Compound Interest Calculator

The compound interest calculator shows how capital grows with regular contributions: enter an initial amount, a monthly deposit, a term and an expected return – the calculator builds a year-by-year growth chart, splits the result into what you invested and what you earned, and shows the power of compounding over the long run.

What compound interest is

Compound interest is return on return: each period the interest is added to the capital, and the next period interest is earned on the larger amount. Investors call it the main secret of private wealth. The difference from simple interest is small in the first years and dramatic over time: at a 10% annual return, $100 a month becomes roughly $76,000 in 20 years, and almost two thirds of that is interest on interest.

How to use the calculator

Enter the initial amount (if you have one), the monthly contribution, the term in years and the expected return. The result shows the final capital, your initial amount, the total of your contributions and the interest earned; the green pill shows how much the capital exceeded what you paid in. The chart with the dashed line shows the growing gap between “just saving” and “saving with interest”: it widens with every year.

The compound interest formula

Each month the capital grows as K = K × (1 + r/12) + contribution, where r is the annual return. Over n months this gives the classic future-value formula: FV = PV × (1 + r)ⁿ + PMT × ((1 + r)ⁿ − 1) / r, where PV is the initial amount and PMT the monthly contribution. The calculator works month by month, like a real brokerage account.

What return to assume

Base your plan on long-term averages, not the best years: the global stock market returned about 7–10% a year in nominal terms over the last 50 years; bonds 3–6%; high-yield savings vary with central bank rates. A conservative scenario (5–7%) shows a realistic plan, an optimistic one (10–12%) the potential. Past performance does not guarantee future results – that is a rule, not a disclaimer.

How to boost the effect

Three levers drive the result. First, time: starting at 25 instead of 35 nearly doubles the outcome for the same contribution. Second, consistency: an automatic deposit on payday works better than “I will invest what is left”. Third, fees and taxes: the gap between a fund’s 10% return and your personal 8% after costs is easy to ignore – over 20 years it amounts to thousands. Review your plan once a year.

Limitations of the calculation

The calculator models a smooth return, while real markets are volatile: along the way there will be both −20% and +30% years. For horizons under 5 years savings accounts and bond funds are safer than stock funds; over 15+ years volatility has historically smoothed out. The calculation ignores inflation: at 4% a year, a $76,000 capital in 20 years “weighs” like about $35,000 today.

This calculation is for information only and is not individual investment advice.