# Stock and flow Stocks are level variables measured at a point in time (accumulations); flows are rates measured over intervals that fill or drain stocks. The fundamental relationship Δstock = net flow underpins continuous and discrete dynamical models, accounting identities, conservation laws, and [[System_dynamics|system-dynamics]] diagrams in [[Economics|economics]], ecology, and engineering. <!-- LEGACYSIM:BEGIN v1.5 — generated by g03_mint_wave.py; three.js first; do not hand-edit inside --> ## Microsims (promoted from legacy — three.js first) ### MicroSim spec - **Recommended sim type:** stock & flow - **Microsimmability score:** 90/100 - **Layout:** drawing region (canvas) on top; control region (sliders/buttons) below. ### Parameters (tunable controls) - `Inflow rate` - `Outflow rate` - `Initial stock` ### What animates A reservoir fills and drains as inflow and outflow rates are tuned, tracing the stock over time. ### Learning objective Relate the level of a stock to the balance between its inflows and outflows. ### MicroSim spec - **Recommended sim type:** stock-and-flow - **Microsimmability score:** 90/100 - **Layout:** drawing region (canvas) on top; control region (sliders/buttons) below. ### Parameters (tunable controls) - `Inflow rate` - `Outflow rate` - `Initial stock` ### What animates A tank fills and drains as inflow and outflow rates change, showing the stock level over time. ### Learning objective Relate the rate of change of a stock to the difference between its inflows and outflows. <!-- LEGACYSIM:END --> ## Reveal %%REVEAL:p5%% %%REVEAL:svg%% --- *Concept aligned with [Wikipedia](https://en.wikipedia.org/wiki/Stock_and_flow); adapted text, where present, is licensed [CC BY-SA 4.0](https://creativecommons.org/licenses/by-sa/4.0/).* ## Overview The bathtub is the canonical picture: water level as stock, faucet and drain as flows—an [[Integral|integral]] in plain sight, with each [[State_variable|state variable]] a [[Conserved_quantity|conserved quantity]] under its [[Dimensional_analysis|dimensional analysis]]. [[Jay_Wright_Forrester|Jay Wright Forrester]] made the pairing the accounting spine of [[Systems_thinking|systems thinking]], and any stock-flow map compiles directly into a [[Dynamical_system|dynamical system]]. Economics runs on the distinction: [[Capital_(economics)|capital]] and [[Capital_accumulation|its accumulation]], [[Income|income]] versus wealth, [[Inventory|inventory]] versus [[Inventory_investment|inventory investment]], [[Gross_domestic_product|GDP]] as pure flow, the [[Debt-to-GDP_ratio|debt-to-GDP ratio]] mixing the two, with [[Depreciation|depreciation]] draining and [[Human_capital|human capital]] accumulating. Earth systems keep the same books: the [[Carbon_cycle|carbon cycle]], [[Greenhouse_gas|greenhouse-gas]] stocks, and [[Earth's_energy_budget|Earth's energy budget]] within [[Earth_system_science|Earth system science]]. ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Stock_and_flow) : [Wikitube](https://en.wikitube.io/wiki/Stock_and_flow) ## Previous hub tags Hubs: `Systems`. Portals: [[PORTAL_Systems]], [[PORTAL_Stock_and_flow]].