Compound interest calculator

Enter the principal, the nominal annual rate as a percentage, the number of compounding periods per year, and the duration in years. The result reports both the final amount and the interest earned, making it easy to separate contributed capital from growth. The same deterministic formula is useful for savings projections, investment comparisons, and transparent financial examples.

Enter the four inputs consistently

Begin with the principal, which is the amount available before any interest is applied. Enter the annual rate as a percentage, so a value of 5 represents five percent rather than the decimal 0.05. Frequency is the number of times interest is compounded during one year: 1 is annual, 4 is quarterly, 12 is monthly, and 365 is daily compounding. Years is the full duration of the calculation and may include a fractional part when the time horizon is not a whole number of years. Keep the principal and the returned amounts in the same currency, because the calculator performs arithmetic without converting monetary units. It accepts zero for principal, annual rate, or years, which can be useful for boundary checks, but frequency must be a positive integer. Before comparing scenarios, confirm that every option uses the same rate convention and time horizon. A nominal annual rate compounded monthly is not entered as a monthly rate; the calculator divides the annual percentage across the declared periods.

Understand the compound interest formula

The calculation uses the standard periodic compounding formula: final amount equals principal multiplied by one plus the annual rate divided by the compounding frequency, raised to the power of frequency multiplied by years. The annual percentage is first divided by 100 to convert it to a decimal. Interest earned is then calculated as the final amount minus the original principal. This method assumes that the rate remains constant, every scheduled compounding event occurs, and all credited interest stays in the balance. It does not model deposits, withdrawals, fees, taxes, changing rates, or inflation. More frequent compounding generally produces a slightly larger final amount when principal, nominal annual rate, and time stay fixed, because credited interest begins earning interest sooner. The outputs retain useful numeric precision for further calculations while avoiding distracting floating-point artifacts. For financial decisions that require currency posting rules, apply the institution's required rounding schedule after reviewing its terms, since periodic cent rounding can differ slightly from a direct formula.

Interpret and compare the results

Use final amount when you need the projected balance at the end of the selected period. Use interest earned when you want to isolate growth from the original principal. For a meaningful comparison between accounts, keep the principal and duration fixed, then change one assumption at a time. Comparing frequencies alone shows the effect of compounding intervals, while comparing annual rates shows the much larger effect of the quoted return. Remember that this is a projection, not a promise: real savings and investment products can have variable rates, fees, minimum balances, contribution schedules, early withdrawal rules, or market risk. The result is especially helpful as a transparent baseline that can be reproduced in a spreadsheet or audit note. Automated callers can submit the same inputs through the API for $0.002 per request, and browser users can explore scenarios without network-dependent calculations. If a quoted product uses an effective annual yield rather than a nominal annual rate, do not enter it as though it were nominal; first confirm the convention or compare effective yields directly.