Compound Interest Calculator
See how a starting amount plus steady monthly contributions can grow. Adjust your rate and time horizon to watch compounding do the heavy lifting.
Assumes monthly compounding and contributions at the end of each month.
Projected value
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Why compounding is the investor's best friend
The single biggest driver of long-term wealth isn't picking the perfect stock — it's time. Because compound growth earns returns on your past returns, the curve starts slow and then bends sharply upward the longer you stay invested. Small, consistent monthly contributions often end up contributing more to the final figure than the starting lump sum, simply because they have years to compound.
Use this to pressure-test a plan: try different contribution amounts, rates, and time horizons, and notice how much of the final value is interest earned versus money you put in. That gap is the reward for patience.
Frequently asked questions
What is compound interest?
Compound interest is interest earned on both your original money and on the interest it has already earned. Over time this 'interest on interest' is what makes long-term investing so powerful — growth accelerates the longer you stay invested.
How is compound interest calculated?
This calculator compounds monthly. Each month your balance grows by (annual rate ÷ 12), your monthly contribution is added, and the process repeats. Future value = starting amount grown over all months, plus the grown value of every contribution.
What return rate should I use?
Use a realistic long-run figure for your plan — historically broad stock-market index returns have averaged roughly 7–10% annually before inflation, but returns vary year to year and are never guaranteed. Try a few rates to see the range.
Does the calculator account for inflation or taxes?
No — it shows nominal growth before inflation, taxes, or fees. Your real spending power will be lower. Treat the result as a projection of gross growth, not a promise.
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Educational tool only — not financial advice. Projections are not guarantees. See more free tools.