Sequential work creates a chain
If Task B cannot start until Task A finishes, their elapsed times largely add together. A three-hour task followed by a two-hour task creates about five hours of work before considering handoffs or waits.
Dependencies are the reason a project with 100 hours of total effort does not necessarily take 100 clock hours—or only 12.5 eight-hour days. Some work can overlap; some work forms a critical chain that controls the finish.
Parallel work needs real independence
Putting two tasks on a chart at the same time does not make them parallel. They need independent inputs, available people or equipment, and no shared bottleneck that forces one to wait. Coordination overhead can also reduce the theoretical gain.
The Project Duration Calculator lets you experiment with parallel work lanes, but its result is an idealized planning estimate. If dependencies are complex, build a proper task network or project schedule.
Watch the bottleneck
When several parallel tasks feed one review, machine, specialist or approval point, that shared resource can become the real duration driver. Improving a non-bottleneck step may reduce effort without changing the completion date.
A good schedule therefore asks not only “How long does each task take?” but also “What must happen before this task, what does it block, and what shared resource can delay it?”