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Project Duration Calculator

Estimate project duration from effort, team size and parallel efficiency.

About Project Duration Calculator

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.

Formula

Project Duration

Duration = effort ÷ (team size × parallel efficiency), plus contingency

parallel efficiency
Coordination overhead means teams never scale linearly
person-day
One person working for one day

Worked example

With the default values below, Project Duration Calculator returns Project Duration of 15.360. Change any input above and every figure here is recalculated in your browser.

Inputs used

Total effortminimum 0 person-days
240 person-days
Team sizeminimum 0.1
5
Parallel efficiencyaccepts 0–100 %
75 %
Working days per weekaccepts 0.5–7 days
5 days
Schedule contingencyaccepts 0–100 %
20 %

Results

Project Duration
15.360 weeks
Duration in Working Days
76.8 days
Duration in Calendar Days
108 days
Effective Team Size After Coordination Loss
3.750
Contingency Included
2.560 weeks

Step by step

  1. 1Values used
  2. 2Project Duration
  3. 3Project Duration= 15.360 weeks
  4. 4Duration in Working Days= 76.8 days
  5. 5Duration in Calendar Days= 108 days
  6. 6Effective Team Size After Coordination Loss= 3.750
  7. 7Contingency Included= 2.560 weeks

Assumptions and limits

  • The result is derived only from the 5 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

How is Project Duration calculated?

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.

Why does Project Duration matter?

Effort and duration are different quantities, and treating them as interchangeable is the root of most schedule overruns.

What values do I need to enter?

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.

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