What Is Inflation? CPI, RPI and What the UK Measures Actually Mean
Inflation is one of those economic concepts that manages to be simultaneously everywhere and poorly understood. Politicians invoke it. Newspapers splash it across front pages. The Bank of England holds emergency meetings because of it. Yet for most people, the actual mechanics — how it is measured, why there are different versions of it, and what the numbers genuinely mean for a household budget — remain opaque.
This guide cuts through the jargon. Whether you are trying to understand why your energy bill keeps rising, wondering how your pension will be uprated, or simply want to hold your own at a dinner table argument about interest rates, here is what you actually need to know.
What Inflation Is and Why It Matters
At its most basic, inflation is the rate at which prices across an economy are rising. If a basket of goods costs £100 today and £103 in a year's time, that is 3% inflation. The purchasing power of your pound has fallen — the same note buys less than it did twelve months ago.
Deflation, the opposite condition, sounds appealing but is generally considered worse. When prices fall, consumers delay purchases expecting further falls, businesses cut investment, wages drop, and economies can spiral into prolonged stagnation. Japan spent much of the 1990s and 2000s trapped in exactly this pattern.
A modest, predictable level of inflation — around 2% — is widely considered healthy. It encourages spending over hoarding, allows wages to rise in nominal terms even when real increases are modest, and gives central banks room to cut interest rates when a recession bites. This is precisely why the Bank of England's remit from the government is to keep CPI inflation at 2%.
Understanding the Basket of Goods
The UK's inflation figures are produced by the Office for National Statistics (ONS), which tracks the prices of roughly 700 different goods and services — the so-called "basket". Each item is weighted according to how much of a typical household's budget it represents, based on the Living Costs and Food Survey.
The basket is updated every year to reflect changes in spending habits. In recent years, items such as streaming subscriptions and plant-based milks have been added, while others that have fallen out of mainstream use have been removed. This matters because a basket that does not reflect how people actually spend money would produce a misleading inflation figure.
Prices are collected from thousands of shops, websites and service providers across the country — around 180,000 individual price quotes each month. The result is a single headline number that attempts to capture the experience of an average household, even though no household is truly average.
CPI, RPI and CPIH: What Sets Them Apart
The UK currently publishes three main inflation measures. Understanding their differences is not merely academic — it can directly affect your income, your debts and your rights as a consumer.
CPI — Consumer Prices Index is the government's official target measure and the one you will most often see quoted. It covers a wide range of consumer goods and services but does not include housing costs such as mortgage interest payments, council tax or house depreciation. CPI uses a geometric mean calculation, which tends to produce slightly lower figures than an arithmetic mean approach.
RPI — Retail Prices Index is the older measure, dating back to 1947, and tends to run higher than CPI — typically by one to two percentage points. The key difference is that RPI includes mortgage interest payments, council tax and some other housing costs that CPI excludes. RPI uses an arithmetic mean. The ONS stripped RPI of its National Statistics designation in 2013, meaning it is no longer considered a high-quality measure. Nevertheless, it remains legally embedded in a large number of contracts and statutory instruments. Rail fares in England are traditionally uprated by RPI. Many index-linked gilts use RPI. Some private pension schemes and commercial leases are tied to it. The government has been working to align RPI with CPIH methodology from 2030, a move that will significantly reduce payouts on RPI-linked gilts and is deeply contentious in financial markets.
CPIH — Consumer Prices Index including owner-occupiers' housing costs is the ONS's preferred measure. It adds owner-occupiers' housing costs (OOH) to CPI using a rental equivalence approach — asking, in effect, what rent would a homeowner pay to live in their own property. CPIH generally sits slightly above CPI. The ONS considers it more representative because homeownership is the tenure of the majority of UK households, yet CPI ignores its costs entirely. CPIH is increasingly cited in official releases, though the government's formal 2% inflation target is still expressed in CPI terms.
How Inflation Affects Your Day-to-Day Finances
The inflation number you read in a headline is a national average that may bear little resemblance to your personal experience. A retired homeowner who pays no mortgage, drives rarely and already has a full wardrobe will feel inflation very differently from a young renter who commutes by car and is building a household from scratch.
That said, there are several direct transmission mechanisms worth understanding.
Wages and benefits. Many annual pay reviews, state benefits and the State Pension triple lock are linked to one of the headline inflation measures. A higher inflation reading generally translates to a larger uplift — but only if your income is formally index-linked. If it is not, high inflation quietly erodes your real pay.
Savings. When inflation runs above the interest rate on your savings account, your money is losing purchasing power in real terms even as the nominal balance grows. The Bank of England rate influences how much banks offer savers, but competition and commercial decisions mean savings rates often lag the policy rate in both directions.
Mortgages and debt. Inflation indirectly drives mortgage rates through the Bank of England's base rate. Variable rate and tracker mortgage holders feel rate changes quickly. Fixed-rate holders are insulated temporarily, but when their deal expires they face whatever rate environment exists. Counter-intuitively, inflation erodes the real value of outstanding fixed debts — good news for borrowers with large mortgages, less so for those saving to clear debt.
Rents. Private sector rents are not formally linked to a national inflation index, but landlords' own costs — mortgages, maintenance, insurance — tend to rise with inflation, and rental market pressure translates this into higher asking rents.
Rail and utility prices. Train fares have historically been set by RPI plus a fixed percentage, though the government has intervened in some years to limit increases. Energy prices are regulated through Ofgem's price cap, which is recalculated quarterly based on wholesale energy costs rather than a retail price index — but broader inflationary pressures on operating costs still feed through.
What the Bank of England Does When Inflation Rises
The Bank of England's Monetary Policy Committee (MPC) meets eight times a year to set the base rate. Its principal tool for controlling inflation is raising interest rates: by making borrowing more expensive, the MPC attempts to cool demand and take upward pressure off prices. Higher rates also attract foreign capital, which strengthens sterling and makes imports cheaper — a second disinflationary channel.
When inflation overshoots the 2% target by more than one percentage point, the Governor is required to write an open letter to the Chancellor explaining the deviation and the Committee's response. These letters become part of the public record and are carefully scrutinised by financial markets.
Rate rises are a blunt instrument. They slow demand across the whole economy, which can mean lower growth, higher unemployment and genuine hardship for those with variable-rate debt — even if the underlying inflation is driven by supply-side factors such as energy prices or supply chain disruption, which interest rates cannot directly fix. This tension was at the heart of debates during the inflation spike of 2022–23, when CPI peaked above 11%.
How to Use Inflation Data in Your Own Financial Planning
Understanding which measure applies to your situation is the starting point for practical action.
Check the index on any contract you sign — a lease, a pension transfer, an index-linked savings certificate. If it references RPI, model the difference it would make compared to a CPI-linked alternative, particularly post-2030 when the ONS reform takes effect.
For savings, compare the real return — the nominal interest rate minus CPI — rather than the headline rate. In periods of elevated inflation, cash savings can quietly shrink in real terms even in competitive accounts.
If you are reviewing a salary or freelance rate, the ONS's personal inflation calculator (available on its website) lets you input your own spending patterns and receive a more personalised inflation estimate than the headline figure provides.
Finally, watch for the distinction between headline and core inflation. Core CPI strips out volatile food and energy prices to reveal the underlying inflationary trend. When energy prices spike, headline CPI surges but core may remain relatively stable — suggesting a temporary rather than structural shift. Central banks and analysts tend to weight core inflation heavily when forecasting where rates are heading.
Inflation is not a single, fixed truth. It is a statistical construction designed to approximate a complex, constantly shifting reality. The more precisely you understand which measure matters for which decision, the better placed you are to make your money work in any economic climate.