Stock and Flow

Remember our bathtub? The amount of water in the bathtub is the stock. Water can accumulate in the bathtub by entering (inflow) through the faucet or leave by going down the drain (outflow). If the inflow is greater than the outflow, the stock (in this case, the water) rises.

Whereas stock is the level (or amount) of water in the bathtub, flow is the rate (or speed) at which the bathtub can be filled or emptied. Think of the stock of Lake Erie relative to the flow of Niagara Falls. Here are some other examples:

Suppose you own a mountain or a forest full of trees. Stock is the number and volume of trees that you own. Outflow is the time it takes to harvest the lumber and turn it into toothpicks or paper. Inflow is the time it takes for your next generation of trees to be planted and mature.

Think about your recent bank statement. Stock is the balance you have sitting in your account. Flow is the rate at which your earnings are deposited and your expenses are debited.

In a large urban centre, there is number (stock) of people who do not have adequate housing. There is an outflow rate at which people find or receive adequate housing and an inflow rate at which people enter the vulnerable position of homelessness.


Cite this page:
Shelley, James. (2020). 'Stock and Flow' (in System Thinker Notebook). Originally published on August 5, 2020. Accessed on September 29, 2020. Licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. Permalink: https://jamesshelley.com?p=17087
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