A free compound interest estimator — see how your money multiplies over time, and how much of your wealth comes for free.
Final Balance
after 20 years at 7% p.a.
Total Contributed
principal + additions
Interest Earned
compounded growth
Money Multiplier
final ÷ contributed
Rule of 72
to double at this rate
Balance Breakdown Over Time
Principal, contributions, and interest earned year by year
Rate Comparison
Final balance at different annual return rates
| Year | Balance | Year Contributions | Cumul. Contributions | Year Interest | Cumul. Interest | Interest Share |
|---|
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This free compound interest estimator shows how money grows exponentially over time. Enter your starting balance, monthly contribution, annual return, and time horizon to see your final balance broken down into what you contributed versus what compound interest added for free. It supports daily, monthly, quarterly, and annual compounding, contribution growth, tax and fee drag, inflation adjustment, and side-by-side rate comparisons — and every calculation produces a shareable link.
Compound interest means you earn interest not only on the money you deposit, but also on the interest you have already earned. In year one, a $10,000 deposit at 7% earns $700. In year two, you earn 7% on $10,700 — and so on. Each year's earnings join the principal and start producing earnings of their own, which is why the growth curve bends upward instead of rising in a straight line. Albert Einstein is often (perhaps apocryphally) quoted calling it the eighth wonder of the world; the year-by-year table above shows exactly when interest overtakes everything you paid in.
For a single lump sum, the standard compound interest formula is:
A = P (1 + r/n)nt
where A is the final amount, P the principal, r the annual rate as a decimal, n the number of compounding periods per year, and t the time in years. Regular contributions complicate the closed-form math, because every deposit compounds from a different starting date — so this calculator simulates your balance month by month instead, which also makes contribution growth and fee drag straightforward to model.
The more often interest compounds, the more you earn at the same nominal rate: 5% compounded annually is exactly 5% per year, while 5% compounded daily works out to about 5.13% (the effective annual rate). The difference is real but modest — switch the compounding frequency under Advanced Assumptions to see it on your own numbers. Time in the market and the size of your contributions matter far more.
A handy mental shortcut: divide 72 by your annual return to estimate how many years it takes your money to double. At 8% that's roughly 9 years; at 6%, roughly 12. The calculator shows the exact figure for your rate in the Rule of 72 card above.
Compound interest is interest earned on both your original deposit and on the interest that has already accumulated. Unlike simple interest, which only pays on the principal, compound interest makes your balance grow exponentially because each period's earnings start earning their own interest.
Use the formula A = P(1 + r/n)nt for a lump sum. With regular contributions, each deposit compounds from the moment it is added, which this calculator simulates month by month for an accurate result.
More frequent compounding earns slightly more at the same nominal rate — 5% compounded daily yields about 5.13% per year versus exactly 5% compounded annually. The gap is real but small compared to the impact of time and contribution size.
Divide 72 by the annual return to estimate the years needed for your money to double. At 8% your money doubles roughly every 9 years; at 6% roughly every 12.
It depends on where the money sits. High-yield savings accounts have historically paid roughly 1–5%. Broad stock market index funds have returned about 7% per year after inflation (around 10% nominal) over long periods, though returns vary widely year to year. Use a conservative figure for planning — past performance does not guarantee future results.
Yes. Set an inflation rate under Advanced Assumptions to see the inflation-adjusted real value of your final balance (the CPI Calculator helps you pick a realistic figure), and use the tax & fee drag field to subtract an annual percentage for taxes, fund fees, or advisory costs.
Compound growth is the engine behind most long-term financial goals. Once you know how fast your money can grow, work out how much of your income to set aside with the Savings Rate Calculator, check whether your nest egg is on track with the Retirement Calculator, or see when investments alone could cover your lifestyle with the FIRE Calculator and the Coast FIRE Calculator.
This tool is for educational purposes only and does not constitute financial advice. Results are estimates based on the assumptions you enter.