Cash is not safe. It is a slow leak.

Most people think of cash as the safe option. The defensive position. The place you put money when you don’t want to lose any of it.

This is a comforting story. It is also wrong over any horizon that matters.

What cash actually is, over ten or twenty or thirty years, is a slow, almost invisible erosion of your purchasing power. The headline balance grows. The number on the statement creeps up. And year after year, that number buys you less.

This isn’t a trick of the markets. It isn’t bad luck. It’s arithmetic. The same arithmetic that has quietly made cash savers poorer for as long as money has existed.

i.The number on the statement isn’t the number that matters.

A pound in your savings account today and a pound in your savings account in twenty years are not the same pound.

The first one buys a coffee. The second one buys half a coffee. Maybe less.

The reason is inflation — the steady, year-on-year increase in the cost of the things you actually buy. Over the twenty years to the end of 2025, UK consumer price inflation averaged around 2.9% a year. Some years far higher: it reached 10.5% in December 2022. It does not stop, and it compounds, just like interest does — except in the wrong direction.

So when someone tells you their savings account is paying 4.5%, the question is not “is that good?” The question is: what is left after tax and inflation?

If inflation is running at 3.5% and your account pays 4.5%, you are earning 1% in real terms. Before tax.

Now add the tax. A higher-rate taxpayer pays 40% on savings interest above their allowance, so 4.5% becomes 2.7%. Set that against 3.5% inflation and you are losing 0.8% of your purchasing power every year. Quietly. Without anyone telling you.

The statement shows progress. The reality is that you are treading water, and falling behind on anything you might actually want to buy with the money.

ii.The headline-rate trap.

This is the part most savings advertising counts on you not noticing.

“5.1% AER” is supposed to sound exciting. In nominal terms — the number going up on the statement — it is. But there are three layers of erosion between the headline rate and the actual increase in what you can buy.

The first is tax. Outside an ISA, interest above your Personal Savings Allowance is taxable income. That allowance is £1,000 a year for a basic-rate taxpayer, £500 for a higher-rate taxpayer, and nothing at all for an additional-rate taxpayer. Above it you pay income tax at your marginal rate — 20%, 40% or 45%. At current rates, a higher-rate taxpayer with meaningful savings loses two fifths of the headline rate before anything else happens.

The second is general inflation. Whatever is left after tax is then measured against the cost of living, which is climbing in the background.

The third is the gap between headline inflation and your inflation. The figure announced each month is an average across a basket of goods. Your basket — if you are saving for retirement, a house, or a child’s education — may be moving faster than that average. Over the past thirty years UK house prices have risen considerably faster than consumer prices, and the same is true of private school fees and university costs. Worth saying plainly, though: that has not been true of the past decade, when house prices have roughly tracked inflation rather than outpacing it. The point is not that property always wins. It is that the average is not your average.

Add those three layers together, and the “safe” account is often losing you ground in real terms.

iii.So when is cash safe?

Cash is the right tool for one specific job: money you need to spend in the next two or three years.

An emergency fund. A house deposit you are saving for. The next two years’ school fees. The cash buffer that stops you having to sell investments in a falling market during the first years of retirement. Money that cannot take a fall in value, because you might need it before any fall has time to recover.

For that job, cash is exactly right. Sitting in an instant-access account, ideally inside an ISA where the interest is tax-free. The fact that it is losing a little to inflation is a fair price for the certainty that it will be there when you need it.

The problem isn’t cash itself. The problem is using cash for jobs it was never designed to do. Like funding a retirement that is twenty years away, or building wealth your grandchildren might one day inherit.

iv.What cash is not designed to do.

Cash is not designed to grow your wealth. It is not designed to outpace inflation over decades. It is not designed to compound at a rate that meaningfully changes your life.

If you want money to do those things, it needs to take some risk — measured, diversified, long-term risk in productive assets. Shares in companies. Property. The bonds of governments and businesses. Things that, in aggregate and over time, have tended to grow faster than the cost of living. That is what investment management is for.

This is not gambling. Gambling, in this context, is keeping your retirement savings in a bank account and hoping inflation goes away. The maths there is close to fixed: over the long run you are very likely to lose ground in real terms. With investing, the maths is uncertain in any given year and has historically been positive over longer horizons, because you own a slice of an economy that grows.

Past performance is no guarantee of the future, and investments can fall as well as rise. But accepting a near-certain real-terms loss in exchange for the comfort of a familiar bank balance is a strange definition of safety.

v.A test you can run yourself.

I built a calculator for exactly this purpose. Put in your starting balance. Pick an interest rate and an inflation assumption. Choose a horizon. See what your money is actually worth at the end.

Most people who run this calculation for the first time are quietly horrified. That is not a flaw in the calculator. It is the truth about long-horizon cash that most people have never had laid out for them in plain numbers.

You can try it yourself, free, with the cash erosion calculator — or see the other side of the same coin with the compound growth calculator.

vi.The conversation worth having.

If you have cash sitting in a savings account for any meaningful period — five years, ten years, more — the right question is not “which account pays the most.” The right question is: should this money be cash at all?

For most clients, the answer for some of it is yes. For most of it, the answer is no. Working out which is which, and getting the wrappers and allowances right around it, is what financial planning is for.

If you would like an honest read on what cash is doing to your savings, the calculator is there. If you would rather have the conversation in person, you know where to find me.