Calculate compound interest with monthly and annual contributions, years and months, tax, inflation, and nine compounding options.
Calculator
Growth Summary
Compare contributions, interest, taxes, and inflation-adjusted buying power.
Balance Composition
Balance by Year
Interest Rate Scenarios
A sensitivity range for planning, not a prediction of future returns.
| Scenario | Annual rate | Ending balance | Net interest | Today's buying power |
|---|
Growth Schedule
| Period | Contributions | Gross interest | Est. tax | Ending balance |
|---|
How compound interest is calculated
Compound interest earns a return on the original principal and on interest already added to the balance. This calculator supports daily, weekly, biweekly, semimonthly, monthly, quarterly, semi-annual, annual, and continuous compounding, plus monthly and annual contributions.
A = P(1 + r / n)^(nt)
In the formula, P is the starting principal, r is the nominal annual rate as a decimal, n is the number of compounding periods per year, and t is time in years. Continuous compounding uses A = Pe^(rt). Recurring contributions are added separately at the selected beginning or end timing.
Worked example
A $10,000 initial investment earning 6% annually, compounded monthly for 10 years with $200 deposited at the end of every month, grows to about $50,970 before taxes and fees. Total contributions are $34,000 and estimated interest is about $16,970.
Contributions and annual increases
Monthly additions are made every month. Annual additions are made once for each complete year. The optional annual contribution increase raises both amounts at the start of each new 12-month period, which can model a planned savings increase after a raise. The SEC Investor.gov compound interest tool likewise models an initial investment, recurring monthly contributions, time, rate, and compounding frequency.
Compounding frequency and APY
At the same nominal annual rate, more frequent compounding produces a higher effective annual yield. The Consumer Financial Protection Bureau explains that increasing compounding frequency, earning a higher rate, and adding principal can accelerate savings growth. Contribution size and time often have a larger effect than small frequency differences.
Tax and inflation estimates
The optional tax rate is applied to positive interest as it accrues, not to contributed principal. Actual tax timing, tax rates, account rules, losses, and deductions can differ. The inflation result discounts the ending balance into today's estimated purchasing power. The U.S. Bureau of Labor Statistics explains that purchasing power falls as prices rise and that price-index ratios can convert nominal amounts to constant dollars.
How to use the rate scenarios
The lower, entered, and higher results change only the annual interest rate by the range selected in More Options. They keep the time, deposits, contribution growth, tax, inflation, and compounding assumptions the same. Use the range as a sensitivity check, not a best-case or worst-case forecast. Actual returns can vary and this calculator does not include account fees or market volatility.
Frequently asked questions
What is the difference between APR and APY?
APR or a nominal annual rate does not itself include intra-year compounding. APY is the effective annual yield after the selected compounding frequency.
Does contribution timing change the answer?
Yes. A beginning-of-period contribution has more time to earn interest than an otherwise identical end-of-period contribution.
What is the Rule of 72?
Dividing 72 by an annual percentage rate gives a rough estimate of the years needed to double money. It is a shortcut, not an exact projection.