market.
What moves the result
- Time is the most powerful lever — an early start is hard to catch up to because the earliest contributions compound the longest.
- Contribution size matters, but adds linearly; compounding rewards consistency over decades more than a big one-off.
- Rate of return has an outsized long-run effect, but higher expected returns come with more risk and year-to-year swings — use a realistic average.
Methodology & assumptions
Future value = starting amount × (1+i)n + contribution × ((1+i)n − 1) ÷ i, where i is the monthly rate (annual rate ÷ 12) and n is the number of months; contributions are treated as made at the end of each month. It assumes a constant return compounded monthly with no fees or taxes — real investment returns vary and can be negative, so treat the figure as an illustration, not a promise. For a guaranteed rate, see the fixed-deposit calculator; for market-style projections, the SIP calculator. See our methodology and editorial policy.
Frequently Asked Questions
Monthly or annual compounding?
This tool compounds monthly, which matches how most regular-contribution plans work and is slightly higher than annual compounding at the same stated rate.
Is the return guaranteed?
No — markets fluctuate. Use a realistic long-term average and remember the result is an illustration.
What rate should I use?
Use a figure you can justify for your mix of investments and expect lower, steadier numbers than the headlines. A lower assumption gives a safer plan.
Planning for a goal or retirement?
See how much to invest each month to hit a target, adjusted for inflation.
Open the retirement calculator →Related calculators
Sources
Uses the standard future-value-of-an-annuity formula with monthly compounding. Projections are illustrations, not guarantees — investment returns vary and can be negative. See our methodology and editorial policy.