UK inflation figures can be difficult to interpret because several measures describe changing prices in different ways. This guide explains CPI, CPIH and RPI, shows how annual and monthly movements are calculated, and provides a repeatable method for comparing official inflation data with your own household costs.
Overview
Inflation is the rate at which prices are changing over time. It does not mean that every item becomes more expensive by the same amount, nor does a fall in the inflation rate usually mean that prices have returned to an earlier level. If inflation slows, prices may still be rising, but at a slower pace.
In the UK, the most widely used headline measure is the Consumer Prices Index, or CPI. The Consumer Prices Index including owner occupiers’ housing costs, known as CPIH, adds an estimate of the cost faced by people who own and live in their homes. RPI, or the Retail Prices Index, is an older measure with different coverage and calculation methods. It remains visible in some contracts and financial arrangements, but it is not the main measure used for assessing consumer price inflation.
| Measure | What it broadly shows | Important point |
|---|---|---|
| CPI | Changes in the prices of a basket of goods and services purchased by households | The main headline measure commonly used in economic reporting and policy discussion |
| CPIH | CPI plus an estimate of owner occupiers’ housing costs and some related housing costs | Useful when housing costs are an important part of the comparison |
| RPI | An older measure covering a range of household goods, services and housing-related costs | Its formula and coverage differ from CPI and CPIH, so the figures are not directly interchangeable |
The inflation basket is a model of household spending rather than a list of every product bought by every person. It contains categories such as food, transport, energy, clothing, recreation and household services. Items receive weights intended to reflect their importance in overall household spending. Those weights and the basket itself can be reviewed over time, so comparisons should always be read alongside the relevant methodology notes.
How to estimate the change
To understand a published inflation figure, first identify the period and measure. A statement such as “CPI inflation was X%” normally refers to the percentage change in the CPI price index over the previous 12 months. It is an annual rate, not a monthly bill increase and not a forecast of what one household will pay.
The basic annual calculation is:
Annual inflation rate = ((current index ÷ index 12 months earlier) − 1) × 100
For a shorter period, use the same principle with the relevant index points:
Percentage change = ((later price or index ÷ earlier price or index) − 1) × 100
A monthly rate compares one month with the previous month. It can be useful for identifying recent movement, but one month can be affected by seasonal patterns, temporary discounts, energy changes or other factors. The annual rate provides a longer comparison, although it can also be influenced by what happened 12 months earlier. This is sometimes called a base effect.
When reading an ONS release, work through it in this order:
- Check the reference month and publication date.
- Confirm whether the figure is CPI, CPIH or RPI.
- Read whether the change is annual or monthly.
- Look at the largest upward and downward contributions.
- Check the accompanying tables, charts and methodology notes.
- Compare the result with your own spending pattern without treating it as a personal inflation rate.
Inputs and assumptions
For a household estimate, collect your spending in the same period before and after a price change. Useful inputs include rent or mortgage-related costs, energy, food, transport, insurance, childcare, communications and other regular services. Keep one-off purchases separate from recurring costs so that an unusual month does not distort the comparison.
A simple personal inflation estimate is:
Personal rate = ((new total household cost ÷ old total household cost) − 1) × 100
This calculation assumes that the two totals cover comparable items and time periods. It will not reproduce CPI or CPIH unless the household spending pattern happens to match the official basket and its weights. For a more useful comparison, calculate each category separately and then identify which changes contributed most to the overall result.
Do not combine CPI, CPIH and RPI figures in one calculation. Choose one measure for the comparison and label it clearly. Also record whether a figure is unadjusted, seasonally affected or revised, where those details are provided in the official release.
Worked examples
Example 1: annual versus monthly change. Imagine a basket costs £100 in January of one year and £104 in January of the following year. The annual change is ((104 ÷ 100) − 1) × 100, which equals 4%. If the same basket costs £103 in December and £104 in January, the monthly change is approximately 1%. These figures describe different intervals: one year and one month. They should not be added together or presented as competing versions of the same rate.
Example 2: a household total. Suppose a household’s comparable monthly spending rises from £2,000 to £2,080. Its simple personal estimate is ((2,080 ÷ 2,000) − 1) × 100, or 4%. If most of the increase comes from one category, such as transport or energy, the household may experience a noticeably different change from the official headline rate. Another household with lower spending in that category could see a smaller increase even during the same month.
Example 3: why a lower rate does not reverse earlier increases. If an item rises from £100 to £110, its price is 10% higher. If it then rises from £110 to £112.20, the later annual or period increase is 2%, depending on the comparison used. The rate has slowed, but the price remains above £100. This distinction is central to understanding cost of living news.
When to recalculate
Revisit the calculation after each monthly inflation release if you are tracking the latest UK inflation rate meaningfully, or whenever a major household cost changes. Recalculate after an energy tariff change, a rent or mortgage review, a pay change, a benefits uprating decision, or a significant change in commuting, childcare or food costs.
Editors and publishers should check the latest ONS release date, headline rates, contribution tables, basket updates and methodology notes before refreshing an article. Avoid carrying an old figure into a new story, and state the reference month beside every rate. If the ONS changes its methods or coverage, explain the change before comparing the new series with older data.
For readers, the practical routine is simple: identify the measure, identify the period, read the annual and monthly changes separately, check the main contributors, then compare those contributors with your own spending. That workflow helps turn a headline into useful cost of living news without confusing an official average with a personal bill.
