How To Calculate Annuity Payments

This is the standard amortisation identity run backwards. The same formula that turns a mortgage balance into a monthly payment turns a retirement balance into a monthly withdrawal, because both are the same question: given a present sum, a rate and a number of periods, what constant payment exactly consumes it? Reversing the direction merely changes who receives the payment and who is paid the interest.

The mechanics have a useful sanity check baked in. A $500,000 balance earning 5% generates $2,083.33 per month forever if nothing is ever withdrawn from principal. Drawing it down over 25 years instead allows $2,922.95 a month. The difference between the two — $839.62 monthly — is precisely the return of your own capital, spread across 300 payments. When the payment noticeably exceeds the pure interest income, that excess is your balance coming back to you, and it is why the account reaches zero right on schedule.

Payment frequency changes the answer more subtly than people expect, because it changes both the rate and the number of periods at once. Quarterly withdrawals of the same balance produce $8,787.14 per quarter against $2,922.95 per month, and over the full term the quarterly pattern yields $878,713.94 versus the monthly total of $876,885.06 — about $1,829 more in aggregate, because each instalment stays invested slightly longer before it is taken out. The effect is small but consistently in that direction: fewer, larger, later withdrawals total slightly more.

Timing within the period matters in the same way. The ordinary form above assumes each payment comes out at the end of the period, giving the money a full period of growth first. An annuity-due, drawn at the start of each period, loses that one-period cushion on every payment and so supports a lower amount. Over hundreds of periods the two conventions differ by roughly one copy of the periodic rate, which for monthly drawdowns is a fraction of a percent.

What the calculation assumes is where its real value lies, because each assumption is a way the plan can fail. It presumes a constant return every single period. Actual portfolios deliver variable returns, and variation during distribution is not the benign thing it was during accumulation: withdrawing from a balance that has just fallen crystallises losses and compounds badly. This is sequence-of-returns risk, and two retirees with identical average returns and different ordering can face wildly different outcomes — one may run out of money years earlier than the arithmetic suggests.

It also presumes payments are level in nominal terms, which quietly means they decline in purchasing power. A fixed $2,922.95 monthly payment loses roughly a third of its real value over twenty-five years at 2.5% inflation. Sustainable-spending research therefore usually works with inflation-adjusted payments instead of level ones, which lowers the safe starting figure and then increases it annually — the pattern behind the widely cited four percent framework, in which the first-year withdrawal is four percent of the balance and every later year is that amount indexed to prices.

Finally this is the mathematics of a self-funded drawdown, not of a commercial annuity contract. Buying a single-premium immediate annuity from an insurer replaces the whole question: the insurer pools longevity risk across many annuitants and pays until death rather than until exhaustion, so a 65-year-old purchasing one often receives a higher monthly amount than self-managing would safely allow, precisely because someone who dies early subsidises someone who lives to 100. The trade is liquidity and bequest for certainty — and for longevity protection that self-funding cannot replicate at any return assumption.

Taxes sit outside every version above. Distributions from tax-deferred accounts are ordinary income, while taxable accounts may trigger capital gains on each rebalancing sale. Whatever the arrangement, the formula computes a gross figure; what arrives depends entirely on account type and jurisdiction.