Finance
Compound Interest Calculator
See how your savings grow with compound interest. Adjust starting amount, rate, contributions, and frequency. No signup, no ads.
Investment details
Balance after 20 years
$40,387
Total contributed
$10,000
Interest earned
$30,387
How does compound interest work?
Compound Interest Formula
Variables
Final amount-Balance after all compounding periods
Principal-Starting amount
Annual rate-Yearly rate as a decimal (8% = 0.08)
Periods per year-1 for annual, 12 for monthly, 365 for daily
Years-Investment duration
Interest Rate Comparison
Growth of $10,000 over 20 years
| Rate | Total Interest | Ending Balance |
|---|---|---|
| 4% | $11,911.23 | $21,911.23 |
| 6% | $22,071.35 | $32,071.35 |
| 8% | $36,609.57 | $46,609.57 |
| 10% | $57,275.00 | $67,275.00 |
Frequently Asked Questions about the Compound Interest Calculator
What is compound interest?
Compound interest is interest earned on both your original deposit and the interest already accumulated. Because each period's earnings become part of the base for the next period, growth accelerates over time. A $10,000 deposit at 6% compounded monthly reaches about $18,194 after 10 years, compared to $16,000 with simple interest.
How is compound interest different from simple interest?
Simple interest is calculated only on the original principal every period. Compound interest is calculated on the principal plus all previously earned interest, so the balance grows faster the longer you leave it. The gap between the two widens significantly over decades.
What compound frequency should I use?
Match the frequency your account actually uses. Most high-yield savings and money-market accounts compound daily, while CDs often compound daily or monthly and bonds typically pay interest semi-annually. For a 401(k) or brokerage account, monthly is a reasonable working assumption.
Does this account for taxes or inflation?
No. The result is a pre-tax, nominal return. For a rough inflation-adjusted figure, subtract your expected inflation rate from the annual rate before entering it (for example, 7% return minus 3% inflation gives a 4% real rate). After-tax estimates require subtracting your effective tax rate from the gross return.
Is 7% a realistic stock-market return?
It depends on the time horizon and what you measure. The S&P 500 has returned roughly 10% per year nominally over long multi-decade periods, or about 7% after adjusting for inflation. Short windows can vary dramatically in either direction, so enter whatever rate fits your own assumptions and risk outlook.
Can I model monthly contributions?
Yes. Enter a monthly contribution amount and the calculator adds it at the start of each month before applying interest, for every month in the time period. The final balance, total contributions (including your starting principal), and total interest earned all reflect those ongoing deposits.
What is the rule of 72?
Divide 72 by the annual interest rate to estimate the number of years it takes to double your money. At 6%, money doubles in about 12 years; at 9%, in about 8 years. The rule assumes no additional contributions and is most accurate for rates between 6% and 10%.
How accurate is this calculator?
The underlying math is precise to the cent. The calculator runs a month-by-month simulation and rounds each yearly snapshot to two decimal places. The real uncertainty is your inputs: actual returns, contribution amounts, and compounding frequency in the future are estimates, not guarantees.