How to Calculate Your Personal Inflation Rate

What a personal inflation rate actually is

The Consumer Prices Index (CPI)Consumer Prices Index: The ONS measure of UK inflation used by the Bank of England for its 2% target. It tracks prices of about 730 goods and services weighted by average household spending, excluding owner-occupied housing costs. - 2.8% in the 12 months to May 2026 - is a national average. It tracks a fixed basket of around 730 goods and services, weighted by what the average UK household spends. Your household is not the average, so your own rate almost certainly differs.

Your personal inflation rate is the price change of your basket: the same idea as CPI, but weighted by your spending instead of the nation's. Two things it is not. It is not a flat average of the price rises you have noticed - that throws away the fact that some categories matter far more to your budget than others. And it is not the change in your total spending - that also moves when you buy more, switch brands, or have a one-off expense, none of which is price inflation. A personal inflation rate isolates pure price change for your mix.

The mistake most guides make

Search "how to calculate your personal inflation rate" and you will mostly find one of two methods. The first tells you to track your own basket year-on-year from receipts. The second - the one that turns up in calculators and explainers - tells you to work out the percentage rise for each thing you buy, add them up, and divide by the number of items. That second method is mathematically wrong, and it is wrong in a predictable direction.

A flat average gives a tin of beans the same say as your rent. Take a renter with no car whose spending splits roughly: 59% housing and energy, 25% food, 12% everything else, 4% transport. Using the real ONS division rates for May 2026 - housing, water and fuel 1.2%, food 2.2%, transport 6.8%, and roughly the 2.8% headline for the rest - the two methods diverge sharply:

• Flat average (wrong): (1.2 + 2.2 + 2.8 + 6.8) ÷ 4 = 3.25%. Transport's 6.8% drags the answer up even though this person barely uses transport.
• Spend-weighted (correct): 1.87%, because housing - the biggest slice of the budget - rose only 1.2%, and that low rate counts for most.

Same person, same prices: 3.3% the naive way versus 1.9% done properly. The flat average overstates this renter's inflation by almost a point and a half, purely because it ignores how the budget is split. Do the weighting, or your number is wrong - and you can know the direction in advance.

The fast, correct method - without a year of receipts

You do not need to price your own shopping every month. The ONS already publishes a 12-month inflation rate for each of the 12 CPI divisions - the COICOP categories (Classification of Individual Consumption by Purpose) that make up the basket: food and non-alcoholic drinks; alcohol and tobacco; clothing; housing, water and fuel; furnishings and household goods; health; transport; communication; recreation and culture; education; restaurants and hotels; and miscellaneous goods and services.

The method is one line: take your share of spending on each division, multiply it by that division's published rate, and add them up. In symbols, your rate is the sum of (your share of a category) × (that category's inflation rate), across the categories. That strips out the noise of your own purchase timing and brand-switching and leaves pure price inflation for your mix.

That is the whole method, and the chart shows why weighting matters: transport has by far the highest rate but the smallest effect, while low-inflation housing dominates because it is most of the budget. You can do this on the back of an envelope with four or five rough shares. Or skip the arithmetic entirely: the personal calculator in the Real Cost of Living tracker runs exactly this calculation across all 12 divisions using live ONS data, and pre-fills your shares from 40 real spending profiles, so you do not even need to know your own split precisely.

Does CPI include your mortgage? The owner-occupier trap

Here is a trap that catches almost everyone who compares "their" inflation to the headline. CPI excludes owner-occupiers' housing costs - mortgage interest and the cost of owning the dwelling itself. If you own with a mortgage, your single biggest outgoing is not in the index you are measuring yourself against, so the comparison is apples to oranges.

Two measures do include it. CPIHConsumer Prices Index including owner occupiers' Housing costs: The ONS's headline inflation measure since 2017. It adds a modelled cost of owner-occupied housing (rental equivalence) to CPI, so it is typically slightly higher. adds owner-occupiers' housing costs - around 18% of that index - which is the main reason CPIH (3.0% in the year to May 2026) usually runs above CPI. So do the ONS Household Costs Indices (HCI)Household Costs Indices: An ONS measure of inflation as households actually pay it -- it counts mortgage interest that CPI omits, and is split by income group, tenure and retirement status. Classified as official statistics in development., which use a "payments" approach and count the actual mortgage interest a household pays.

This is why the personal calculator is deliberate about housing. It works across the 12 CPI divisions - so it captures rent, energy, water and maintenance in the housing division - but it excludes the owner-occupier category and hands that part off to the household view rather than pretending CPI covers it. If you are a mortgaged owner, take the housing piece from the household (HCI) view, which includes mortgage interest, and read the companion guide on why your cost of living differs for the tenure story in full.

Why your number has a shelf life

Two things drift over time, so a personal rate is a snapshot, not a fixed figure. First, the official basket is reweighted every year: the ONS updates the division weights each January and February to reflect changing spending. The 2026 update, for instance, lifted the housing weight and the transport weight materially. Second, your own shares drift too - a house move, a new car, or children change your mix. Recompute occasionally, especially after the annual reweighting or a change in your circumstances.

This is also why any tool that holds weights constant across a long history is making a disclosed simplification. The personal calculator does exactly that for its historical series - and shows the vintage of the spending data and flags where figures are suppressed - so you can see the assumption rather than take the number on faith. That honesty is the point: a rate you understand the limits of is more useful than a precise-looking one you do not.

The other free calculators, briefly

A couple of well-known tools answer a different question. The Bank of England and Hargreaves Lansdown inflation calculators tell you the historical purchasing power of a pound - what £100 in 1990 is worth today. Useful, but that is not your personal rate.

The ONS's own personal inflation calculator (the dvc1833 visualisation) is closer: it asks a handful of questions - housing situation, income, food and energy spend - and was built for the 2022 cost-of-living episode. It is a good, simple starting point, but it is coarse and of its time. The Real Cost of Living personal calculator is the granular, current option: all 12 CPI divisions, your rate now and its full history against CPI, a contribution breakdown showing which categories drive your number, and the pound-per-month impact.

What to do with your number

A personal inflation rate is only worth calculating if you do something with it. Most of the figures pegged to "inflation" in your life use the headline CPI: pay rises, most benefit upratings, the State Pension triple-lock floor, many drawdown and annuity assumptions. If your own rate runs above that headline, those upratings are a real-terms cut for you specifically; if it runs below, you get a small real gain.

So the practical step is to put your number next to whatever is pegged to inflation and check the gap. If it is a pay question, the guide on whether your pay has kept up walks through which measure to use. For why your rate differs from the headline and who is hit hardest, see the cost-of-living companion guide. And because small gaps compound, the compound interest calculator shows how a couple of points a year mounts up over time.

None of this is financial advice. It explains the mechanics of how a personal inflation rate is calculated so you can work out your own and make your own assessment. If a specific decision - a salary negotiation, a drawdown plan, an investment - turns on it, speak to a qualified adviser.

Primary sources: ONS Consumer price inflation, UK: May 2026; ONS Consumer price inflation, updating weights: 2026; ONS Household Costs Indices methodology. Per-category rates reconciled against the live ONS division series used by the Real Cost of Living tracker.