The formula
How to calculate retirement savings needed
Retirement planning starts with an income target rather than a pot. Once you know the income you want and the part already guaranteed, the savings requirement is whatever fills the difference.
The replacement ratio is below 100% for good reasons: pension contributions stop, commuting stops, the mortgage is often repaid and tax is usually lower. Somewhere between 60% and 80% is the conventional range.
What to enter:
- Current annual income
- Income to replace (%) — most households need 60–80% of pre-retirement income
- Guaranteed pension income — state pension plus any defined benefit scheme
- Withdrawal rate (%)
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.
The order the fields are filled in makes no difference to the result — the calculator recomputes the whole formula from whatever is currently in every field, not step by step. That means it is safe to adjust one number, watch the result change, and adjust it back, without worrying about resetting anything first.
Why retirement savings needed 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 retirement savings needed 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.
It is also worth being clear about what a single figure like this can and cannot settle on its own. It answers the specific question the formula was built to answer, and nothing more — a favourable retirement savings needed result does not automatically mean a decision is a good one overall, since plenty of other factors that a formula cannot capture, from personal circumstances to how comfortable a commitment feels, usually matter just as much as the arithmetic. Use the number as one solid input among several rather than the whole of the decision.
In practice, most people arrive at a page like this one having already tried a version of the calculation by hand or in a spreadsheet, and use the calculator here to confirm it rather than replace it. That is a reasonable way to use it — the two should agree to the last decimal place if the same inputs and the same formula are used, and if they do not, the formula shown above is the one to check your own working against first.
Worked example
Work through the defaults on this page:
- Current annual income: 55,000
- Income to replace: 70 %
- Guaranteed pension income: 11,500
- Withdrawal rate: 4 %
That gives:
- Savings needed: 675,000
- Annual income the portfolio must provide: 27,000
- Total retirement income targeted: 38,500
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
The state pension does more heavy lifting than people expect. At around £11,500 a year it is equivalent to nearly £290,000 of portfolio at a 4% withdrawal rate — subtracting it first often shrinks the target substantially.
Where this goes wrong. Counting the full state pension without checking your National Insurance record. The full new state pension requires 35 qualifying years, and career breaks or contracted-out periods commonly leave people short.
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.
Around 70% is the usual planning figure. Use more if you plan to travel heavily or still have a mortgage; less if the house is paid off and your current income includes large pension contributions you will stop making.
Yes — subtract what you already hold from the savings needed figure to get what is still to accumulate. This calculator gives the total requirement, not the remaining gap.
The answer it gives you is savings needed. With 55,000 current annual income, 70 % income to replace and 11,500 guaranteed pension income, that comes to 675,000. 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.