How to make precise work time estimates

Marija Kojic

Last updated on: June 9, 2022

Precise time estimates help you plan and parse your work – as a result, you manage your time and errands properly.

But, it’s relatively easy to make a mistake in the estimated time it takes you to complete a project – mostly because people are notoriously bad at guessing the time it takes them to finish something, either out of optimism they’ll finish faster, or out of the urge to give themselves more time than they really need:

  • You may estimate less time, and rush the project in the end – instead of taking a few extra hours for revisions and necessary tweaks.
  • You may estimate more time, finish earlier, and spend the remaining hours before the deadline procrastinating – instead of allocating this extra time to a different project.

Either way, poor time estimates will disrupt your schedule and compromise the quality of your efforts – as a result, this may turn the client you’re working with away from becoming a recurring one. Some clients may even mark the outcome of your project as a failure if you breach the estimated deadline – which shows how much setting precise time estimates is important in cultivating a successful business.

So, to help you become an expert in time estimation, here are some efficient estimation formulas, tips, and tricks:

Make a list of tasks you need to complete

You can’t go far without knowing what you need to do to get there – so, before making time estimates, make sure you define a list of tasks you need to complete during a time period.

The best practice is to make a straightforward list, one where you’ll lay out your errands and assignments for the day:

Call Henry B.

Meeting with the marketing team

Write the project proposal

Email Elinor about Becky’s party

Attend a business brunch with clients

Meeting with Luke S.

Fill out the timesheet

Once you have your errands and assignments laid out this way, it’ll be easier to make estimates for the time it takes to complete them.

Call Henry B. – 10 minutes

Meeting with the marketing team – 25 minutes

Write the project proposal – 4.5 hours

Email Elinor about Becky’s party – 10 minutes

Attend a business brunch with clients – 1.5 hours

Meeting with Luke S. – 25 minutes

Fill out the timesheet – 5 minutes

In the end, you can easily calculate your total estimated time for the day by adding up these estimated hours (e.g. 7 hours and 15 minutes).

Put extra effort into time estimates for your priorities

One other important aspect of proper time estimation are your priorities – by rule of thumb, you should always allocate the most of your time to priority errands and assignments. And, in order to get that right and minimize time wastage, you’ll have to put in extra effort into defining these time estimates.

Your priorities are your most important and most urgent tasks – tasks that have a specific deadline you need to fulfil, or tasks that are crucial to the goal you want to accomplish for the day.

For example, you have to write a ~1,000 word Project Proposal for tomorrow – your ability to estimate time for this task may influence whether you manage to complete it or not.

The first thing you need to do is make an outline for your proposal, and define the subheadings – they can serve as separate tasks, which you should then set separate time estimates for:

  • Estimate the number of words you’ll write for each subheading:

(e.g. “Overview – 100-150 words”, “Goals – 200-300 words”, etc.)

  • Estimate the time it takes you to write and edit each subheading, in relation to the number of words:

(e.g. “100-150 words – 30 minutes”, “200-300 words – 1 hour”, etc)

Estimate time for parsed priority task
  • Once you’re done estimating time for the subheadings, add these numbers up – this is your total estimated time for the priority task.
Total time estimate in Clockify

On an additional note, if you estimate the longest amount of time to your priority task, you’re essentially signaling yourself that you should focus all your energy on this task.

To-do list with priority task highlighted

Use the Time Estimation Formula

When looking for a formulaic approach to defining the number of days it’ll take you to finish a project, you don’t need to look further than the Time Estimation Formula, based on the The Program Evaluation and Review Technique (also known as PERT).

PERT is a method of calculating project time – it involves a time estimation formula that takes into account your:

  • Optimistic time estimate (A)
  • Most likely time estimate (B)
  • Pessimistic time estimate (C)

The optimistic time estimate (A)

This is the shortest possible amount of time you think will take you to finish a type of project – it’s also known as the best-case estimate, and it implies smooth sailing throughout the project.

The optimistic time estimate applies when everything is going perfect:

  • Everyone in your team finishes their work on time
  • No revisions necessary
  • No problems with the budget
  • No problems with the materials and equipment

For example, past experiences have taught you that the optimistic time estimate for a type of project in your team is 225 days, meaning that A = 225.

The most likely time estimate (B)

This is the realistic amount of time you think will take you to finish a type of project – it’s also known as the most likely estimate, and it implies an average voyage throughout the project.

The most likely time estimate applies when everything about the way your project is progressing is average:

  • Everything goes generally according to plan
  • Most of the people finish their tasks on time
  • There’s an average number of revisions needed

For example, past experiences have taught you that the most likely time estimate for a type of project in your team is 360 days, meaning that B = 360.

The pessimistic time estimate (C)

This is the longest possible amount of time you think will take you to finish a type of project – it’s also known as the worst-case estimate, and It implies icebergs everywhere throughout the project.

The pessimistic time estimate applies when you encounter several problems before finally completing the project:

  • Most of your team members have issues finishing before the deadline
  • A lot of revisions are needed before submitting the project for final review
  • Major problems with your materials and equipment
  • Major problems with your budget

For example, past experiences have taught you that the pessimistic time estimate for a type of project in your team is 375 days, meaning that C = 375.

The actual calculation

The Time Estimation Formula takes into account that all these types of estimates are equally possible, and the formula itself (E) goes as follows:

E = (A+4B+C)/6

If we take the listed optimistic (A = 225), most likely (B = 360), and pessimistic time estimate (C = 375), here’s how the numbers stack up:

E = (225+4*360+375)/6

E = 340

So, you’re most likely to finish said project within 340 days. But, as complications happen, you’ll need to expand your calculation – this is where standard deviation kicks in.

The standard deviation formula (SD = (C-A)/6) takes into account only the optimistic and pessimistic time estimates and tells you how accurate your total time estimate really is:

SD = (375-225)/6

SD = (150)/6

SD = 25

If the standard deviation is 25 days, that means that the project can take anywhere between 25 days more than what the time estimation formula tells you, and 25 days less – so, in the end, this type of project can take anywhere between 315 and 365 days.

Use a project duration calculator

A project duration calculator is based on the previously talked about Time Estimation and Standard Deviation formulas.

You’ll still get the rough estimated number of days you need in order to finish the project – but, you’ll get it faster, because the calculation will be handled automatically.

Project Duration Calculator

Here’s how you can create a simple project duration calculator in Excel, by using the Time Estimation Formula (E = (A+4B+C)/6) and the Standard Deviation Formula (SD = (C-A)/6):

  1. For the “Time estimation formula” cell, write: = ROUNDUP((C4+C5*4+C6)/6)
  2. For the “Standard Deviation” cell, write: = ROUNDUP((C6-C4)/6)
  3. For the “Shortest estimated project duration” cell, write: = ROUNDUP(C8-C9)
  4. For the “Longest estimated project duration” cell, write: = ROUNDUP(C8+C9)

Once you enter your data into the “Optimistic time estimate”, “Most likely time estimate”, and “Pessimistic time estimate” cells, the data showing the estimated time, standard deviation, and the shortest and longest estimated duration will be calculated automatically.

Using the ROUNDUP function is optional, but highly recommended because you’ll round up the estimated number of days to a whole number.

In order to use the same formulas for multiple projects, simply hold and drag your cursor at the edge of the cells containing the formulas:

Project Duration Calculator in action

So, if you enter “145”, “234”, “345”, you’ll get that your estimated time to finish the project is 238 days, with a standard deviation of 34 days – meaning you’ll likely take between 204 and 272 days to finish this type of project.

💡Clockify pro tip
Looking for an easy way to calculate your work hours in Excel? Then check out our guide on how to create functional timesheets in Excel.

Track your time

A project duration calculator tells you how many days you’ll likely need to finish a project, which is useful for giving a rough estimate to the client.

But, you’ll also need a more precise estimate, in order to plan and parse your work in detail, as well as create a functional schedule to follow while working on said project – this is where a time tracker becomes crucial.

A time tracker makes estimating time into an exact science – it tells you how much time you’ve spent on previous projects in total, in hours, minutes, and seconds, and it lets you identify the tasks that took most of your time. It also tells you what type of work you’ve spent most of your time on – for example, “front-end” or “back-end” tasks.

List of time entries in Clockify

By keeping such time tracking records, you’ll be able to make time estimates for a type of project that are based on your completely verifiable past experiences with the same type of project.

To gather your time tracking data, start a timer each time you start working on a task, and associate it with the right project – once you’re done working on this task, stop the timer. By doing so, you’ll be adding this time to your total time for that project.

Adding time to total project time

By the end of the project, you’ll have the exact time you’ve spent on it now, and are likely to spend on the same type of projects in the future – so, you’ll be able to give a precise estimate to your client.

In addition, you’ll be able to distinguish between billable and non-billable time, as well as understand how much money you’ve earned working on the project.

You can even turn hours into days and see how accurate your project duration calculator really is – for example, if you’ve spent 283:39 hours on a project, that translates to 35.5 days you have spent working on this project for 8 hours a day.

Total tracked time in Clockify

But, this isn’t the only way to manage and define time estimates within a time tracker.

Clockify also offers an estimation system where you’ll be able to define estimates for tasks and the project overall, before comparing your real time tracking results with these estimates.

Time estimates per task in Clockify

So, once you start tracking time, you’ll be able to compare your real-time progress with your estimates as you work. You’ll also be able to see the percentage of completed work compared to your estimates at all times.

Estimated time vs Real time in Clockify

This way, you’ll not only be able to see that your original time estimates were off, you’ll also be able to see by how much.

Calculate the time you’ll waste

When making time estimates, it’s easy to overlook the time you’re bound to waste while officially working on said project – so, don’t forget to account for interruptions, distractions, and idle procrastination.

First, list the activities you likely waste time every day at work – then, assign them with a time estimate:

Going out for a smoke every 2 hours – 25 minutes

Going out to the nearby bakery to buy scones for an afternoon snack – 15 minutes

Going to fix a cup of coffee every 3 hours – 15 minutes

Chatting with colleagues – 45 minutes

You can even add the time you spend on meetings and sorting out your email inbox to this wasted time – after all, these activities don’t contribute to finishing the project and only take you away from real priorities:

Meetings – 1 hour and 20 minutes

Inbox management – 1 hour

You can make time estimations at the top of your head for these activities, or you can make a weekly experiment and track the exact time it takes you to finish these activities per day – for example, if you track that meetings take you:

  • 40 minutes on Monday
  • 45 minutes on Tuesday
  • 30 minutes on Wednesday
  • 45 minutes on Thursday
  • 40 minutes on Friday

You can take 40 minutes as your average time for meeting in future estimates:


In the end, to get the total wasted time per day, just add up the numbers – for example, all the listed time wasters, from coffee breaks to meetings, add up to 4 hours.

So, if you work 8 hours per day, and waste 4 hours, you essentially spend only about 4 hours focused on the project. This is a time-per-day which translates the 283:39 hours you’ve previously tracked for work to 71 days for the time estimate, instead of the 35.5 you’d spend if you were to put the remaining 4 hours to better use.

Schedule additional time, just in case

The data you get from your time tracker shows the exact time you’ve spent on similar projects before, and how much you’re likely to spend on it next time.

So, in 9 out of 10 cases, if you’ve spent 71 days on a project (283:39 hours for 4 hours/day), you’ll likely spend a near similar amount of time on it next time.

But, just in case, allocate some additional time to this project, for unexpected problems and shortcomings that usually come with the 10% of such projects. You never know when this percentage might increase, and when you’ll need additional days and hours to round up the project to the complete satisfaction of your client.


In the end, making precise time estimates isn’t that difficult – you’ll just need to:

  • Parse your work into daily tasks and make estimates for these separate tasks
  • Put extra energy and concentration into correctly estimating time for priority tasks
  • Track the time you spend on projects and tasks
  • Account for the time you waste everyday
  • Account for additional time you may need in order to finish the project

For extra flair, you can even use PERT’s Time Estimation Formula and rely on a simple Project Duration Calculator you’ll make yourself to calculate daily estimates. As a result of following these tips, you’ll make your time estimates precise, make planning your schedules easier, and make sure your clients view you as reliable professionals who keep to their word.

Author: MarijaKojic

Marija Kojic is a productivity writer who's always researching about various productivity techniques and time management tips in order to find the best ones to write about. She can often be found testing and writing about apps meant to enhance the workflow of freelancers, remote workers, and regular employees. Appeared in G2 Crowd Learning Hub, The Good Men Project, and Pick the Brain, among other places.

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