A stock is the amount of a Product existing at a specified point in time. Unlike flows, which measure the movement or transfer of products over a period of time, stocks represent a snapshot measurement of accumulated product quantities at a single moment.
In Life Cycle Assessment and economic modelling, the distinction between stocks and flows is fundamental. Whilst flows capture dynamic processes such as production rates, consumption rates, or emission rates over time, stocks capture static quantities that exist at a particular moment. For example, the amount of steel in buildings at the end of 2025 is a stock, whereas the amount of steel produced during 2025 is a flow.
Stock accounting plays an important role in understanding material accumulation in the economy and environment. Products can accumulate as stocks in various forms: raw materials in warehouses, goods in retail inventory, products in active use (such as buildings, vehicles, or infrastructure), and materials in landfills or other disposal sites. Changes in stock levels over time result from the difference between inflows (such as production or imports) and outflows (such as consumption, exports, or disposal).
In consequential Life Cycle Assessment, understanding stock dynamics can be particularly relevant when analysing systems where production decisions are influenced by inventory levels or where the timing of product availability matters. Stock changes can also be important when assessing the life cycle of durable products with long lifetimes, as these products represent accumulated environmental burdens that are stored in the stock until end-of-life treatment occurs.
The concept of stocks extends beyond physical products to include stocks of natural resources (such as fossil fuel reserves or forest biomass), stocks of emissions in environmental compartments (such as carbon dioxide in the atmosphere), and stocks of economic value (such as capital goods).
