Project Timeline Calculator
How long a project takes from the effort it needs, the people on it and the coordination overhead that adding people creates.
Effort divided by people gives an elapsed time only if people were interchangeable and communication were free.
How the project timeline calculator works
Effort divided by people gives an elapsed time only if people were interchangeable and communication were free. They are not. Communication paths grow as n(n−1)/2, so each person added contributes less than the last.
That is Brooks's observation in arithmetic form. Past a certain team size the overhead exceeds the added capacity and the project gets slower, which this shows as an optimal team size rather than as advice.
Formula: elapsed = effort / (people × efficiency); efficiency falls with team size
Worked examples
| Inputs | Elapsed working days | Note |
|---|---|---|
| 240 person-days across five people | 81.63 | about 71 days |
| A large team | 228.57 | slower than a smaller one |
| Full availability | 57.14 | a third faster |
FAQFrequently asked questions
Why does adding people not halve the time?
Because coordination costs grow with the square of team size while capacity grows linearly. Past a point the next person makes the project slower.
What is the ideal team size?
For most software and creative work, five to nine. This calculator finds the size that finishes soonest given your own overhead figure.
What availability should I assume?
Sixty to eighty per cent for people who also have meetings, support duties and another project. A hundred per cent is fiction.
Is this Brooks's law?
It is the arithmetic behind it. Brooks's point was about adding people to a late project, where ramp-up makes it worse still.
Does the effort estimate include testing?
Only if you put it there. Effort estimates that count development but not review, testing and deployment are the usual reason projects overrun.
Where these figures come from
- ISO 8601 — Date and time — Representations for information interchange — the yyyy-mm-dd format and the week-numbering rules used here
- US Naval Observatory — Calendars — the Gregorian leap-year rule
- US Office of Personnel Management — federal holidays — federal holidays (not applied here)
Last checked: September 2026. The Gregorian calendar rules do not change; working-day counts exclude Saturdays and Sundays only.