Compound Interest Calculator
See a starting amount and monthly savings grow together, with a live annual chart and schedule—without sending your numbers anywhere.
- Fewer clicks, keyboard friendly
- More accurate results
Change any value to update the projection, annual chart, and schedule instantly.
Projected value
Annual growth chart
Estimate only: assumes a constant rate and monthly deposits. Taxes, fees, changing returns, and account rounding are not included.
Annual projection
Compare the balance, your deposits, and accumulated interest at every year-end.
How to use
- Enter the amount already saved or invested as the starting amount.
- Enter the amount you plan to add each month, or enter 0 for a lump sum.
- Set a constant annual rate, number of whole years, and compounding frequency.
- Choose whether monthly contributions arrive at the beginning or end of each month.
- Review the final balance, interest earned, annual chart, and year-by-year projection as they update.
Compound interest formula and monthly contributions
For a single starting amount, the standard nominal-rate formula is A = P × (1 + r ÷ n)^(n × t). P is the starting amount, r is the annual rate as a decimal, n is the number of compounding periods in a year, and t is the number of years. Monthly deposits have their own dates, so this calculator converts the selected annual growth factor into an equivalent monthly factor: (1 + r ÷ n)^(n ÷ 12). That keeps annual, quarterly, monthly, and daily choices comparable while allowing each deposit to start growing in the month it arrives. It reports the final balance, total amount contributed, and the difference as projected interest.
Annual rate, compounding frequency, and effective rate
The annual interest rate in this calculator is a nominal annual rate. Its effective annual rate is (1 + r ÷ n)^n − 1, where n is the selected compounding frequency. At the same 5% nominal rate, annual compounding has a 5.00% effective rate while monthly compounding is about 5.12%. The difference is real but usually much smaller than the effect of the rate itself, regular saving, and time. Daily compounding here uses 365 compounding periods per year; a bank may use a different daily-balance convention.
How to read a compound-growth estimate
A projection is a scenario, not a guarantee. This tool holds the rate constant and assumes that every planned monthly contribution is made. It does not subtract tax, account fees, fund expenses, withdrawals, inflation, or changes in investment value. Choose Beginning when the deposit reaches the account at the start of each month and End when it arrives after that month’s growth; the beginning option gives each deposit one additional month of growth. Compare more than one rate and contribution amount before using a result for a financial decision, then check the actual account’s terms and crediting method.
Frequently asked questions
What is the compound interest formula?
For a starting amount with a nominal annual rate, the compound-interest formula is A = P × (1 + r ÷ n)^(n × t), where P is the starting amount, r is the annual rate as a decimal, n is compounds per year, and t is years. Monthly contributions are separate deposits that begin growing when they are added.
Does monthly compounding pay more than annual compounding?
At the same positive nominal annual rate, monthly compounding produces a slightly higher effective annual rate than annual compounding. For example, 5% compounded annually stays 5.00% effective, while 5% compounded monthly is about 5.12% effective before taxes and fees.
Should I use beginning or end of month for contributions?
Use Beginning when each contribution reaches the account before that month’s growth and End when it arrives after the month’s growth. Beginning-of-month deposits receive one additional month of modeled growth, so their projected final balance is higher at any positive rate.
How does the calculator handle monthly deposits with annual or quarterly compounding?
The calculator converts the selected nominal annual compounding rate into an equivalent monthly growth rate, then applies each monthly deposit on its actual month. This preserves the selected annual growth factor while avoiding a grouped-deposit approximation when deposits are made monthly.
Does this calculator account for tax, inflation, or fees?
No. This calculator uses a constant gross rate and excludes tax, inflation, account fees, fund expenses, withdrawals, and rate changes. Those factors can materially reduce or change a real account’s outcome, so the result is an estimate rather than financial advice.
Are my savings figures sent to a server?
No. The projection runs in your browser. The numbers are saved only in that browser’s local storage so the form can be restored when you return; they are not sent to a server.
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Updated 2026-10-14