Calculation details
Planning estimate only, not financial, tax, or legal advice. Verify assumptions and current rules before making decisions.
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Use this result
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What does this calculator estimate?
Compound interest is interest earned on both your principal and the interest it has already generated — the engine of long-term growth. This calculator projects a future value by compounding your principal at an assumed rate. Enter the principal, rate, and time to see the power of compounding.
- Interest compounds on interest
- FV = P × (1 + rate/n)^(n×years)
- More frequent compounding = slightly more growth
How compounding works
With compound interest, you earn interest on your original money AND on the interest you've already accumulated. Over time this snowball effect grows an investment faster than simple interest, and the longer the horizon the bigger the difference.
Limitations to watch for
The projected value depends on an assumed constant rate that isn't guaranteed. Taxes and fees reduce real returns. More frequent compounding helps only slightly, and the calculator's big variable is the rate, not the frequency. Returns around inflation are real growth; below it you lose purchasing power.
How to use it in practice
Start early and stay invested — time is the biggest lever in compounding, far more than the rate. Understand if you are projecting a nominal or real (inflation-adjusted) value. Compare compounding to simple interest to see the difference and to appreciate the value of not interrupting the growth.
- Enter the principal (starting amount).
- Enter the rate and compounding frequency (if adjustable).
- Enter the number of years; FV = P × (1 + rate/n)^(n×years).
How this calculator works
Formula
Future value = principal × (1 + rate/n)^(n×years), where n is the compounding frequency per year. Interest is earned on the principal AND on previously earned interest — the source of compounding growth.
Worked example
A $10,000 investment at 7% compounded annually for 20 years grows to $10,000 × 1.07^20 ≈ $38,697 (vs $24,000 at simple interest).
Assumptions to verify
- A constant annual return and fixed compounding frequency.
- No additional contributions, taxes, or fees.
- The projection is a mathematical estimate, not a guarantee.
Frequently asked questions
What is compound interest?
Interest earned on both your principal and on previously earned interest, so growth accelerates over time.
How do I calculate future value?
FV = principal × (1 + rate/n)^(n×years), where n is the compounding frequency per year.
What does compounding frequency mean?
How often interest is added (annually, quarterly, monthly, daily). More frequent compounding produces slightly more growth.
Is the projected value guaranteed?
No. It assumes a constant rate. Real returns vary with market performance, and fees/taxes reduce the actual amount.
Why does time matter so much?
Compounding grows exponentially, so most of the final value comes from the later years. Starting earlier makes a dramatic difference even with the same rate.
What is simple vs compound interest?
Simple interest is earned on the principal only; compound interest is earned on principal plus accumulated interest. The difference grows over time.
What rate should I use?
Use a realistic, conservative expected return (historically ~7–9% nominal for broad stocks) and account for inflation if you want real growth.
Cite this tool
BoringToolsKit. “Compound Interest Calculator.” boringtoolskit.com/compound-interest/ (reviewed 2026-08-25). Free to reference in articles, syllabi, and answer posts with a link.
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