THE NUMORIX GUIDE
How to use the Interest calculator
Last reviewed September 14, 2026
What this calculator does
The projection converts the selected compounding convention into an effective annual rate and then into an equivalent monthly rate, because the schedule advances month by month.
Formula and method
The projection converts the selected compounding convention into an effective annual rate and then into an equivalent monthly rate, because the schedule advances month by month. Monthly contributions are deposited every month, annual contributions are added in the engine's annual-contribution rows, and beginning timing deposits before that month's interest while end timing deposits afterward. Monthly interest is reduced by the entered tax percentage, and buying power divides the ending balance by the inflation factor.
Variables and inputs
Enter initial investment, annual contribution, monthly contribution, contribution timing, annual rate, compound choice, years, additional months, tax rate, and inflation rate. Amounts are currency; years and months define the horizon; tax, return, and inflation inputs are annual percentages.
Worked example
With $10,000 initially, $200 deposited at the end of each month, 6% compounded monthly, no tax, and one year, the monthly rate is 0.06 / 12 = 0.005. The first month is 10,000 x 1.005 + 200 = $10,250. After 12 months the balance is 10000 x 1.005^12 + 200 x ((1.005^12 - 1) / 0.005) = about $13,084.
How to interpret the result
The ending balance separates the starting principal and contributions from modeled interest. Depositing at the beginning gives each contribution one more period of exposure than depositing at the end. Buying power is a present-dollar comparison based only on the inflation percentage supplied.
Common mistakes to avoid
Match contribution timing to the account's actual deposit date. Do not treat the annual rate as a monthly rate, and do not confuse the tax rate on interest with a tax rate on withdrawals or investment gains. Include both years and months when reproducing a result.
Assumptions and limitations
The model assumes one constant return, tax percentage, and inflation percentage. It does not simulate losses, fees, withdrawals, changing tax brackets, asset allocation, or the timing of the annual contribution beyond the engine's schedule convention. Investment returns can be negative in real life even though this form validates nonnegative rates.