How Inflation Quietly Rewrites Your FIRE Number

Inflation is the assumption people set once and never revisit, and it is the one with the longest lever. Over a 20-year accumulation followed by a 40-year retirement, you are compounding it for sixty years. Small differences at that length are not small.

What it actually does to the target

Two effects, working in the same direction.

It raises what your life costs. At 3%, $50,000 of annual spending becomes $90,306 in twenty years and $163,102 in forty. Nothing about your life improved; the same basket now carries a bigger price tag.

It shrinks what your return is worth. A 5% nominal return against 3% inflation is not 5% of purchasing power. It is 1.94% — and that is the number that has to fund your spending.

Together, they set the corpus. For $50,000 a year over a 40-year retirement at a 5% nominal post-retirement return:

From 3% to 4% — one percentage point, well within the range of plausible long-run outcomes — the target moves by $266,955. At a $42,000-a-year savings rate that is more than six extra years of work, decided entirely by an assumption you typed once.

The bottom row is worth staring at. When inflation equals your nominal return, the real return is zero and the corpus is simply spending × years: $50,000 × 40 = $2,000,000. No compounding is helping you at all. That row is what a sustained inflationary decade looks like on a conservatively invested portfolio.

Divide, don’t subtract

The real return is not nominal − inflation. It is:

real return = (1 + nominal) / (1 + inflation) − 1

At 5% and 3%, subtraction gives 2.00% and division gives 1.9417%. The gap looks like a rounding error and is worth $13,840 on the 40-year target above — small, but it runs the wrong way (the shortcut always flatters your plan), and it grows with both the rates and the horizon. The calculator divides.

The mistake that flatters every DIY spreadsheet

This is the single most common arithmetic error in home-made FIRE models, and it is bad enough to invalidate the whole plan:

Projecting a future portfolio balance in future money, then comparing it to a target expressed in today’s money.

Our example target is $1,394,107 in today’s money. By the time our 30-year-old reaches 50, twenty years of 3% inflation have moved the same requirement to $2,517,913. A spreadsheet that projects a $2.1M balance and compares it to $1.4M reports a comfortable surplus for a plan that is actually $400,000 short.

The error scales with the horizon: at 3% inflation it flatters a 20-year plan by a factor of 1.8 and a 30-year plan by 2.4.

There are two valid conventions, and either works as long as you never mix them:

• Everything in today’s money. Use real returns everywhere, keep spending constant, compare a real balance to a real target. Easier to reason about, because every number means something you can picture.
• Everything in future money. Use nominal returns, inflate spending each year, inflate the target to the retirement date. Harder to read, but matches the numbers you will see on a statement.

This site uses the first convention for the headline and shows the second where it matters — the target inflated to your retirement age, so you can compare it to a projected balance without doing the conversion yourself.

Where inflation is not 3%

The single national figure is a weighted average of a basket that is not yours. Two adjustments deserve thought:

Your basket is not the index. Retiree spending skews toward healthcare, insurance and services, and those categories have persistently outpaced headline inflation in most developed countries. Goods that have gotten cheaper — electronics, clothing, imported manufactures — are a smaller share of a retired household’s budget. It is common for retirees to experience an effective inflation rate half a point to a point above the published one.

Housing changes the picture in both directions. A fixed-rate mortgage is a large nominally-fixed expense that inflation erodes in your favour, and a paid-off house removes the fastest-inflating line item in most budgets entirely. A renter has the opposite exposure: an expense that tracks or exceeds inflation for the entire retirement, with no end date.

Some income is indexed and some is not. Social Security and many public pensions carry inflation adjustments; most private annuities and defined-benefit payments do not, and a nominal fixed pension loses about 45% of its purchasing power over twenty years at 3%.

What to assume

There is no correct answer, only defensible ones. Three practical guidelines:

Use 3% as a base case, not 2%. Central bank targets around 2% describe an intention over a policy horizon, not a realised long-run average — and your plan spans several policy regimes. Assuming the target will be hit for sixty consecutive years is an optimistic choice presented as a neutral one.

Stress-test at 4%. If your plan works at 3% and breaks at 4%, it is more fragile than its headline suggests, and you want to know that now rather than in year fifteen.

Assume a higher rate for healthcare-heavy budgets. If medical costs are a large share of your projected spending, modelling the whole budget at the headline rate understates what you need.

Defences that actually help

• Equity exposure through the whole plan. Over long horizons, ownership of productive assets is the most reliable inflation hedge available to a retail investor. This is a genuine argument against de-risking too aggressively at 50.
• Inflation-linked bonds for the portion of the portfolio you cannot afford to see eroded — they trade real yield for the removal of exactly this risk.
• Flexible spending. The same defence that works against sequence risk works here: a budget that can absorb a bad decade is worth more than a target with an extra $100,000 in it.
• Owning your home outright by retirement, which converts the largest and most reliably-inflating line in the budget into a fixed cost.

Set your own inflation rate on the calculator and watch both the target and the “money lasts to” age move. The sensitivity is larger than almost anyone expects, and seeing it directly is more persuasive than any argument about what the right number is.