How long will your money last? Financial longevity calculators

Each tool runs a month-by-month simulation of a single balance. Growth or interest is applied for the month, money is added or taken out, and the process repeats until the balance either reaches zero or the projection window ends. Nothing is smoothed, averaged over decades, or borrowed from a lookup table — you can follow the logic step by step, and each page explains its own version of it in the methodology section.

That design has a deliberate consequence: the answer is only as good as the assumptions you feed it. A retirement projection at 7% and the same projection at 4% can differ by more than a decade. Rather than hide that behind a single confident number, every input is editable and every result updates the moment you change something, so the sensitivity is visible instead of theoretical.

They are for anyone doing arithmetic on the back of an envelope: a person weighing an earlier retirement, someone deciding how much of a windfall they can responsibly spend, a household sizing an emergency fund, or a homeowner wondering what an extra $200 a month does to a mortgage. No accounts, no email capture, no data leaving your browser — every calculation runs locally on your device.

What they are not is a forecast. No model knows next year's returns, your future tax bill, or the boiler that fails in year six. Treat every result here as a well-reasoned estimate that shows the shape of a decision, not a prediction of how it will turn out.

The honest answer is frustration. Searching for something as basic as "how long will $500,000 last in retirement" tends to return one of three things: an article that never gives a number, a calculator wrapped in a lead-capture form, or a dense planning suite that wants twenty inputs before it will show you anything. The underlying arithmetic is not complicated. The barrier is presentation.

So the rule here is that the answer comes first. You land on a page, the default scenario is already calculated, and you adjust it toward your own situation. The reading material sits below the tool for the moment you want to know why the number moved — because understanding the mechanism is what lets you make your own decision instead of deferring to a calculator.

We also try to be candid about the limits. Every page states what its model ignores, and none of them will tell you what to do with your money. That is not modesty for its own sake; a tool that overstates its certainty is worse than no tool at all.

1. Start with today's reality. Enter the balance you actually have and the amount you actually spend, not the numbers you hope to reach. An honest baseline is the only useful starting point.
2. Set your assumptions. Return rates, interest, and inflation are inputs, not facts. Pick figures you could defend to a sceptical friend, then check the page's notes on what the defaults represent.
3. Read the projection. The chart shows the path, not just the endpoint. A balance that falls gently for twenty years and then drops off a cliff behaves very differently from one that declines steadily.
4. Test the scenario that worries you. Lower the return by two points. Raise spending by $500. Add a year of retirement. If the plan survives the pessimistic version, it is far more robust than one that only works at the optimistic settings.
5. Use it as a starting point. The output is a conversation opener — with a partner, or with a qualified professional who can account for your taxes, benefits, and circumstances.

Curious about the mechanics behind all of this? The methodology page explains the shared modelling approach, and each calculator repeats the specifics for its own case.