The formula
How to calculate retirement nest egg
This sizes a pot that is meant to be spent, not preserved. Unlike the 25× FIRE number, which assumes the portfolio lasts forever, this runs the balance down to nothing over a defined number of years.
It is the present value of an annuity, computed with a real return so the spending figure stays in today's money and rises with inflation automatically.
Here is what each field means:
- Annual spending
- Years in retirement (years)
- Real return (%)
Everything recalculates as you type, and the numbers in the address bar update with it, so a link to this page carries your figures with it.
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 retirement nest egg 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.
Most people who look up a retirement nest egg calculation already have a specific number in mind — a quote, an offer, a target — and want to check it rather than learn the theory behind it. This page is built for that: enter your own figures, see the result immediately, and change any field to see how the answer moves without redoing the arithmetic from scratch each time.
It is also useful as a sense check before signing anything. A quote, an offer letter or a spreadsheet from someone else can contain an error, an optimistic assumption, or simply a different convention for rounding — running the same inputs through an independent calculator is a quick way to confirm a number before relying on it.
This kind of calculation rarely stands entirely alone. A retirement nest egg figure usually feeds into a wider decision — how it compares with a competing offer, whether it fits inside a monthly budget, what it does to a longer-term plan — and the value of having it as an exact number rather than a rough guess is that those follow-on comparisons stop being guesswork too. Once one figure in a decision is precise, it is worth making the effort to get the others precise as well, rather than mixing an exact calculation with several estimates and treating the result as equally reliable.
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:
- Annual spending: 36,000
- Years in retirement: 30 years
- Real return: 3.5 %
That gives:
- Nest egg required: 662,113.63
- Required with no growth at all: 1,080,000
- Saved by investing rather than holding cash: 417,886.37
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
Depletion is materially cheaper than perpetuity. Thirty years of £36,000 needs around £662,000 at a 3.5% real return, where the perpetual version at a 4% withdrawal rate needs £900,000 — the difference is what leaving an estate costs.
Where this goes wrong. Choosing the number of years by guesswork. A 65-year-old today has a meaningful chance of reaching 95, and a plan built for exactly 30 years leaves nothing if that happens. Add a margin, or keep a guaranteed income to fall back on.
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.
Use this one if you are comfortable spending the capital and have a defined horizon, typically retiring at a conventional age. Use the FIRE number if you are retiring early, want to leave an estate, or dislike the idea of a plan that ends at zero.
Retirement portfolios usually hold more bonds, so 2.5–3.5% after inflation is a reasonable planning range. Assuming an equity-like real return while holding a defensive portfolio is the mistake that quietly undersizes the pot.
It returns nest egg required. With 36,000 annual spending, 30 years years in retirement and 3.5 % real return, that comes to 662,113.63. 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.