400 hours of work is not a delivery date

The short answer

Effort measures the total work. Capacity measures how much work the available people can perform. Duration also depends on the order of tasks, specialist constraints, waiting time and uncertainty. Dividing effort by headcount only answers a simplified resource question.

First check the available hours

Suppose three people each work eight hours per day across 20 working days. Nominal capacity is 480 hours. At 70% project availability, after other commitments and non-project work, the project receives only 336 hours. An estimated 400 hours exceeds that capacity by 64.

The resource-only duration is 400 ÷ (3 × 8 × 0.70) ≈ 23.8 working days. To fit the same divisible effort into 20 days at that availability, the arithmetic needs four whole people. Use the project capacity calculator to test availability and preserve a baseline.

Then inspect the dependency path

Consider five days of design, followed by two implementation tasks of six and nine days that can run in parallel, followed by three days of integration testing. If separate people are available for the parallel work, the deterministic completion path is 5 + max(6, 9) + 3 = 17 working days. Total effort across the tasks is 23 person-days.

If the same person must perform both implementation tasks, the parallel assumption is false and the sequence takes 23 working days before other constraints. An additional person who cannot perform those tasks does not remove the bottleneck. Check the constrained skill rather than treating every team hour as interchangeable.

Keep external waiting time separate

A client approval can add elapsed time with little hands-on effort. If the review needs three calendar days after the work is ready, adding another developer does not necessarily make it arrive sooner. Identify who owns the dependency, what starts the waiting period and what happens if the input is late.

Read dependencies and deadline confidence to connect that distinction to a project model. The software estimating guide brings scope, task ranges, capacity and discrete risks into one workflow.

Uncertain branches can change which task controls completion

In a range-based plan, the branch that finishes last can vary between simulated outcomes. Adding branch means or task P80s does not reproduce the distribution of the project finish. See PERT and Monte Carlo and why task P80s do not add up.

What the capacity result can support

A capacity shortfall is evidence that the plan does not fit its entered resource assumptions. Sufficient capacity is a reason to continue checking the schedule, not proof of a delivery promise. The calculator uses working days without a calendar; the quick deadline tool uses elapsed days for its date mode. Keep units explicit when moving between them. The GAO Schedule Assessment Guide is a broader reference on integrated schedule assessment.

Prepared with AI assistance. These are illustrative calculations checked against the calculator code, not client case studies or market benchmarks.