Worked example: three people do not provide 480 project hours
Three people over 20 working days at eight hours per day provide 480 nominal hours. At 70% availability, only 336 belong to this project. An estimated 400 hours therefore leaves a 64-hour shortfall. With the same assumptions, you need four whole people or about 23.8 working days.
This is a capacity floor, not a schedule forecast
The calculation assumes divisible work, interchangeable people and constant availability. A task waiting on a client, a specialist bottleneck or a dependency chain can make elapsed delivery much longer. Adding a person does not automatically divide duration. Read effort versus duration and task dependencies before turning the result into a date.
Check the constrained skill as well as the whole team
Run a second check for the designer, reviewer or integration specialist whose work cannot be reassigned. Keep a baseline and reduce availability to see the effect of another commitment. The software estimating guide explains how to combine this capacity check with task dependencies and a deadline confidence check.
Frequently asked questions
Are the calculated days business days?
They are working days under your hours and availability assumptions. The tool does not apply a calendar, weekends, holidays or leave dates.
Can I promise the calculated duration?
It is only a resource arithmetic check. Dependencies, skill constraints, waiting time, uncertainty and coordination can require more elapsed time.