Parkinson's law states that "work expands so as to fill the time available for its completion."
It was first articulated by C. Northcote Parkinson in the book Parkinson's Law: The Pursuit of Progress, (London, John Murray, 1958) based on extensive experience in the British Civil Service. The scientific observations which contributed to the law's development included noting that as Britain's overseas empire declined in importance, the number of employees at the Colonial Office increased.
According to Parkinson, this is motivated by two forces: (1) "An official wants to multiply subordinates, not rivals" and (2) "Officials make work for each other." He also noted 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".
"Parkinson's law" is also used to refer to a derivative of the original relating to computers: "Data expands to fill the space available for storage"; buying more memory encourages the use of more memory-intensive techniques. It has been observed over the last 10 years that the memory usage of evolving systems tends to double roughly once every 18 months. Fortunately, memory density available for constant dollars also tends to double about once every 12 months (see Moore's Law); unfortunately, the laws of physics guarantee that the latter cannot continue indefinitely.
"Parkinson's Law" could be more generalized still as: "The demand upon a resource always expands to match the supply of the resource."
- Parkinson's Law, or The Pursuit of Progress, C. Northcote Parkinson, 1957. Parkinson's Law quoted in full