Estimate project duration from effort, team size and parallel efficiency.
Adding people to a project raises coordination overhead, so effective team size grows more slowly than headcount. That is why doubling the team does not halve the schedule. Effort and duration are different quantities, and treating them as interchangeable is the root of most schedule overruns.
Project Duration
Duration = effort ÷ (team size × parallel efficiency), plus contingency
Duration = effort ÷ (team size × parallel efficiency), plus contingency Adding people to a project raises coordination overhead, so effective team size grows more slowly than headcount. That is why doubling the team does not halve the schedule.
Effort and duration are different quantities, and treating them as interchangeable is the root of most schedule overruns.
This calculator takes 5 inputs: Total effort, Team size, Parallel efficiency, Working days per week, Schedule contingency. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.