Effort vs. Duration: Why Eight Hours Can Take a Week

Hours of work and space on the calendar

A drawing review needs eight hours of a senior engineer’s time. That sounds like a day. The engineer has two hours available on Monday, none on Tuesday, two on Wednesday, two on Thursday, and two on Friday. The review takes the working week.

Effort is the amount of labor a task needs, usually measured in person-hours or person-days. Duration is the working time between its start and finish under the schedule’s calendar. Elapsed time also counts nonworking time, including nights and weekends. Eight hours of effort tells you little about the finish date until you know when someone can do the work.

Ask “How many hours will this take?” and “When can those hours happen?” separately. The answers solve different planning problems.

A useful first calculation

For one person doing work at a steady rate, divide the effort by the hours they can give it each working day. Eight person-hours at two hours per day needs four working days. At eight hours per day, it needs one.

That calculation assumes the inputs are ready, the available hours are usable, and there is no waiting or rework. In the example above, Tuesday’s missing availability pushes completion into a fifth day. A weekly average can hide exactly the gap that matters to a deadline.

Scheduling software represents this relationship through work, duration, and resource assignments. Microsoft’s guide to resource units explains how availability enters that calculation. Before adjusting a percentage in the software, check what time the person can actually reserve.

Waiting needs a place in the plan

Suppose the review also needs a client decision. The engineer can complete the first four hours, then must wait for confirmation of an access location before checking the remaining drawings. That wait may add no engineering effort, but it can still move the finish date.

Show the decision as a dependency with an expected response date and someone responsible for following up. Adding labor hours to cover the wait confuses the fee estimate and still does not explain what is holding up the review.

The same issue appears outside engineering: a draft can be ready for editing while its editor is booked, or a purchase can be ready to place while approval is outstanding.

Another person helps when the work can be shared

Two reviewers might divide a drawing set. They may also need time to agree on assumptions and reconcile comments. Some checks must follow others, and one person may still need to approve the complete package.

Before halving a duration, name the work that can happen in parallel and the coordination it adds. The effect of adding people depends on those details. More available hours help only where the task can use them.

Turn the estimate into a commitment

For a task coming up this week, record the remaining effort, the person doing it, their available time, and the inputs needed to start or continue. Include review and correction if they are part of the promised deliverable.

A useful update sounds like this: “The review has eight hours remaining. We have two hours booked on Monday, Wednesday, Thursday, and Friday, so we expect to finish Friday. We need the access decision before Thursday’s session.”

If Friday is too late, there is now something specific to change: protect an earlier review slot, obtain the decision sooner, divide a suitable portion of the work, or agree to a different delivery. Check the critical path before assuming an earlier task finish also moves the project finish.