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What does this calculator estimate?
Annuity calculator: compute annuity payments, future value, or present value from rate, term, and payment frequency. Free, editable rates, no signup needed.
How this calculator works
Formula
FV = payment x (((1 + r)^n - 1) / r) x (1 + r) for payments due at the start of each period, r the periodic rate, n the number of periods.
Worked example
$500 a month for 20 years at 6%: about $231,020 of accumulated value.
Cite this tool
BoringToolsKit. “Annuity Calculator.” boringtoolskit.com/annuity-calculator/ (reviewed August 2026). Free to reference in articles, syllabi, and answer posts with a link.
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A worked example: $100 a month for 10 years
Take the future value mode with $100 per payment, a 6% annual rate, monthly compounding, and a 10-year term. The rate per period is 6% divided by 12, or 0.5% (0.005), and the total number of periods is 12 times 10, or 120. The growth factor is (1.005 to the power of 120 minus 1) divided by 0.005, which works out to (1.81940 minus 1) divided by 0.005, or 163.8794. Multiplying by the $100 payment gives a future value of $16,387.93. That is $12,000 of your own money plus $4,387.93 in compound interest, so interest makes up about 27% of the ending balance.
The compounding frequency is doing real work in that number. Change the compounding dropdown from monthly to yearly while leaving everything else alone, and the rate per period becomes the full 6% over just 10 periods. The growth factor is now (1.06 to the power of 10 minus 1) divided by 0.06, which equals 13.1808, so the future value drops to $13,180.79. The same $100 payments, the same 6% headline rate, but $3,207 less at the end, because each deposit sits for an average of about half a year longer before it starts earning interest under monthly compounding.
The payout mode runs the same inputs in reverse. With $100 as the starting amount available per period rather than the deposit, the calculator solves for what an account worth the given principal can sustain: $100 divided by ((1 minus 1.005 to the negative 120) divided by 0.005) is roughly $1.11 per period if you read the payment as the balance, which is why the mode's inputs matter. A cleaner reading: a $16,387.93 balance paying out monthly over 120 periods at 0.5% per period supports exactly the $100 payment that built it.
Where the model bends: zero rates, caps, and negative inputs
The formula has a hard edge at 0%. Plugging a 0% rate into the standard annuity expression would mean dividing by zero, so the JavaScript branches: with no interest, the future value is simply payment times periods ($100 a month for 10 years is exactly $12,000, interest contributes nothing), and the payout mode divides the balance evenly ($12,000 over 120 months is $100.00 per month, versus $100.00 per year at yearly compounding). Because interest can never grow the balance at 0%, payout-per-period is always at or below the payment in that branch.
Small positive rates are where the smooth curve starts. At 0.01% annual, monthly compounding on the $100/10-year example adds only pennies of interest; by 1% annual the same deposits end near $12,561. Every extra compounding period per year increases the exponent and shrinks the per-period rate, which raises the result slightly but never dramatically: moving from monthly (12 periods a year) to daily-style frequencies converges within a few dollars on ordinary inputs.
Input validation blocks the mathematically broken cases outright. A negative payment, negative rate, zero years, or a compounding frequency below 1 produces the 'Check inputs' error instead of a number, because a negative rate or negative term has no meaningful annuity interpretation. The compounding frequency must be a whole number; 12.5 compounds a year is rejected rather than silently floored. Real-world caveats remain that the tool does not model: fixed annuity results assume the rate never changes, actual annuity payouts are taxed as ordinary income when distributions begin, insurers subtract fees and mortality charges that can shave roughly 1% to 3% off the effective rate, and payments are assumed to occur at the end of each period, so results will differ slightly from annuity-due products that pay at the start.