Estimating Time Accurately

Calculating Realistic Project Timelines

Be realistic.

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Have you ever been on a project where the deadline was way too tight?

Chances are that tempers were frayed, sponsors were unhappy, and team members were working ridiculous hours. Chances are, too, that this happened because someone underestimated the amount of work needed to complete the project.

People often underestimate the amount of time needed to implement projects, particularly when they're not familiar with the work that needs to be done.

For instance, they may not take into account unexpected events or urgent high priority work; and they may fail to allow for the full complexity of the job. Clearly, this is likely to have serious negative consequences further down the line.

This is why it's important to estimate time accurately, if your project is to be successful. In this article, we look at a process for making good time estimates, and we explore some of the estimating methods that you can use.

Why Estimate Time Accurately?

Accurate time estimation is a crucial skill in project management. Without it, you won't know how long your project will take, and you won't be able to get commitment from the people who need to sign it off.

Even more importantly for your career, sponsors often judge whether a project has succeeded or failed depending on whether it has been delivered on time and on budget. To have a chance of being successful as a project manager, you need to be able to negotiate sensible budgets and achievable deadlines.

How to Estimate Time Accurately

Use these steps to make accurate time estimates:

Step 1: Understand What's Required

Start by identifying all of the work that needs to be done within the project. Use tools such as Business Requirements Analysis  , Work Breakdown Structures  , Gap Analysis   and Drill-Down   to help you do this in sufficient detail.

As part of this, make sure that you allow time for meetings, reporting, communications, testing and other activities that are critical to the project's success. (You can find out more on these activities in our article on Project Management Phases and Processes  .)

Step 2: Order These Activities

Now, list all of the activities you identified in the order in which they need to happen.

At this stage, you don't need to add in how long you think activities are going to take. However, you might want to note any important deadlines. For example, you might need to get work by the finance department finished before it starts work on "Year End."

Step 3: Decide Who You Need to Involve

You can do the estimates yourself, brainstorm   them as a group, or ask others to contribute.

Where you can, get the help of the people who will actually do the work, as they are likely to have prior experience to draw upon. By involving them, they'll also take on greater ownership of the time estimates they come up with, and they'll work harder to meet them.


If you involve others, this is a good time to confirm your assumptions with them.

Step 4: Make Your Estimates

You're now ready to make your estimates. We've outlined a variety of methods below to help you do this. Whichever methods you choose, bear these basic rules in mind:

  • To begin with, estimate the time needed for each task rather than for the project as a whole.
  • The level of detail you need to go into depends on the circumstances. For example, you may only need a rough outline of time estimates for future project phases, but you'll probably need detailed estimates for the phase ahead.
  • List all of the assumptions, exclusions and constraints that are relevant; and note any data sources that you rely on. This will help you when your estimates are questioned, and will also help you identify any risk areas if circumstances change.
  • Assume that your resources will only be productive for 80 percent of the time. Build in time for unexpected events such as sickness, supply problems, equipment failure, accidents and emergencies, problem solving, and meetings.
  • If some people are only working "part-time" on your project, bear in mind that they may lose time as they switch between their various roles.
  • Remember that people are often overly optimistic, and may significantly underestimate the amount of time that it will take for them to complete tasks.


The most reliable estimates are those that you have arranged to be challenged. This helps you identify any assumptions and biases that aren't valid.

You can ask team members, other managers, or co-workers to challenge your time estimates.

Methods for Estimating Time

We'll now look at different approaches that you can use to estimate time. You'll probably find it most useful to use a mixture of these techniques.

  • Bottom-Up Estimating

    Bottom-up estimating allows you to create an estimate for the project as a whole. To analyze from the "bottom up," break larger tasks down into detailed tasks, and then estimate the time needed to complete each one.

    Because you're considering each task incrementally, your estimate of the time required for each task is likely to be more accurate. You can then add up the total amount of time needed to complete the plan.

    Tip 1:

    How much detail you go into depends on the situation. However, the more detail you go into, the more accurate you'll be.

    If you don't know how far to go, consider breaking work down into chunks that one person can complete in half a day, for example. Sure, this is a bit circular, but it gives you an idea of the level of detail you should aim for.

    Tip 2:

    Yes, this does take a lot of work, however, this work will pay off later in the project. Just make sure that you leave plenty of time for it in the project's Design Phase.

  • Top-Down Estimating

    In top-down analysis, you develop an overview of the expected timeline first, using past projects or previous experience as a guide.

    It's often helpful to compare top-down estimates against your bottom-up estimates, to ensure accuracy.


    Don't assume that the bottom-up estimates are wrong if they differ widely from the top-down ones. In fact, it's more likely that the reverse is true.

    Instead, use the top-down estimates to challenge the validity of the bottom-up estimates, and to refine them as appropriate.

  • Comparative Estimating

    With comparative estimating, you look at the time it took to do similar tasks, on other projects.

  • Parametric Estimating

    With this method, you estimate the time required for one deliverable; and then multiply it by the number of deliverables required.

    For example, if you need to create pages for a website, you'd estimate how much time it would take to do one page, and you'd then multiply this time by the total number of pages to be produced.

  • Three-Point Estimating

    To build in a cushion for uncertainty, you can do three estimates – one for the best case, another for the worst case, and a final one for the most likely case.

    Although this approach requires additional effort to create three separate estimates, it allows you to set more reasonable expectations, based on a more realistic estimate of outcomes.


In the early stages of project planning, you often won't know who will do each task – this can influence how long the task will take. For example, an experienced programmer should be able to develop a software module much more quickly than someone less experienced.

You can build this into your estimates by giving best, worst, and most likely estimates, stating the basis for each view.

Preparing Your Schedule

Once you've estimated the time needed for each task, you can prepare your project schedule  . Add your estimates to the draft activity list that you produced in the second step, above.

You can then create a Gantt Chart   to schedule activities and assign resources to your project; and to finalize milestones and deadlines.


If your project is complex, you might find that identifying the critical path   on your plan is helpful. This will help you highlight the tasks that cannot be delayed if you're going to hit your deadline.

Key Points

You need to estimate time accurately if you're going to deliver your project on time and on budget. Without this skill, you won't know how long your project will take, and you won't be able to get commitment from the people required to help you achieve your objective.

More than this, you risk agreeing to impossibly short deadlines, with all of the stress, pain, and loss of credibility associated with this.

To estimate time effectively, follow this four-step process:

  1. Understand what's required.
  2. Prioritize activities and tasks.
  3. Decide who you need to involve.
  4. Do your estimates.

Use a variety of estimating methods to get the most accurate time estimates.

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Comments (11)
  • DavidAJSmith wrote Over a month ago

    I agree we will not be able measure a task accurately. Especially when human factor is involved (and I'm not considering the unknown-unknown) It is a guessing game however we improvise as the event unfolds.

    In our company we are moving from waterfall to Agile methodology. Due to this strategic change I feel the team is facing issue in coping with the new process.

    As per the new process for a release we have planned for 5-6 sprints and each sprint is of 2 weeks. We have structure in place however the issue here is estimating the work which need to be completed within a sprint becomes a big challenge. If there is a backlog that puts tremendous pressure on the team.

    The Project Management (PM) software provides various options to schedule a task. I was seeking help to apply the above guidance to optimise the effort estimation efficiently.

  • Midgie wrote Over a month ago
    Hi David,
    Thanks for the link to help with task calculation. Effective project management and calculating time accurately is sometimes a 'guessing game' so having some sort of structure or process to use helps!
  • DavidAJSmith wrote Over a month ago

    Supporting information to my earlier question. The three task calculation types fixed-work, fixed-duration, and fixed-units which is used for scheduling the tasks. Reference URL

    - David
  • DavidAJSmith wrote Over a month ago

    This is a very good article. It explains various approach in calculating and scheduling the task. However I have a question:

    How efficiently can we use the different calculation types available in the project management software?

  • Midgie wrote Over a month ago
    HI Adee1066,
    Welcome to the Club and thanks for the suggestion. I will forward this to our editorial team to consider.

    So, what has brought you here to Mind Tools? Is there any particular skill area you wish to develop and improve further?

    If you want to post a question up in the Cafe area, you will get lots of ideas, suggestions and input from members from all around the world and from all different backgrounds and professions. We'd also value your input as well to others discussions.

    Hope to see you around and please let me know if there is anything I can help with.

  • Adee1066 wrote Over a month ago
    Critical Chain and the Theory of constraints adressess the whole issue of padding when estimating task durations in a Project schedule. I'd love to see Mind tools do an article on this.
  • Midgie wrote Over a month ago
    Hi Jack,
    Thanks for the reminder about doubling the estimate of time to complete certain tasks. I'm currently feeling quite heavy with all the things to do and I often underestimate the time allowed!

    I know, I know ... I should know better as the end result is that I start feeling overwhelmed by the sheer number of tasks that I want done by the end of the day or week. Yet, if I simply decided to allocate sufficient time, doubling the estimate of how long it may take, then I may be able to breath a sigh of relief. Some tasks can indeed wait!!

    Does anyone else experience this sense of 'overwhelm' of all the things on their list of things to do and how do you manage it?

  • Dianna wrote Over a month ago
    I tend to agree Jack!! Particularly when my husband says he'll take on a little project around the house... it'll only take 1/2 an hour he says... in that case I double it and add 10!

  • jack hobbs wrote Over a month ago
    I find that the rule of two sometimes works, depending on the project: whatever time you think it's going to take- double it!
  • James wrote Over a month ago
    Hi Everyone

    We’ve given this popular article a review, and the updated version is now at:

    Discuss the article by replying to this post!


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