Compound Interest Calculator
Compound interest is the most powerful force in personal finance — the growth your money earns starts earning its own growth, so over decades the gains on your gains can dwarf the contributions you actually made. This calculator projects how an initial investment, combined with regular contributions and a steady rate of return, grows over time, letting you see the long-term payoff of starting early and contributing consistently. Enter your starting balance, the monthly contribution you plan to add, the expected annual return, the number of years, and the compounding frequency, and the calculator returns your projected balance, total contributions, and the interest earned on top. Adjusting the compounding frequency shows how monthly versus annual compounding affects your final balance, with more frequent compounding producing a slightly larger result because interest is credited and starts earning sooner. The math is unforgiving in a good way: a dollar invested at 25 has roughly four decades to compound, while a dollar invested at 45 has less than half that runway, which is why early starters with modest contributions often out-accumulate late starters with large ones. Use a realistic return assumption — a broadly diversified U.S. stock portfolio has returned about 10% per year before inflation, or roughly 7% after inflation — and remember that real returns fluctuate year to year rather than growing smoothly. This calculator is the foundation of retirement and long-term savings planning: use it to see how increasing your contribution rate, extending your time horizon, or earning a higher return changes your outcome, and to understand why the single most important variable in building wealth is simply time in the market.
Compound Interest Calculator
See how your money grows with compounding returns and regular contributions.
Total balance
$386,158
After 30 years
Total contributions
$100,000
Interest earned
$286,158
This calculator assumes monthly compounding. Compound interest is the return earned on both your principal and accumulated interest — the longer your time horizon, the more powerful the effect.
About This Calculator
Compound interest is the most powerful force in personal finance — the growth your money earns starts earning its own growth, so over decades the gains on your gains can dwarf the contributions you actually made. This calculator projects how an initial investment, combined with regular contributions and a steady rate of return, grows over time, letting you see the long-term payoff of starting early and contributing consistently. Enter your starting balance, the monthly contribution you plan to add, the expected annual return, the number of years, and the compounding frequency, and the calculator returns your projected balance, total contributions, and the interest earned on top. Adjusting the compounding frequency shows how monthly versus annual compounding affects your final balance, with more frequent compounding producing a slightly larger result because interest is credited and starts earning sooner. The math is unforgiving in a good way: a dollar invested at 25 has roughly four decades to compound, while a dollar invested at 45 has less than half that runway, which is why early starters with modest contributions often out-accumulate late starters with large ones. Use a realistic return assumption — a broadly diversified U.S. stock portfolio has returned about 10% per year before inflation, or roughly 7% after inflation — and remember that real returns fluctuate year to year rather than growing smoothly. This calculator is the foundation of retirement and long-term savings planning: use it to see how increasing your contribution rate, extending your time horizon, or earning a higher return changes your outcome, and to understand why the single most important variable in building wealth is simply time in the market.
Disclaimer: This calculator provides estimates for educational purposes only and does not constitute financial, tax, or investment advice. Results depend on assumptions that may not reflect your actual situation.
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Written by
James MitchellSenior Financial Analyst & Personal Finance Expert
Certified Financial Planner (CFP) • MBA in Finance, University of Chicago Booth School of Business