Estimating project duration: a beginner’s guide

Project managers estimate the duration of a project, as this gives them an idea of how long they have to complete it. Find out how with our beginners’ guide.

Estimating the duration of a project is like drawing up a life plan, a total project duration with, above all, an end date. You know where you want to go, you know that problems will probably arise, but you still need to establish a timetable to reach your objectives.

If there’s one thing you need to know about estimating the duration of a project, it’s this: there are many pitfalls to avoid, known as the critical path of the project.

But with the right preparation, you can make this an easy experience.

What is duration in project management?

The duration of a project corresponds to the total time needed to complete it, with a start and end date. It is measured in working days, hours, weeks or months. It is indicated on the project delivery schedule, whether it takes five days or five years. It generally depends on the availability of your resources.

The difference between effort, duration and elapsed time in project management

In simple terms, effort focuses on the units of work (hours) needed to complete a task or project, while duration focuses on the time needed to complete it. Duration is generally higher than estimated effort hours, because your team doesn’t work without interruption.

Elapsed time is more related to project progress: it measures the time elapsed between the moment you assigned a project to someone and its completion.

Ultimately, it also indicates the efficiency of your work: will you meet the promised deadlines?

How is the project duration calculated?

Top-down estimation

The PMBOK (Project Management Body of Knowledge) explains top-down estimating, also known as analog estimating, as “a technique for estimating the duration or cost of an activity or project using historical data from a similar activity or project”.

In other words, here you need to look at your historical data and compare the new project to something similar that has already been done, assuming that the new project will take about as much time and resources to complete.

Bottom-up estimation

With bottom-up estimating, you move from a detailed view to an overview, from task to project. The rule is simple: if you can’t estimate a project precisely, break it down into units you can estimate correctly, such as milestones or even individual tasks.

Parametric estimation

Parametric estimation essentially involves taking analog estimation to a higher level. What’s more, the analysis of historical data only increases in numerical precision, introducing a kind of “statistical relationship”.

In other words, you need to find a comparable project in your historical data, then customize the calculations according to the numerical parameters of your new project.

Three-point estimate

PMBOK explains three-point estimating as “a technique used to estimate cost or duration by applying an average or weighted average of optimistic, pessimistic and high-probability estimates when there is uncertainty with individual activity estimates.”

Here, you can reduce your risk by taking into account several scenarios that your project could follow.

Best practices in project duration for a project manager

Imagine you need to create a website and you’ve estimated that it will take you around 40 hours of work (effort). However, the site won’t be ready in 40 hours for a number of reasons: you may have other projects on the go, you can’t devote all your time to creating the website, you need to take a few days off in the middle of the project, and so on.

Consequently, the effort will be 40 hours, but the duration will be longer. For example, if you decide to devote 5 hours a day to the project, it will take you 8 days to complete it (your duration), but if your colleague comes to help you and offloads half the workload, the project will take you only 4 days.

But whether you’re working alone or with back-up, here are a few things you can do to boost efficiency.

Create a resource calendar for planning

When a project involves several people working on it, you need to create a resource calendar for greater transparency and visibility.

Here you can see when your resources are free, full or overloaded, and by how much. This is the best way to manage your resources and follow the “less is more” principle.

With Teambook’s resource scheduling, you simply click, drag and drop the workload when you want to allocate it to a specific person. You can extend, reduce, transfer and distribute work between your resources to meet everyone’s needs.

Include free time

With resource planning, it’s a good idea to take into account not only availability, but also absences. Vacations, days off, sick leave, etc. can delay the delivery of your project if you don’t take them into account when estimating its duration. By including these factors in your planning from the outset, you’ll avoid unpleasant surprises and unnecessary hassle.

Add a contingency reserve

No project is perfect. There will be risks, delays, additional costs, budget overruns and so on.

With an emergency reserve, you can be prepared for anything that comes up, and continue to carry out your project as planned (even if it’s not the most optimistic one).

Don’t underestimate

We have a natural tendency to be over-optimistic. In project management, this can lead to failure.

Have you ever moved house? It never takes as long as you expect: there’s always a problem that causes delays, and you end up sleeping on a mattress for two weeks.

In a way, projects can be similar. That’s why allowing for room for maneuver, target drift, ad hoc requests, etc., can help you to estimate the duration of a project realistically.

Start a free Teambook trial to see how you can optimize and simplify your project planning and estimating with just a few clicks!