What Will Your Money Be Worth in 10 Years? Inflation Explained

Inflation Is Compound Interest Running Backwards

Inflation is usually described as prices going up. It is more useful to think of it as your money shrinking, because that is the side of it you can act on.

The arithmetic is the compound interest formula, used in both directions:

What something will cost: price × (1 + i)ⁿ
What today's money will be worth: amount ÷ (1 + i)ⁿ

Here i is the annual inflation rate as a decimal and n is the number of years. Both compound, which is why inflation that sounds mild is severe over a long horizon and why a single year's figure tells you very little.

What £10,000 Is Worth in Ten Years

Cash held under a mattress, at three different inflation rates:

At 2% — the Bank of England's target — it buys what £8,203 buys today
At 3%, £7,441
At 5%, £6,139

The gap between the first and last line is the important part. An extra three percentage points of inflation costs more than a fifth of the money over a decade, which is why the difference between 2% and 5% is treated as a serious economic question rather than a rounding detail.

The Rule of 72

For a quick estimate without a calculator, divide 72 by the inflation rate and you get the years for prices to double — equivalently, for your money to lose half its purchasing power:

At 2%: 36 years
At 3%: 24 years
At 6%: 12 years

It is an approximation and a good one across the range that matters: 3% compounded for 24 years multiplies prices by 2.03, and 6% for 12 years by 2.01. The same rule works for investment growth, where it tells you how long money takes to double rather than to halve.

Real Returns: Do Not Just Subtract

A return quoted by a bank or a fund is nominal. What you care about is the real return, after inflation has taken its share.

The common shortcut is to subtract: 5% growth minus 3% inflation equals 2%. That is close, and it is not exact. The correct relationship divides:

Real return = (1 + nominal) ÷ (1 + inflation) − 1

For 5% against 3% inflation, the exact figure is 1.94%, not 2%. A small discrepancy, and it widens as the numbers grow: 12% growth against 9% inflation gives 2.75%, not the 3% that subtraction suggests — the shortcut overstates the result by nearly a tenth.

Subtraction is fine for a rough answer at low rates. Use the division whenever the figures are large or the horizon is long, because the error compounds along with everything else.

A Pay Rise Below Inflation Is a Pay Cut

This follows directly from the formula above and is worth stating plainly, because the headline number is the one people react to.

A 2% pay rise in a year of 3% inflation is a real-terms change of (1.02 ÷ 1.03) − 1 = −0.97%.

The figure on the letter went up and the amount of goods it buys went down. To stand still you need a rise that matches inflation exactly; to gain anything you need to beat it. It is the same arithmetic behind the phrase "real-terms cut" in public spending stories, where a budget rises in cash and falls in what it can actually purchase.

Where It Bites Hardest: Cash Savings

An account paying less than the inflation rate loses money in real terms every year, even while the balance rises. The rising balance is what makes this easy to miss.

£10,000 at 1.5% interest grows to £11,605 over ten years. With inflation at 3% over the same period, that balance buys what £8,636 buys today — a real loss of about 14%, despite the statement showing a gain of £1,605.

Nothing here argues against holding cash. An emergency fund needs to be safe and immediately available, and a real loss of a percent or two is the price of that. The point is to recognise the cost, and not to leave money intended for a decade's growth sitting where it is quietly shrinking.

CPI, RPI and Your Own Rate

The UK publishes more than one inflation figure and they do not agree.

CPI is the headline measure and the one the Bank of England's 2% target refers to.
CPIH is CPI plus owner-occupiers' housing costs, and is the ONS's preferred measure.
RPI is an older calculation that typically runs above CPI, and it still governs some rail fares, index-linked gilts and student loan interest — which is why the measure used in a contract is worth reading.

All of them track a representative basket of goods, and none of them is your basket. Someone spending heavily on rent and fuel experiences a different rate from someone who owns their home outright and walks to work. When inflation is uneven across categories, the headline figure can understate what a particular household is feeling by a wide margin.

Long Horizons Are Where the Damage Is

Over a year or two inflation is an irritation. Over a working life it dominates the arithmetic, which is why retirement planning has to be done in real terms.

An income of £30,000 a year today needs to be £62,813 in 25 years to buy the same things, at 3% inflation. The target has more than doubled without anyone's standard of living improving at all.

This is the strongest argument for holding long-horizon money in assets that have historically outpaced inflation rather than in cash, and the reason a pension projection quoted in future pounds flatters itself. When you see a large figure for decades ahead, divide it back to today's money before deciding whether it is enough.

Conclusion

Inflation is the compound interest formula pointed the other way. Multiply to find what something will cost; divide to find what your money will be worth. Divide rather than subtract when working out a real return, and apply the same test to a pay rise, a savings rate and a pension projection alike.

The single habit worth keeping is to convert any long-dated figure back into today's money before judging it. An Inflation Calculator does that in one step, and the rule of 72 will get you close enough in your head.

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