How to Estimate Project Duration

Project managers often underestimate the amount of time needed to implement projects, which results in missed deadlines or teams rushing and overworking themselves to get the job done. Knowing how to estimate project duration can help teams become more efficient and improve on-time and on-budget delivery. 

When it comes to project management time estimation, there are a couple of likely scenarios. Either your deadline is too ambitious and there’s a risk of disappointing your customer, losing business, or burning out your team or you’re too conservative when you estimate project duration, which can result in people standing around idle.

So, how do you strike a happy balance? Read on to learn how to achieve realistic project duration estimates for your future projects. 

Why is it important to estimate project duration?

Project management is a game of precision. If you cannot estimate project duration, you won’t know how long your overall project will take or when you will reach certain stages or milestones. Having a milestone trend analysis is crucial to your project as it helps your whole team have a clear vision of the future.

This inability to estimate project duration can lead to multiple problems, including:

  • Not having resources available when you need them
  • Not ordering materials early enough to arrive on time
  • Upset customers wondering when they’ll receive deliverables
  • Inefficient employees since there is no pressure to complete work by a set time
  • Idle employees if projects finish and no other work has been lined up

It’s also difficult for senior leadership and clients to determine whether a project has been successful when there’s no estimate to compare it to. Suppose your team completed a project in eight months. Is this good or bad? 

It’s impossible to say without knowing how long the project should have taken. If six months is a realistic estimate, then your team underperformed. If nine months is a realistic estimate, your team overperformed and should be rewarded for their efforts.  

How is project duration calculated?

There are multiple ways to conduct project management time estimation, but most methods can be grouped into one of two buckets. You can either estimate project duration from the top down or the bottom up. 

Top-down approaches create an overall project duration estimate, and they are typically based on benchmarks and past projects. If you’ve done many similar projects in the past, and the average completion time was ten months, you may estimate this project will take ten months. 

Bottom-up approaches estimate each task or activity and then combine those task durations to create an overall project duration. This approach is more detailed and time-consuming but often results in more accurate estimates.

You must make sure you don’t add all the durations together as some tasks will overlap and be completed during the same timeframe. To achieve an accurate bottom-up estimate of project duration, you need to identify your critical path and assess its length. 

You can also combine the two approaches to arrive at a duration somewhere in the middle. If the top-down estimate is ten months and the bottom-up estimate is twelve months, you may go with a final estimate of eleven months. 

What factors to involve in a project management time estimation

Many factors can impact your project and task duration estimates. Some of the major ones include:

  • Project scope: If a past project didn’t include user training, but your new one does, then your estimate needs to take into account that difference, or you won’t have enough time planned.   
  • Experience: The level of experience your team members have will impact how long it takes them to complete tasks.  
  • Availability: Work hours, holidays, and vacations all need to be factored into your estimates. If your company always shuts down for a week at Christmas, make sure your project duration accounts for that lost time. 
  • Uncertainty: If there are things you cannot accurately estimate, you should factor in a contingency or buffer to help cover these variables. 
  • Governance requirements: The level of oversight and control in your organization can impact how long a project takes. Any reports, meetings, and reviews must be accounted for in your duration estimates.  
  • Estimation biases: If the person performing the work has provided the estimate, they may have been motivated to increase the duration. It’s important to compare estimates against past actuals and question potential biases. 
  • Project priority: If your project isn’t strategically important for your company, you could find yourself facing delays whenever projects conflict. For instance, if a vital project runs into trouble and needs your lead engineer to help, you could suddenly find yourself working without them. 
  • External forces: There are many factors outside of your company that could also impact your estimates, including long delivery times for materials, and weather delays. If you know a part will take eight weeks to ship, or that winter storms could mean lost workdays, these insights need to factor into your estimates. 

What is analogous estimating?

Analogous estimating uses the data from similar projects completed in the past to estimate the duration of upcoming projects. 

Often, this historical data will be compiled and assessed by experienced individuals such as the project manager, sponsor, and senior team members. The assessment may involve comparing scopes of work, considering lessons learned, and adjusting for other known factors such as team experience and resource availability. 

Suppose you’re about to do a new marketing project. Last year, a similar project took twelve weeks. But, that was your first one of this kind, and you’ll be using the same team this year. Based on these insights, you may assume this project will only take ten weeks. 

Analogous estimates are typically quick top-down estimates that can be done when senior management or clients want a rough idea about how long a project will take. They can also be done as bottoms-up estimates, where you determine each task’s length based on the past duration of similar tasks. 

What is parametric estimating?

Parametric estimating is a detailed, bottoms-up approach to estimating where one unit or task is assessed and estimated in detail and then scaled up to arrive at an overall estimate. 

If you’re creating six similar ads, you’ll estimate in detail how long it will take to complete one ad. You may use historical data, subject matter expertise, vendor quotes, or other relevant information to create the detailed estimate. Then, instead of going through the estimation process again for the other five ads, you simply multiply your first duration by six. 

Often, parametric estimating will be used once a project has already started to refine the initial estimate based on work already completed

Imagine your project is to complete an e-book with seven chapters, all of roughly the same length. After the first chapter is completed, you see that it took your team five business days. Based on parametric estimating, the entire book should be complete in 35 business days. 

If your initial top-down estimate was 30 days, parametric estimating helps you discover early that something may need to change for your team to still hit your deadline. 

How Wrike can help with estimating project duration

With Wrike, you can easily track all of your task and project durations in one central location. Then, when it’s time to estimate your next project, everything is right at your fingertips. You’ll be able to see any information about why things took longer than expected, what lessons were learned, and what can help your project go even smoother. You’ll also have access to who worked on the project, what tasks were involved, and what changes occurred throughout. 

As you plan your own project in Wrike, you can easily set task durations, account for weekends and vacations, monitor your critical path, and leave comments to note assumptions you made around duration estimates. Start your free trial of Wrike today and discover how it will help you achieve more realistic project durations.

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