A price index level has no useful scale until readers know what it is being compared with. A base period supplies that anchor, conventionally setting its index to 100. Later numbers express how the measured basket’s price level relates to that reference. The base is a measuring point, not a claim that prices were objectively normal or affordable then. To interpret an index, separate its level from the percentage change between two levels; each answers a different question.
What the number 100 means
An index compresses price movement into a relative scale. If a representative basket costs $100 in its reference period, a later index of 110 corresponds to a basket costing $110 on that same constructed basis: 10 percent above the reference. The Bureau of Labor Statistics uses 1982–84 as the reference base for most CPI-U and CPI-W measures, expressed as 1982–84=100 Bureau of Labor Statistics. That convention lets the series move above or below 100 without making 100 a price ceiling, target, or current-dollar amount.
The CPI itself tracks average changes in prices paid by urban consumers for a representative basket of goods and services. It uses weights reflecting their relative importance in spending, rather than treating each item as equally important Bureau of Labor Statistics. A base period gives the resulting measure a starting scale. It does not say that every household bought the same basket, or that a particular person’s cost of living moved exactly with the national index.
A hypothetical calculation
Suppose, purely as a hypothetical example, a price index for a made-up basket is 125 in one year and 130 the next. The index-point increase is five. To find the percentage change, divide the change by the earlier index and multiply by 100: 5 ÷ 125 × 100 = 4 percent. The basket’s measured price level rose 4 percent over that interval. The current level, 130, is 30 percent above the base-period level of 100; it does not mean prices rose 130 percent.
The starting level matters when converting index points into a rate. A five-point rise from 100 to 105 is 5 percent, while five points from 125 to 130 is 4 percent. BLS explains why movements are usually expressed as percentages: point changes depend on the index’s position relative to its base, while percentage changes do not Bureau of Labor Statistics. The calculation is simply a ratio, but choosing the right earlier period is essential to the answer.
The time window changes the question
A month-to-month percentage change describes movement over a short interval; a 12-month change compares with the same month a year earlier. Neither is interchangeable with the index level. For September 2026, the CPI-U rose 0.6 percent on a seasonally adjusted monthly basis, while the unadjusted 12-month increase was 3.7 percent Bureau of Labor Statistics. Those rates answer different questions about price movement. They are not alternate readings of a single monthly change.
Seasonal adjustment also affects what is being compared. Adjusted figures aim to remove recurring seasonal patterns and are useful for short-term trend analysis; unadjusted figures reflect prices without that adjustment and are often relevant to consumers and escalation agreements Bureau of Labor Statistics. The base period itself does not turn a monthly rate into an annual one. Readers need to check both the comparison period and whether the series is adjusted.
What the index can and cannot say
An index level can show how the measured average has changed relative to a reference, and a percentage change can summarize movement across a chosen interval. Neither directly tells a shopper the price of a particular item today. Nor does an aggregate CPI necessarily match an individual’s experience: spending patterns differ, and costs in one household’s budget may move faster or slower than the average Bureau of Labor Statistics.
Base periods can also be changed mathematically to make a series easier to read, without changing its underlying pattern of relative movement. Whatever scale is used, the useful questions remain: What population and basket does the index represent? Which two periods are being compared? Is the reported change monthly, annual, adjusted, or unadjusted? Once those are clear, 100 becomes what it is meant to be: a reference mark that makes the index interpretable, not a standalone verdict on prices.
Sources & further reading
- Consumer Price Index questions and answers ↗Bureau of Labor Statistics
- September 2026 Consumer Price Index ↗Bureau of Labor Statistics
