What is the Consumer Price Index (CPI)?

What is the Consumer Price Index (CPI)?

As the Office for National Statistics (ONS) has just announced that inflation, as measured by the Consumer Price Index (CPI), rose once again by 3% in the 12 months to January 2025, we thought it would be useful to take a look at exactly how the CPI works and what it does.

The UK CPI is a key measure of inflation, tracking changes in the price of a basket of goods and services over time. It is calculated by the ONS and is used by the government and the Bank of England to guide economic policy, including interest rates.

How the CPI Works

Basket of Goods and Services

The CPI is based on a representative "shopping basket" of goods and services that UK households commonly purchase.

This basket includes food, clothing, transport, housing costs (excluding mortgage interest), recreation and other consumer expenses.

The composition of the basket is reviewed annually to reflect changing consumer habits.

Price Collection

Each month, the ONS collects thousands of price points from retailers, service providers and online sources across the UK.

Prices are taken for identical or comparable products to ensure consistency.

Weighting System

Not all items in the basket contribute equally to the CPI.

Each category is weighted based on how much households typically spend on it.

For example, if people spend more on energy bills than on cinema tickets, energy prices will have a greater impact on the CPI.

Calculation

The index is calculated by comparing the total cost of the basket in the current month with the cost in a base year (typically set to 100).

The percentage change represents inflation or deflation.

CPI vs Other Inflation Measures

CPIH – This is the CPI plus owner-occupiers' housing costs, providing a broader measure of inflation.

Retail Price Index (RPI) – An older measure that includes mortgage interest payments. It tends to be higher than CPI and is still used for things like rail fare increases and some pension calculations. However, it is no longer an official national statistic.

Why the CPI Matters

Monetary Policy – The Bank of England uses CPI to set interest rates. If CPI inflation exceeds the 2% target, interest rates may rise to cool the economy.

Wages & Benefits – Some salaries, state pensions, and benefits are adjusted in line with CPI.

Investment Decisions – Investors monitor CPI to assess inflation risk and adjust portfolios accordingly.

Clearly, a rise in the CPI will not affect everybody the same way as we don't have the same 'basket' in our weekly shopping, or the same number of people to shop for. More importantly, a consequential hike in interest rates will have a negative impact on anyone with a mortgage and a positive one on those people with cash savings.

So the knock on consequences of inflation above 2% can often be more significant than the costs of the weekly shop.

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