Parkinson’s law
Etymology
Named for the British naval historian C. Northcote Parkinson, who opened a satirical 1955 essay in The Economist with the line and expanded it in his 1957 book Parkinson’s Law. He drew it from observing bureaucracies that kept adding staff even as their actual workload shrank. Though written as comedy, it endures as a genuine rule of thumb about deadlines and organizational bloat.
noun phrase · /PAR-kin-sunz law/
The principle that work expands to fill the time available for its completion.
What it names
Why a task you could finish in a morning somehow takes the whole week you gave it — slack gets absorbed rather than saved.
Example
I gave the report a two-day deadline instead of two weeks precisely because of Parkinson’s law — otherwise it would swallow all the time I had.
Related idea
Bikeshedding (Parkinson’s law of triviality) is a separate observation by the same author; the original Parkinson’s law is specifically about time and effort expanding to fit their container.