Compound Interest Calculator
Project how your money grows over time with compound interest and regular contributions.
Amount you invest upfront.
How long you plan to invest (0–100 years).
Amount added every month.
Expected average annual return (0–20%).
Estimated value of your investment after 10 years.
Compound interest is the eighth wonder of the world. He who understands it, earns it... he who doesn't... pays it.
About this calculator
This compound interest calculator estimates how an investment can grow over time by combining an initial amount, regular monthly contributions, and an expected annual return. It’s designed to give a clear, high-level projection — not a precise prediction — of long-term investment growth.
Use it to explore how time, consistency, and rate of return work together to compound wealth.
What each input means
- Initial investment is the amount of money you start with. This is sometimes called the principal and begins earning interest immediately.
- Length of time (years) is how long you plan to invest. Longer time horizons generally have a much larger impact on growth than starting with more money.
- Monthly contribution is the amount you add to your investment each month. Regular contributions significantly increase long-term results because each new dollar also compounds over time.
- Annual interest rate is your expected average yearly return, expressed as a percentage. This calculator assumes the rate is constant over time, which simplifies comparisons but does not reflect real-world market volatility.
- End value is the estimated total value of your investment after the selected time period, including contributions and compounded growth.
How does compound interest work?
Compound interest means you earn interest not only on your original investment, but also on the interest your money has already earned.
Each compounding period builds on the previous one. Over time, this creates a curve that starts slowly and accelerates — often dramatically — in later years. This is why compounding is most powerful over long time horizons.
In this calculator:
- Monthly compounding – Interest compounds monthly.
- End-of-month contributions – Contributions are added at the end of each month.
- New deposits start next month – Each new contribution begins compounding in the following period.
A simple example: If you invest $1,000 at a 5% annual return and make no additional contributions, your balance grows to about $1,629 after 10 years. That extra growth comes from earning interest on prior interest — not just on the original $1,000.
When monthly contributions are added, the effect becomes even stronger. Each deposit has less time to grow than earlier ones, but together they meaningfully increase the final result.
Key levers that drive growth
- Time – Time is the most powerful variable. Starting earlier often matters more than starting with a larger amount.
- Rate of return – Small changes in return compound dramatically over long periods. This is why fees, taxes, and asset allocation matter.
- Consistency – Regular contributions smooth out market ups and downs and steadily increase the amount of money earning interest.
- Volatility & inflation (not modeled) – This calculator assumes steady growth and does not account for market swings or inflation. For long-term planning, consider viewing results in today’s dollars and comparing multiple return scenarios.