The formula
How to calculate income replacement ratio
The income replacement ratio measures retirement income against what you earned before. It is the standard benchmark pension schemes and advisers use, precisely because most households need less than 100% to maintain their standard of living.
The reason the target sits below 100% is that a chunk of pre-retirement income never reached your standard of living: pension contributions, National Insurance, commuting and often a mortgage all stop.
The calculator asks for:
- Pre-retirement income
- Expected retirement income
- Target ratio (%)
The result updates on every keystroke. The URL updates too, which makes the filled-in version easy to bookmark or send to someone else.
Some of the fields above will accept figures that seem unusual for your own situation, and that is deliberate: the formula behind income replacement ratio works the same way regardless of scale, so the calculator does not stop you testing a hypothetical scenario a long way from your actual numbers — often the fastest way to see which input the result is most sensitive to.
Why income replacement ratio 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.
A calculation like this usually gets used at a decision point rather than out of curiosity — comparing two real options, checking a number a lender or adviser has quoted, or working out whether a plan that sounded fine in conversation still holds up once it is written down with actual figures. The maths itself is rarely complicated; what is hard is remembering which figures to use and in what order, which is exactly what a dedicated calculator is for.
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 income replacement ratio 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.
A calculator like this one is often bookmarked and returned to repeatedly over months rather than used once, particularly for anything tied to an ongoing plan such as a mortgage, a savings goal or an investment being tracked. Because the figures live in the web address rather than only in memory, coming back to the same page with updated numbers is quicker than starting from a blank spreadsheet each time.
Worked example
Take the figures the calculator starts with:
- Pre-retirement income: 58,000
- Expected retirement income: 36,000
- Target ratio: 70 %
That gives:
- Replacement ratio: 62.07 %
- Annual income gap to target: 4,600
- Capital needed to close it at 4%: 115,000
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
Between 60% and 80% is the usual planning range. Lower earners typically need a higher ratio, because a larger share of their income goes on essentials that do not fall in retirement; higher earners often manage on less.
Where this goes wrong. Comparing gross with net. If pre-retirement income is gross salary and retirement income is what lands after tax, the ratio is understated by a wide margin. Use the same basis on both sides.
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% for most households. Push it towards 80% if you plan to travel extensively or will still be paying a mortgage; 60% can be enough if the house is paid for and your current income includes heavy pension saving.
It should. The state pension is retirement income like any other, and for households on average earnings it can supply a third or more of the target on its own.
The headline figure is replacement ratio. With 58,000 pre-retirement income, 36,000 expected retirement income and 70 % target ratio, that comes to 62.07 %. 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.