Stocks and Flows

A stock is an accumulation, the amount of something present in a system at a moment in time. Flows are the rates that fill or drain it. Stocks are the memory of a system; they change only through their flows.

Why it Matters

Stocks change gradually even when flows change abruptly, which is the source of inertia, buffering, and delay in every system. Confusing a stock with a flow leads to wrong expectations about how fast a system can respond.

Signals

• A quantity measured as a level ("how much") versus a rate ("how fast").
• Something that cannot change instantly no matter how hard you push.
• Two flows, one in and one out, jointly setting whether a level rises or falls.

Benefits

Gives precise vocabulary for what accumulates versus what merely passes through, and explains momentum: a stock keeps moving in its current direction until the flows are rebalanced.

Risks

Treating a stock as if it reacts instantly to a changed flow; watching the visible flow while ignoring the stock that is the real state; forgetting that a stock keeps rising while inflow still exceeds outflow.

Tensions

Flows are visible activity and grab attention, but the stock is what matters and it lags. Cutting an inflow does not lower a stock if the outflow is even smaller.

Examples

A bank balance is a stock; deposits and withdrawals are flows. A work backlog is a stock; arrival and completion rates are its flows.