PiggyCompound Interest Calculator
See how an initial deposit plus monthly contributions grow over time — and how much of the final balance is pure interest.
Your plan
Future value
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Total contributed
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Interest earned
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Growth over time
Balance vs. what you actually put in
Total balance
Contributions
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The math behind compound growth
This calculator compounds monthly: each month your balance earns annual rate ÷ 12, then your contribution is added. Over long horizons the interest-on-interest effect dominates — which is why the gap between the two lines above widens every year.
Frequently asked questions
What return should I assume?
The S&P 500 has historically averaged about 10% per year before inflation (roughly 7% after). Conservative planners often model 5–7% for diversified portfolios. Past performance never guarantees future results.
Does compounding frequency matter?
Less than most people think. The difference between monthly and daily compounding on the same APR is typically a few dollars per thousand per year. Contribution amount and time horizon matter far more.
What's the Rule of 72?
Divide 72 by your annual return to estimate the years needed to double your money. At 7%, money doubles roughly every 10.3 years.