Stockflow

Stockflow is a concept used in systems thinking and economics to describe how stock variables and flow variables interact within a dynamic system. A stock represents a quantity measured at a specific point in time, such as the amount of capital, inventory, or population. A flow represents a rate of change over time, such as investment per year, shipments per day, or births per month.

The core relation is that the stock changes in response to inflows and outflows. Mathematically, the rate of change of stock equals inflows minus outflows: dS/dt = Inflows − Outflows. Over a discrete period, the stock at the end equals the stock at the start plus net flow. This simple accounting captures dynamic behavior in many domains.

Stockflow modeling is widely used in system dynamics, macroeconomics, inventory management, and environmental accounting to analyze sustainability, capacity, and policy effects. It helps planners assess how policies or shocks propagate over time, and it highlights the difference between quantities measured at a moment and flows that accumulate them.

Common applications include warehouse inventory management, where stock is on-hand items and flows are receipts and shipments; environmental accounting of carbon stocks and fluxes; capital stock in national accounts; population planning with births and deaths; and energy or resource planning that tracks stock levels and flows of energy, water, or minerals.

Software tools that support stockflow analysis include system dynamics modeling programs, which allow users to build stock-and-flow diagrams and simulate time-based behavior.