A cost index is a dimensionless number that tracks how prices in some category change over time relative to a chosen reference period. The Consumer Price Index (CPI), construction cost indices, and industry-specific equipment indices all work the same way.
The standard formula for scaling a known historical cost forward (or backward) using an index:
where is the cost in the reference year , is the index value in year , is the index value in year , and is the estimated cost in year .
The idea is that even if you don’t know the cost of this specific item in year , you know how the category moved in price between year and year (through the index), and you can apply that same percentage change to your specific item.
Common indices:
- Consumer Price Index (CPI) tracks a basket of consumer goods — used for general inflation, contract escalation, minimum-wage adjustment.
- Producer Price Index (PPI) tracks wholesale prices upstream of consumers.
- Construction cost indices (e.g., Engineering News-Record’s CCI) for building work.
- Chemical Engineering Plant Cost Index for process-industry equipment.
The reference year is arbitrary — what matters is the ratio . Statistics agencies usually set some recent year’s index to 100 and rebase periodically.
Caveats: an index reflects the average movement of a category. Specific items can move differently — a particular alloy might double in price while the broader metals index moves 20%. Indices work well for order-of-magnitude and budgetary estimates of items closely tracking the category, less well for specialised or highly volatile items.
Cost indices are also the basis for contract escalation clauses (lock in today’s price plus a CPI-tied adjustment over the life of the contract) and for converting historical bid data into today’s-dollars estimates for new bids. See Cost estimate classes for where this technique fits in the estimation hierarchy and Inflation for the macroeconomic version of the same phenomenon.