Work expands so as to fill the time available for its completion.
It was later reprinted together with other essays in the book Parkinson's Law: The Pursuit of Progress (London, John Murray, 1958). He derived the dictum from his extensive experience in the British Civil Service.
The current form of the law is not that which Parkinson refers to by that name in the article. Rather, he assigns to the term a mathematical equation describing the rate at which bureaucracies expand over time. Much of the essay is dedicated to a summary of purportedly scientific observations supporting his law, such as the increase in the number of employees at the Colonial Office while Great Britain's overseas empire declined (indeed, he shows that the Colonial Office had its greatest number of staff at the point when it was folded into the Foreign Office because of a lack of colonies to administer). He explains this growth by two forces: (1) "An official wants to multiply subordinates, not rivals" and (2) "Officials make work for each other." He notes in particular that the total of those employed inside a bureaucracy rose by 5-7% per year "irrespective of any variation in the amount of work (if any) to be done".
- Data expands to fill the space available for storage.
- Storage requirements will increase to meet storage capacity.
"Parkinson's Law" could be generalized further still as:
The demand upon a resource tends to expand to match the supply of the resource.
An extension is often added to this, stating that:
The reverse is not true.
This generalization has become very similar to the economic law of demand; that the lower the price of a service or commodity, the greater the quantity demanded.
Some define Parkinson's Law in regard to time as:
The amount of time which one has to perform a task is the amount of time it will take to complete the task.
Related efficiency 
Parkinson also proposed a rule about the efficiency of administrative councils. He defined a coefficient of inefficiency with the number of members as the main determining variable.
The Coefficient of Inefficiency is a semi-humorous attempt of Parkinson to define the size of a committee or other decision-making body at which it becomes completely inefficient.
In the book Parkinson's Law: The Pursuit of Progress, London: John Murray, 1958 one of the chapters is devoted to the basic question of comitology: how committees, government cabinets, and other such bodies are created and eventually grow irrelevant (or are initially designed as such).
Empirical evidence is drawn from historical and contemporary government cabinets. Most often, the minimal size of a state's most powerful and prestigious body is five members. From English history, Parkinson notes a number of bodies that lost power as they grew:
- The first cabinet was the Council of the Crown, now the House of Lords, which grew from an unknown initial number of members, to 29, to 50 before 1600, by which time it had lost much of its power.
- A new body was appointed in 1257, the "Lords of the King's Council", numbering fewer than 10. The body grew, and eventually ceased to meet when it numbered 172 members.
- The third incarnation of the English cabinet was the Privy Council, initially also numbering fewer than 10 members, rising to 47 in 1679.
- In 1615, the Privy Council lost power to the Cabinet Council, initially with 8 members, rising to 20 by 1725.
- Around 1740, the Cabinet Council was superseded by an inner group, called the Cabinet, initially with 5 members.
At the time of Parkinson's study (the 1950s), the Cabinet was still the official governing body. Parkinson observed that, from 1939 on, there was an effort to save the Cabinet as an institution. The membership had been fluctuating from a high of 23 members in 1939, down to 18 in 1954.
A detailed mathematical expression is proposed by Parkinson for the Coefficient of Inefficiency, featuring many possible influences. In 2008, an attempt was made to empirically verify the proposed model. Parkinson's conjecture that membership exceeding a number "between 19.9 and 22.4" makes a committee manifestly inefficient seems well justified by the evidence proposed. Less certain is the optimal number of members, which must lie between three (a logical minimum) and 20. That it may be eight seems both justified and ruled out by observation: no contemporary government in Parkinson's data set had eight members, and only the unfortunate king Charles I of England had a Committee of State with that membership.
He also wrote the book Mrs. Parkinson's Law: and Other Studies in Domestic Science.
In popular culture 
A "generalization" of Parkinson's Law, is mentioned in an episode of British comedy series Yes Minister, The Skeleton in the Cupboard, originally aired on November 25, 1982. In that episode, an undersecretary of the Department, played by Ian Lavender, explains to the Minister that a certain county "has the smallest establishment of social workers in the U.K." Answering the Minister's question "Is that supposed to be a good thing?", he replies, "Oh yes, sign of efficiency. Parkinson's Law of social work you see; it's well known that social problems increase to occupy the total number of social workers to deal with them".
See also 
- Parkinson's law of triviality
- Hofstadter's law
- Jevons paradox
- List of eponymous laws
- Peter Principle
- Snackwell effect
- Student syndrome
- Time management
- Darwin Award
- Parkinson, Cyril Northcote (November 19th, 1955). Parkinsons Law. The Economist.
- Fowler, Elizabeth M (May 5, 1957). It's a 'Law' now: Payrolls grow. The New York Times.
- O'Sullivan, John (June 2008). "Margaret Thatcher: A Legacy of Freedom". Imprimis (Hillsdale College) 37 (6): 6.
Further reading 
- Parkinson, Cyril Northcote (1957), Parkinson's Law, or The Pursuit of Progress.