The formula
How to calculate safe withdrawal rate
A safe withdrawal rate is the share of a portfolio you can take each year without running out. This computes it directly from your horizon and expected real return rather than relying on the 4% rule of thumb.
The formula assumes a steady real return and a balance that ends at exactly zero. That is deliberately less conservative than the historical simulations behind the 4% rule, which are built to survive the worst sequence on record rather than the average one.
The inputs, one by one:
- Portfolio value
- Years the money must last (years)
- Real return (%)
The result updates on every keystroke. The URL updates too, which makes the filled-in version easy to bookmark or send to someone else.
Where a figure is not immediately to hand — a precise interest rate, an exact balance — a reasonable estimate is a perfectly good starting point. Because every result updates instantly, refining a rough guess into the real figure once you have it takes a moment, and nothing about the calculation depends on getting it exactly right on the first attempt.
Why safe withdrawal rate matters
FIRE stands for Financial Independence, Retire Early — reaching a portfolio large enough that investment income covers your living costs, so paid work becomes a choice rather than a necessity. It is not a specific account or product, just the point at which the figures below cross over.
The formula behind safe withdrawal rate is standard and has not changed in decades; what changes is the situation it gets applied to. Two households can run the identical calculation and land on very different conclusions once their own numbers — income, rate, term, balance — are dropped in, which is why a generic textbook example is less useful than a calculator you can adjust to match your own circumstances.
Beyond a one-off check, the same calculation is worth revisiting whenever the underlying numbers change — a new interest rate, a change in income, a different term. Because the figures live in the page's own web address, coming back to update just one field and compare the new result against the old one takes seconds rather than starting again from a blank page.
The reason a page like this exists at all, rather than leaving the calculation to a spreadsheet or a textbook appendix, is that the formula behind safe withdrawal rate is fiddly enough to get wrong by hand but not complicated enough to need specialist software. That middle ground — real enough maths to matter, simple enough to check instantly — is exactly what a dedicated calculator is for, and it is why the same figure recalculated here should match a careful manual calculation almost exactly.
Where the same calculation needs to be run for several different scenarios side by side — three loan offers, two savings plans — the fastest approach is usually to open the calculator in a second browser tab for each one, so that the results can be compared directly rather than overwriting each other in a single set of fields.
Worked example
Here is the calculation with the starting values:
- Portfolio value: 800,000
- Years the money must last: 40 years
- Real return: 4 %
That gives:
- Sustainable withdrawal rate: 5.052 %
- Annual withdrawal it supports: 40,418.791
- Annual withdrawal under the 4% rule: 32,000
These figures are only the calculator's own starting values, included so the working is visible rather than hidden inside the tool above. Replace them with your own numbers and the same arithmetic applies — nothing about the method changes, only the inputs feeding it.
Reading the result
Compare the two outputs. Where this figure sits above 4%, the difference is the cushion the 4% rule keeps back for bad market sequences. Where it sits below, your horizon or your return assumption is telling you that 4% is too aggressive.
Where this goes wrong. Treating a smooth return as equivalent to a real one. Two portfolios with identical average returns can end very differently depending on when the bad years arrive — that is sequence risk, and this formula cannot see it.
A useful check on any unfamiliar result is to compare it against a rough mental estimate first — round the inputs to convenient numbers and see whether the calculator's answer lands in roughly the same territory. A wildly different figure usually means one of the fields was entered in the wrong unit, most often a percentage typed as a whole number where a decimal was expected, or the reverse.
As a starting point, yes, with caveats. It came from US data over 30-year periods with a 50–75% equity allocation. Longer retirements, higher fees and non-US markets all argue for something closer to 3.5%.
A lot. Plans that cut spending 10% after a bad year, or skip the inflation increase, sustain rates around half a percentage point higher than rigid ones. Adaptability is worth more than precision in the initial rate.
The headline figure is sustainable withdrawal rate. With 800,000 portfolio value, 40 years years the money must last and 4 % real return, that comes to 5.052 %. Change any field and the figure moves with it.
Whenever one of the underlying figures changes — a new interest rate, a different balance, an updated term — since the result only reflects what is currently in the fields. There is no need to keep a separate record of past results; the web address for a filled-in version already carries the figures used to produce it.
Not unless a tax rate or a fee is explicitly one of the inputs above. Where it is not, the figure shown is a gross calculation, and any tax due depends on your personal circumstances and current tax rules, which are worth checking separately.
The arithmetic itself is exact — the calculator applies the formula shown above precisely, with no rounding until the final figure is displayed. The uncertainty, where it exists, is entirely in the inputs: an estimated rate or an approximate balance carries that same approximation through to the result.