What it means
Statistical agencies choose a representative basket covering food, housing, transport, energy, clothing, healthcare and leisure, and weight each item by how much households actually spend on it. Prices are collected regularly from thousands of retailers, and the total cost of the basket is expressed as an index number relative to a base period set at 100.
The index itself is not inflation; the change in the index between two dates is. If CPI rises from 104.0 to 106.6 over a year, the inflation rate for that year is 2.5%, and the index level simply records how far prices have moved since the base period.
CPI matters commercially far beyond economics commentary. Pay reviews, commercial rent reviews, pension increases, index-linked contracts and interest rate decisions all reference it, so a single published figure can change millions of dollars of obligations across an economy.
Two variants show up constantly. Headline CPI includes everything, while core CPI strips out food and energy because their prices swing violently for reasons unrelated to underlying demand, and central banks usually watch the core measure when setting policy.
The main criticism is that no single basket matches any real household. A family that rents in a city and drives to work experiences very different inflation from a mortgage-free household in a rural area, which is why the published rate often feels wrong to the people reading it.
In practice
Real-world examples.
Example
A commercial landlord has a lease clause raising rent annually in line with CPI, capped at 4%. When the published rate comes in at 2.5%, the tenant's $180,000 annual rent rises to $184,500 for the following year.
Example
A trade union enters pay talks quoting CPI of 2.5% and argues for a rise above that figure to reflect productivity gains. The employer counters with core CPI, which excludes a temporary energy spike and is running lower.
Example
A treasury team pricing a five-year supply contract insists on annual CPI indexation rather than a fixed price. The supplier accepts, because the clause protects it against input cost inflation without either side having to guess where prices will settle.
Think of it
“CPI measures consumer inflation-how much prices are rising for everyday goods.
Formula
Calculation
CPI = (cost of the basket in the current period / cost of the basket in the base period) x 100.
Inflation rate % = (current CPI - prior CPI) / prior CPI x 100.
Suppose the statistical agency's basket cost $2,000 in the base year, giving a CPI of 100 by definition. A year ago the same basket cost $2,080, and today it costs $2,132.
CPI a year ago = $2,080 / $2,000 x 100 = 104.0.
CPI today = $2,132 / $2,000 x 100 = 106.6.
Inflation over the past year = (106.6 - 104.0) / 104.0 x 100 = 2.6 / 104.0 x 100 = 2.5%.
Now apply it. An employee receiving a 3% pay rise in a year when CPI inflation is 2.5% is roughly 0.5% better off in real terms, because the extra money slightly outpaces the rise in the cost of the basket. A 2% rise in the same year would leave that employee about 0.5% worse off despite the pay packet growing.Case study
Seen in the real world.
The following case is illustrative and fictional. Pemberley Facilities Group is an invented cleaning contractor that signed a four-year contract with a large client at a fixed price, expecting cost increases of about 2% a year. Wages, materials and fuel all rose faster than that, and by year three the contract was losing money on every site.
The commercial director rebuilt the standard contract template around CPI indexation, with an annual adjustment based on the published index each March and a floor of 0% so the price could not fall. Clients pushed back at first, so Pemberley offered a slightly lower starting price in exchange for the clause.
Two years later the approach had settled. Margins were more stable, disputes about mid-contract price rises had almost disappeared, and the sales team found the indexation clause easier to defend than an unexplained buffer built into the opening price.
Watch out
Common mistakes.
- Confusing the index level with the inflation rate. A CPI of 106.6 does not mean inflation is 106.6%; it means prices are 6.6% above the base period, and the annual rate is the change since the previous reading.
- Assuming falling inflation means falling prices. A drop from 5% to 2% means prices are still rising, just more slowly, and only a negative rate, deflation, means prices are actually coming down.
- Applying headline CPI to your own cost base without checking. Your inputs may be dominated by wages, freight or a single commodity that behaves nothing like the household basket.
Questions
People also ask.
What is the difference between CPI and core CPI?
Core CPI excludes food and energy prices because they are volatile and often driven by weather or global supply shocks, which makes core a steadier read on underlying inflation.
How often is CPI published?
In most developed economies it is published monthly, with an annual review of the basket's contents and weightings to reflect changing spending patterns.
Should contracts use CPI or a sector-specific index?
CPI is transparent and widely accepted, but where a contract's costs are concentrated in one area, a construction or labour cost index may track the real exposure far more closely.
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