Two halves, and the second one is the question
Every retirement calculator can tell you what a pot grows to. The number that actually decides anything is what happens afterwards: whether that pot pays the income you want for as long as you need it, and if not, when it stops. This page runs both halves on the same monthly schedule — money going in until the day you stop working, money coming out from that day until the age you name — and reports the turn rather than only the peak.
The withdrawal is taken at the start of each month and what is left earns for the rest of it, which is the way spending actually happens. A month that cannot pay the whole withdrawal pays what is there and the schedule records it. That is what lets the page say "the money runs out at eighty-four" instead of quietly carrying a negative balance to the end of the table.
Everything you type is in today's money
This is the single most useful convention on the page and the one most likely to be got wrong elsewhere. You cannot judge whether an income decades away is enough, because you have no feel for what money will be worth then. You can judge whether it is enough today.
So the income you want and any other income you expect are both entered in today's money, and the page carries them forward at the inflation rate before comparing them with the pot. The withdrawal then rises each year in retirement, so the plan holds its buying power rather than its face value — which is the difference between a plan that works and one that fails slowly over twenty years.
The income the pot supports is shown twice for the same reason: once as the amount that will actually arrive in the first year of retirement, and once deflated back to today's money so you can tell whether it is a life you would want.
Two rates of return, because most people change what they hold
The form asks for a return while saving and a return in retirement. Most people move towards holding less in shares as they stop earning, and a single rate across fifty years quietly assumes otherwise. Both figures are assumptions you are making, not forecasts this site is offering, and the difference between running the page at one pair of rates and another is usually larger than the difference between any two savings strategies.
The inflation field is prefilled with what consumer prices have actually done over the last twenty years, annualised. That is a measurement rather than a prediction, and twenty years is the horizon because a retirement plan runs for decades and last year's figure says very little about the next thirty.
The income the pot supports is found, not formula'd
Rather than applying a withdrawal formula, the page searches the schedule itself for the largest monthly withdrawal that survives to the last month. The answer therefore belongs to the same table you can read underneath: take a dollar more and the money runs out early. This also means it respects the indexation, which a flat percentage rule cannot.
If you leave the target income blank, the plan simply spends what the pot supports. If you fill it in, the page tells you whether the pot reaches it and by how much it misses.
The risk this page cannot show you
A single fixed rate of return hides the thing most likely to break a retirement: the order the returns arrive in. A bad few years immediately after you stop working does far more damage than the same average spread evenly, because you are selling assets to live on while they are down and they are not there to recover. Two plans with identical average returns can end very differently for that reason alone.
Showing that properly needs many simulated paths rather than one line, and it needs a way of presenting a range of outcomes that does not mislead. Until this page does that, treat the single figure it gives you as the middle of a spread rather than as the answer.
What else this does not do
It does not model tax, in either half. It does not model required minimum distributions, fees during retirement, the order you draw from several accounts, part-time work after retirement, care costs, or a partner with a separate plan. It does not know your state pension entitlement — that is what the other income field and the state pension page are for.