Money under the mattress feels safe because the number never changes. But the number is not the point — what that number can buy is. Inflation is the slow erosion of purchasing power, and it means a currency amount that felt like plenty a decade ago quietly buys less each year.
This is why "keep it all in cash to be safe" is one of the most expensive pieces of comfortable advice around. Cash is stable in name and shrinking in substance.
The arithmetic of erosion
At a modest few percent of inflation a year, prices roughly double over a couple of decades. Put differently, cash left idle can lose something like a third to a half of its real value across a working life, without a single number on your statement ever going down. The loss is invisible precisely because it hides in prices rather than in your balance.
This does not mean cash is useless — far from it. It means cash is a tool for short-term needs and safety, not a place to grow long-term wealth.
What cash is genuinely for
Keep cash for its real strengths: an emergency fund you can reach instantly, money for goals within the next few years, and the sheer calm of knowing the rent is covered no matter what markets do. For those jobs, stability is exactly what you want and short-term growth is irrelevant.
The mistake is not holding cash. The mistake is holding all of it in cash, including money you will not need for decades and which inflation is guaranteed to erode.
Outrunning it over time
Historically, owning a slice of the productive economy — through broad, low-cost stock funds — has been the most reliable way for ordinary people to grow money faster than inflation over the long run. Prices rise, but so, over time, do the revenues and profits of the businesses you part-own.
The framing that helps: cash protects you from market volatility, and investing protects you from inflation. A sensible life needs protection from both, which is why you hold some of each.
The published figure is not your figure
National inflation statistics are constructed from a basket of goods and services weighted to represent an average household, and almost nobody is that household. Your own rate depends on what you actually buy, and it can differ from the headline figure substantially and persistently.
The largest source of divergence is housing. Someone with a fixed mortgage payment experiences no increase in their largest cost, while a renter facing annual increases experiences a great deal. Someone who drives a long commute is exposed to fuel prices in a way that a cyclist is not. Households with children face education and childcare costs that move differently from the general basket.
The practical implication is that planning against the published figure can mislead in either direction, and that the useful exercise is to look at your own largest recurring costs and how they have moved over several years. That is a more honest input to any long-term projection than a national average constructed for a different purpose.
Why the erosion is invisible
The reason cash losing value causes so little alarm is that the number does not move. A balance sits at the same figure month after month, and nothing about a statement communicates that the figure buys less than it did. Losses that are visible provoke a response; losses that require a calculation do not.
This is a specific instance of what economists call money illusion: the tendency to think in nominal rather than real terms. It shows up everywhere in financial behaviour, from reluctance to accept a nominal pay cut in circumstances where a real one is accepted without complaint, to satisfaction with a savings rate that is below the rate prices are rising.
The corrective is to occasionally state balances in real terms — what a sum would have bought at some earlier date compared to now. It is a mildly depressing exercise and it is the only way to make the effect visible, since neither the account nor the statement will ever show it.
What has historically outpaced it
Over long periods, broad equity holdings have historically delivered returns above inflation by a meaningful margin, which is the principal reason they feature in long-term plans despite their volatility. This is a historical record rather than a guarantee, and it is the strongest available argument for accepting price variability in exchange for maintained purchasing power.
Conventional bonds have historically done less well against inflation, for the reason discussed elsewhere on this site: their payments are fixed, so unexpected inflation erodes them directly. Index-linked government bonds, where available, address this directly by adjusting payments with a measured price index.
Property has a mixed record that varies enormously by location and period, and the popular belief that it reliably tracks inflation is not well supported across all markets. Commodities and precious metals are frequently proposed as inflation hedges and their record over long periods is less consistent than the argument implies. The dull conclusion is that broad equities have been the most reliable long-run defence, and that no asset provides protection over short periods.
The years when it moved quickly
Long periods of low and stable price increases can make this feel theoretical, and the historical record is a useful corrective. There have been extended stretches in developed economies where prices rose at rates that halved purchasing power within a decade, and several in living memory.
What those periods demonstrated is how quickly the arithmetic becomes serious. At low rates, the erosion is slow enough that a decade of cash holding is a mild cost. At higher rates, the same decade is transformative, and the difference between the two scenarios is not visible in advance.
This is the argument for treating inflation as a risk to be planned for rather than a variable to be forecast. A plan that only works if price increases remain low is a plan resting on an assumption nobody can support, and building in some protection is cheaper than being wrong about it.
Wages, and the half of the equation people forget
The effect on savings is only one side of it. Inflation also interacts with income, and whether a household ends up better or worse off depends on whether wages keep pace. Historically they sometimes have and sometimes have not, and the periods where they lagged were experienced as a sharp fall in living standards regardless of what savings did.
For anyone with a mortgage, there is an offsetting effect that gets little attention: inflation erodes the real value of fixed debt as well as fixed savings. A mortgage balance that stays the same in nominal terms while wages and prices rise becomes progressively easier to service, which historically transferred value from lenders to borrowers.
The net position for any household therefore depends on the balance between its cash holdings, its fixed debt and its income trajectory. Someone with a large mortgage, secure wages and modest cash may be a net beneficiary of an inflationary period, which is the opposite of the usual framing.
What to hold in cash despite all this
None of this argues for holding no cash, and the argument for holding some is unaffected by the erosion. Cash exists to cover near-term needs and emergencies, and its value in that role has nothing to do with its long-run return.
The correct amount is the amount those functions require, discussed at length elsewhere on this site, and the erosion is the price paid for having it available. That price is worth paying, because the alternative — being forced to sell a volatile asset at a bad moment — is considerably more expensive than a modest annual loss of purchasing power.
What is not worth doing is holding substantially more cash than those functions require, over a period measured in years, out of a preference for the stability of a number that is quietly shrinking. That is the specific behaviour the erosion argument is directed at, and it is common among people who correctly built a buffer and then never stopped. None of this is financial advice, and the appropriate balance depends on circumstances only you can assess.
The compounding works the same way in reverse
The most common error in thinking about this is treating the erosion as linear. It is not. A given annual rate applied to a shrinking real value compounds in exactly the manner described in the compounding articles on this site, which means the loss over twenty years is far larger than twenty times the loss over one.
The rule of 72, mentioned elsewhere here as a way of estimating doubling times for investments, works equally well in reverse: divide 72 by the inflation rate to estimate how many years it takes for purchasing power to halve. At modest rates that is a few decades. At the rates seen in some historical periods it is under a decade.
Running that calculation against a cash balance you have been holding for years is an uncomfortable and clarifying exercise. It converts an abstraction into a specific statement about a specific sum, which is the form in which people actually act on information.
Why the rate on cash rarely closes the gap
A reasonable objection to all of this is that savings accounts pay interest, which offsets the erosion. Sometimes they do. Historically, the rate available on ordinary deposits has frequently sat below the rate prices were rising, which means the real return was negative even while the nominal balance grew.
The reason is structural rather than accidental. Deposit rates follow central bank policy rates, and those are set with reference to economic conditions rather than to preserving savers' purchasing power. During periods when policy rates were held low, cash savers experienced years of negative real returns with no realistic alternative at the same risk level.
This does not argue against holding cash for its proper purpose. It argues against expecting the interest to solve the problem, and against the common belief that a competitive rate makes a large long-term cash holding sensible. It reduces the cost; it does not usually eliminate it.
A practical response that does not require forecasting
Nobody can predict the rate of price increases over the next decade, and a plan that requires such a prediction is not a plan. What is available is a structure that behaves acceptably across a range of outcomes, which is a lower bar and an achievable one.
The shape of that structure is described throughout this site: cash sized to its actual functions and no larger, a broad equity holding for money with a long horizon, and whatever stabilising allocation your tolerance requires in between. That arrangement loses modestly to inflation on the cash portion and has historically outpaced it substantially on the rest.
The single most valuable habit alongside it is running every long-term projection in real terms rather than nominal ones. This makes the numbers less flattering and stops the plan being calibrated on figures that will not buy what they appear to. As with everything on this site, this is educational rather than advice, and the right structure depends on circumstances only you can assess.