Compound interest is the most powerful force in personal finance: your interest earns interest. Enter a starting balance, a monthly contribution and an annual return to see what your savings plan grows into — and how much of it is pure compounding, not your own deposits. As a rule of thumb, broadly diversified stock indices have historically returned about 5–8 % per year over long periods (a guideline, not a guarantee). The longer you stay invested, the steeper the curve gets: with 30-year horizons, the final decade typically adds more growth than the first two combined. Results are before tax and assume a constant return.
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Compound Interest Calculator: See Your Money Grow
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What is compound interest?
Interest that is reinvested and then earns interest itself, making your balance grow exponentially rather than linearly.
How accurate is this calculator?
It assumes a constant annual return with monthly compounding. Real markets fluctuate, and taxes and fees reduce results — treat the output as a guideline.
What return should I assume for index funds?
Broad global stock indices have historically averaged roughly 5–8 % per year before tax over long periods. Past performance does not guarantee future results.
Why does starting early matter so much?
Because the largest compounding gains come in the later years. Starting ten years earlier can cut the monthly amount needed for the same goal roughly in half.