THE NUMORIX GUIDE
How to use the Savings Interest Calculator
Last reviewed September 14, 2026
What this calculator does
For a finite compounding frequency n, the engine uses periodic rate = annual rate / 100 / n and APY = ((1 + annual rate / 100 / n)^n - 1) x 100.
Formula and method
For a finite compounding frequency n, the engine uses periodic rate = annual rate / 100 / n and APY = ((1 + annual rate / 100 / n)^n - 1) x 100. Each period credits interest to the balance first; when n is at least 12, it then adds monthlyAddition / (n / 12). Year rows record balance, cumulative interest, and total deposited.
Variables and inputs
Enter initial deposit, annual interest rate, years, compounding frequency of 1, 2, 4, 12, or 365, and a monthly addition. The UI requires a nonnegative deposit/rate and at least one year. The monthly addition is distributed across periods only for frequencies of 12 or more.
Worked example
With a $10,000 deposit, 5% annual rate, monthly compounding, 5 years, and no monthly addition, final balance is 10,000 x (1 + 0.05 / 12)^60 = $12,833.59. Interest is $2,833.59 and effective APY is about 5.1162%.
How to interpret the result
The final balance combines the starting deposit, modeled periodic interest, and any supported additions. APY makes the effect of compounding frequency comparable, while the nominal rate is still the percentage entered in the form.
Common mistakes to avoid
Choose the frequency that matches the account rather than treating APY as the nominal rate. Enter a monthly addition as one month's amount, not the annual total. Do not assume a monthly addition is applied for annual or semiannual frequency; the current engine skips it when n is below 12.
Assumptions and limitations
The model assumes a constant rate and deposits, with no fees, taxes, withdrawals, minimum balances, rate changes, or day-count rules. Years are parsed as an integer by the UI, and the engine does not validate frequency, negative additions, or account-specific compounding conventions.