There’s a £10,000 in a savings account somewhere in your family — being “sensible”, earning a little interest, safe. Except run it forward ten years at 3% inflation and its buying power is about £7,400. Nothing was spent. Nothing was lost to fees or crashes. The pounds are all still there — they just each buy less, and nobody sent a statement about it.

That’s inflation: a tax with no bill, collected from cash by default. And because it compounds — the same machinery as the growth chart, pointed the other way — it’s exactly the kind of slow arithmetic human intuition ignores and a spreadsheet makes undeniable.

The two-line model

nominal  =B2 * (1 + interest)      what the statement says
real     =C2 * (1 + interest) / (1 + inflation)    what it buys

One row per year, copy down twenty, chart both lines. The statement line climbs gently and reassuringly. The buying-power line — the true one — falls whenever interest < inflation, which for ordinary savings accounts is most of most decades.

the statement: £11,046 ✓ what it buys: £8,190 inflation's take 10 years → same account, two truths — only one appears on any statement
The grey line is what the bank reports; the green one is what the money can do. The shaded wedge is a tax nobody invoiced.

The chart usually lands harder than any lecture — especially on the family member who’s been keeping serious money “safe” for a decade. It also explains the odd vocabulary money people use: nominal (the pounds on paper) versus real (the buying power), and real return — interest minus inflation, roughly — as the only return worth quoting.

The habit: build models in today’s money

This is the one that upgrades everything else in the field notes. Every long-range model faces a choice: project in nominal pounds (bigger, flattering, meaningless to your 2046 self) or in real terms — using a real growth rate, so every output is in pounds you can actually judge today.

You’ve already been doing it — the pension countdown insisted on a real rate precisely so “£1,800 a month” still means something at the crossing date. The general rule for any model that spans decades: subtract inflation from the growth assumption, then read every result in today’s money. (The precise form is (1+r)/(1+i)−1; rate-minus-inflation is close enough for planning, and either belongs in a named cell where a Data Table can sweep it — inflation at 2% vs 4% over thirty years is the sensitivity worth staring at.)

What to actually do about it

The model, not this page, hands you the conclusions — but they fall out fast:

  • Cash has a job description: the runway and the sinking funds — money whose availability matters more than its growth, in the best-paying accounts you can be bothered to switch to. Inflation is the fee you pay for certainty; pay it on months of spending, not decades of savings.
  • Long-horizon money needs a real return above zero — historically the reason equities-in-an-ISA and pensions beat deposit accounts for retirement, with their volatility. (The trade-offs are the countdown’s territory, and past a certain pot size, a professional’s.)
  • Debts are on your side here — inflation quietly shrinks a fixed mortgage balance in real terms, one of the few respectable arguments for the invest side of the overpay question.

One division per row — that’s all “real terms” is. But it’s the difference between models that flatter and models that tell the truth, and between money that looks safe and money that is. The quiet tax can’t be repealed; it can only be seen — and seen, planned around. That’s what the spreadsheet is for.