The same savings, asked two ways
Somebody with a retirement account usually has one of two questions. The first is how much it can pay each month if it has to last a given number of years, which is what people mean by the monthly payout of an IRA or a 401(k). The second is the other way round: at the income they have in mind, how long the money lasts. This page answers either, on the same monthly schedule, and the table under the answer shows every year of it.
Each month the payment is taken first and what is left earns for the rest of the month, which is the order spending actually happens in. When a month cannot pay the whole amount the schedule pays what is there and stops, so the page can tell you the month the money runs out rather than carry a negative balance to the end of the table.
Why the payout is not a percentage rule
Rules of thumb express a payout as a share of the starting balance. They are a useful first guess and a poor answer, because the payout a pot can sustain depends on three things the rule fixes in advance: how long it has to last, what the money earns while it waits, and whether the payment has to keep up with prices. Change any of the three and the right share moves a long way.
The page works the other way: it takes the three as inputs and searches for the largest monthly amount that still covers every payment to the end of the period. The result runs the balance down to nothing on the last day, which is exactly what a payout for a fixed period means, and it is why the period you enter should be longer than you expect to need.
A payment that keeps up with prices costs more than it looks
The payment rises once a year by the rate you set, prefilled with what consumer prices have done over the long run. That keeps the payout's buying power level, which is what makes it an income rather than a slowly shrinking allowance. It also means the first payment has to be lower than a flat one would be, because the later ones are larger. Set the rise to nothing to see the flat payment instead, and compare what the last year of each would buy.
What the answer leaves out
The payout is before tax. In the United States, withdrawals from a traditional IRA or 401(k) are generally taxed as income, while qualified withdrawals from a Roth account are generally not, so the same payout can leave very different amounts in your hand. Traditional accounts also require minimum withdrawals from a certain age, which this page does not model.
The largest gap is the order in which returns arrive. The page uses one fixed return every year, and real markets do not. A run of poor years just as withdrawals begin does more damage than the same average return spread evenly, because money is being sold low to pay the bills. A single-rate projection cannot show that, and it is the main reason to treat any payout figure as a starting point rather than a promise.