THE NUMORIX GUIDE
How to use the Annuity Calculator
Last reviewed September 14, 2026
What this calculator does
The projection runs monthly using monthlyRate = annualGrowthRate / 100 / 12.
Formula and method
The projection runs monthly using monthlyRate = annualGrowthRate / 100 / 12. It adds monthly additions either before or after each month's interest according to additionTiming, then adds annualAddition at the end of every year. The schedule totals starting principal and additions separately from return.
Variables and inputs
Enter starting principal, annual addition, monthly addition, addition timing of beginning or end, annual growth rate, and years. Principal and additions are dollars; years is the number of annual schedule rows; growth is an annual percentage.
Worked example
With $20,000 starting principal, no monthly addition, $10,000 annual addition, 6% growth, and end-of-year additions, the first 12 months grow the principal by 20,000 x ((1 + 0.06 / 12)^12 - 1) = about $1,234. The $10,000 addition is then made at year end, leaving about $31,234 and total contributions of $30,000.
How to interpret the result
An end addition is an ordinary-annuity-style deposit and a beginning addition receives the current month's modeled growth. The result is an accumulation scenario; it is not the payout or guarantee of a commercial annuity contract.
Common mistakes to avoid
Do not enter the annual addition again as a monthly addition. Select beginning only when deposits actually occur before the monthly interest credit. Keep the annual growth rate consistent with the period being modeled.
Assumptions and limitations
The route assumes a constant growth rate, monthly crediting, and an annual addition exactly at year end. It does not model insurance charges, surrender fees, mortality credits, taxes, withdrawals, variable subaccounts, or a guaranteed contract benefit.
Practical use and checks
This calculator is useful for a savings projection that combines an opening balance with a monthly addition and an optional annual addition. Enter the amount already saved, the monthly contribution, the annual contribution, the expected growth rate, and the number of years. The timing choice is practical: an end addition models a deposit after that month's growth, while a beginning addition gives each monthly deposit one extra month of exposure. As a simple audit, enter $1,000 starting principal, $100 monthly addition, $0 annual addition, 0% growth, and one year. The end balance should be $2,200 and total contributions should also be $2,200, because twelve deposits were made and no return was earned. With a positive rate, compare the schedule's total return with the total contributions rather than treating the whole balance as investment gain. Use annual additions for a bonus or yearly transfer, not for a monthly amount that has merely been multiplied by twelve. The projection assumes a constant rate and fixed contributions; it does not model taxes, fees, inflation, contribution limits, withdrawals, market losses, or an insurance annuity contract. A beginning-of-month convention, variable deposits, or a rate that changes during the year can materially alter the decision, so test more than one scenario before using the result for a financial commitment.