Annuities Explained: Turning a Lump Sum Into Guaranteed Income

How does an annuity work, mechanically? It’s the mirror image of a retirement-savings projection — instead of asking what a monthly contribution grows into, it asks what lump sum is needed today to fund a fixed monthly payment for a set number of years.

The formula

This is the present value of an ordinary annuity — the reverse calculation of the future-value formula used for retirement savings projections.

A worked example

To generate $2,000/month for 20 years at a 5% assumed return, you’d need roughly $303,000 today. To generate $3,000/month for 25 years at 6%, you’d need roughly $465,621. The Annuity Calculator runs this present-value calculation for your own target payment and timeline.

Why the required lump sum is less than a simple multiplication

The naive approach — payment × number of months — assumes the money just sits still while being paid out, which overstates what’s actually needed. In reality, the remaining balance keeps earning a return throughout the payout period, which means each payment is partly funded by ongoing growth, not solely by drawing down principal. This is exactly why the present-value formula returns a smaller number than a flat multiplication.

Types of annuities, beyond the pure math

This calculator models the pure present-value math behind a fixed, immediate annuity — real commercial annuity products layer additional fees, guarantees, and insurer profit margins on top of this baseline math, meaning an actual insurance company’s quoted premium for the same payment stream will typically be somewhat higher than this pure calculation suggests.

The real tradeoff: guaranteed income vs. flexibility

An annuity’s core appeal is eliminating the risk of outliving your money — the payment continues for the agreed term (or for life, in some products) regardless of how markets perform. The cost of that guarantee is flexibility: once committed, the lump sum is generally no longer available for other uses, unlike a self-managed investment portfolio drawn down at your own discretion.

How the assumed return rate changes the required lump sum

The assumed return rate has a substantial effect on the required lump sum, and it’s worth understanding the direction: a higher assumed return means a smaller lump sum is needed, since the remaining balance is doing more of the work through ongoing growth. This is exactly why real annuity providers, who must actually guarantee the promised payments rather than merely project them, tend to assume conservative returns when pricing a real annuity product — a conservative return assumption requires a larger premium to safely guarantee the same payment stream, which is part of why commercial annuity pricing tends to be less favorable than this pure present-value math alone would suggest.

From accumulation to payout

Annuities represent the payout side of retirement planning — the accumulation side is covered by a retirement savings projection, which shows what a recurring contribution grows into over time. Together, the two calculations cover the full arc: building a balance during working years, then converting it into an income stream during retirement.

Inflation’s effect on a fixed annuity payment

A fixed annuity payment’s purchasing power erodes over time due to inflation — a $2,000/month payment agreed to today buys meaningfully less in year 15 of a 20-year payout than it does in year one, even though the nominal dollar figure never changes. Some annuity products offer an inflation-adjusted (increasing) payment option specifically to address this, typically at the cost of a lower starting payment or a larger required premium — a real tradeoff between payment stability today and purchasing-power protection over the full payout period, worth understanding before assuming a flat fixed payment maintains its value throughout a long payout term.

FAQ

Is the lump sum from this calculator what an insurance company would actually charge for an annuity? No — real annuity products include fees and insurer margins on top of the pure present-value math shown here, so an actual quote will typically be somewhat higher than this calculation.

Why is the required lump sum less than payment × number of months? Because the remaining balance continues earning a return throughout the payout period, partially funding each payment through ongoing growth rather than principal alone.

What happens if I live longer than the annuity’s term? For a fixed-term annuity, payments simply stop at the end of the agreed term — a lifetime annuity, a different product structure, guarantees payments for life regardless of how long that turns out to be, at a correspondingly different cost.

Does inflation reduce the value of a fixed annuity payment over time? Yes — a flat nominal payment buys less in later years of a long payout period; some products offer an inflation-adjusted payment option, typically at a lower starting amount or higher required premium.

Is a lump sum better than an annuity for a large one-time payout, like a lottery win? It depends on individual circumstances — an annuity provides guaranteed structure and protection against overspending a lump sum, while a lump sum offers flexibility and control, including the option to invest it independently at potentially better terms than an annuity’s built-in return assumption.