index proposed by German economist Étienne Laspeyres (1834–1913) for measuring current prices (price) or quantities in relation to those of a selected base period. A Laspeyres price index is computed by taking the ratio of the total cost of purchasing a specified group of commodities at current prices to the cost of that same group at base-period prices and multiplying by 100. The base-period index number is thus 100, and periods with higher price levels have index numbers greater than 100.

The distinctive feature of the Laspeyres index is that it uses a group of commodities purchased in the base period as the basis for comparison. In other words, in computing the index, a commodity's relative price (the ratio of the current price to the base-period price) is weighted by the commodity's relative importance to all purchases during the base period. (Compare Paasche index.)

The Laspeyres price index tends to overstate price increases because, as prices change, consumers typically alter their purchasing decisions by selecting fewer products with large price increases while buying more products that show low or no price increases. If consumers can do this without reducing their total satisfaction, the use of base-period commodity selections tends to overstate declines in the standard of living. Similar to the price index, the Laspeyres quantity index uses base-period prices to compare aggregate production levels in two periods.