Calculating Project Duration: A Beginner Friendly Guide

Project managers estimate project duration because it gives them a sense of how long they have to get everything done. Learn how to do it in our beginner friendly guide.
Steven Male

Estimating project duration is like building a life plan. You know where you want to get, you know something will likely go wrong, but you still need to establish a timeline to reach your goals.

If there is one thing to know about estimating project duration, it will have to be this: there are lots of traps to look out for.

But with proper preparation you can make this into an easy experience.

Here is where you start.

What is duration in project management?

Project duration is the total amount of time it takes to finish a project, which you measure in business days, hours, weeks, or months. The duration can be seen at the timeline for project delivery, whether it is five days or five years. Project duration usually depends on your resource availability.

The difference between effort, duration, and elapsed time

In simple terms, effort is focused on highlighting the work units (hours) you need to complete a task or a project, while duration is focused on the time you need to take to complete it. The duration is usually longer than the estimated hours in effort because your team doesn't work non-stop.

Elapsed time is more about the progress — it looks at how long it took from the moment you assigned someone to a project to the moment they completed it. Eventually, it will also show how effectively you're working — are you going to meet the promised deadlines?

How is project duration calculated?

Top-down estimating

PMBOK explains top down estimating, also known as analogous estimating as “a technique for estimating duration or cost of an activity or a project using historical data from a similar activity or a 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 completed, assuming that the new project will take approximately as much time and resources to complete.

calculating project duration

Bottom up estimating

With bottom-up estimating, you go from detailed to general look — from task to project. The rule is simple — if you cannot make an accurate estimation of a project, dissect it to the units which you can estimate properly, like milestones or even individual tasks.

Parametric estimating

Parametric estimating is basically taking analogous estimating to another level. You also look at historical data only get more accurate when it comes to numbers, introducing something of a "statistical relationships".

That is to say that you need to find a comparable project in your historical data and then customize calculations based on the numerical parameters of your new project.

Three-point estimating

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 most likely estimates when there is uncertainty with the individual activity estimates.” 

Here you get to reduce your risks by accounting for several scenarios your project could end up following.

Project duration best practices

Suppose you need to build a website and you have estimated that it will take about 40 hours of work (the effort). However, the website will not be ready in 40 hours for a number of reasons: you might have other projects running, you cannot devote all of your time to building a website, you need to take some days off in the middle of the project, etc.

As a result, 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 and takes half of the workload off your shoulders, the project will take only 4 days.

But whether you will be working alone or with reinforcements, here are some things you could do to get more efficient.

1. Create a resource schedule

When a project involves more than one person working on it, you need to create a resource schedule for transparency and visibility.

Here you will be able to see when your resources are free, full, or overbooked and by how much. This is your best bet to juggle resources and follow the "less is more" concept.

With Runn's resource scheduling, all you need to do is click, drag, and drop workload when you need to allocate it to someone specific. You can extend, shorten, transfer, and split work among your resources to accommodate everyone involved.

2. Include time off

With a resource schedule, it's wise to account not only for the time when someone is available to work but also when they are not. People take vacations, days off, sick leave, etc. — all of which can delay your project delivery if you don't account for them when estimating project duration. But by adding that time off right into the schedule you can avoid surprises and unneeded headaches.

3. Add a contingency reserve

No project is perfect. There will be risks which may or may not happen, delays that may or may not stall the project, additional costs that may or may not make you go over the budget.

With a contingency reserve, you can be prepared for whatever happens and still keep your project going according to plan (even if it's not the most optimistic one).

4. Don't underestimate

People have a natural tendency to be overoptimistic. In project management, this can lead to project failure.

Have you ever moved houses? It never takes the time you expect it to take — there is always a something causing one delay after another and you end up sleeping on a mattress for two weeks.

In a way, projects can be the same. This is why leaving space for some wiggle room, scope creep, ad hoc requests, and the like can help you realistically estimate project duration.

Start a free trial of Runn to see how you can optimize and simplify your project planning and estimation in a matter of a few clicks!

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