# Parkinson's law

**Parkinson's law** states that work expands to fill the time available. Three weeks to build a feature takes three weeks; three days takes three days. The result is often functionally equivalent because extra time was spent on marginal improvements, over-engineering, and second-guessing decisions.

Estimates become self-fulfilling deadlines. If a developer estimates five days and could do it in three, the estimate makes them take five days. This motivates time-boxing: short, firm deadlines prevent indefinite expansion. [[swe:agile]] sprint cycles enforce this with fixed two-week scopes.

Related to [[swe:goodharts-law]]: once a time target becomes a commitment, behaviour optimises for hitting the target rather than for shipping useful software efficiently.