The plain-English guide
How to estimate a project with uncertainty in mind
Estimate a range of durations for each task, then use the dependencies to work out a likely finish. Keep a visible buffer for work that takes longer than expected.
From the studio behind ganttchart.ai · a 6-minute read
To estimate a project is to work out how long the tasks may take and how their dependencies affect the finish date. A range makes the uncertainty visible, especially when the work is unfamiliar.
A three-point estimate records a range: the fast case, the most likely case, and a pessimistic case. Estimate each task, then total the durations along the longest chain of dependent work: the critical path, which sets the earliest finish date.
Why estimates run low
The planning fallacy
It is easy to estimate the time spent doing a task while forgetting interruptions or time spent waiting for someone else. Underestimating the total time is known as the planning fallacy, and it can persist even when you know about it.
Writing down a pessimistic case makes you consider what could take longer. Record the reasons for that case so the team can review the assumptions and decide how much contingency to allow.
Size it three ways
Show the range of possible durations
The same bakery-website launch, with its three critical tasks sized as ranges. Each band runs from the optimistic day to the pessimistic day; the bright notch is the likely case. The wider the band, the less you really know yet.
Quote the middle — 21 days — and hold the 16-day gap up to pessimistic as one buffer, not as padding hidden inside every task.
The method, in five moves
How to estimate step by step
No software required to do it right — only the discipline to size small pieces, name three numbers instead of one, and keep the uncertainty where you can see it.
- 01
Break the work into estimable tasks
Create a work breakdown before estimating. Split broad items such as "build the website" into tasks a person could finish in a day or a few days, then estimate each task.
- 02
Estimate a range for each task
For each task, record three durations: optimistic, most likely, and pessimistic. Write down the assumptions behind them, including delays or interruptions that could affect the work.
- 03
Add up the likely durations
Total the most likely estimates along each chain of dependent tasks. For a weighted estimate per task, use the PERT formula: (optimistic + 4×likely + pessimistic) ÷ 6.
- 04
Find the critical path
Find the critical path, the longest chain of dependent tasks. A delay on this path affects the finish date. Tasks off the path have slack that can absorb some delay.
- 05
Bank the uncertainty in one buffer
Use the gap between the likely and pessimistic totals to choose a visible buffer before the finish. Track how much remains as work progresses so the team can see when the deadline is at risk.
Want to see honest estimates drawn onto real plans? See the worked examples →
What the range buys you
A number you can defend
A range gives the team more information than a single target date. It shows how much the estimate could vary and gives you a starting point for deciding how much buffer to allow. Work can still take longer than the pessimistic case, so review the estimate as the project progresses.
It also changes the conversation. "Six weeks" invites an argument about whether you are padding. "Most likely six weeks, and here is the run of tasks that could push it to eight" invites a decision — which is what a plan is for. Lay that estimate on a Gantt chart and the argument gets even shorter, because the critical path shows exactly which tasks the whole date is riding on.
Re-estimate as you learn. Completed tasks provide evidence for the work still ahead, while new requirements may widen the range. Keep the assumptions and the estimate up to date.
Common questions
Estimating, briefly
How do you estimate a project, in one sentence?
Break the project into small tasks, estimate optimistic, likely, and pessimistic durations for each, then total the likely durations along the longest chain of dependent work and allow a visible buffer for delays.
Why are my project estimates always wrong?
Estimates can miss interruptions, waiting time, or work that was not identified at the start. The planning fallacy describes the tendency to underestimate how long work will take. Record a range and the assumptions behind it, then revise the estimate as you learn more.
What is a three-point estimate?
A three-point estimate records optimistic, most likely, and pessimistic durations. You can use the range directly or calculate a weighted estimate with the PERT formula: (optimistic + 4×most-likely + pessimistic) ÷ 6. The formula gives the most weight to the likely case.
Should I add a buffer to every task?
For the method in this guide, keep a shared, visible buffer before the finish. This lets the team track contingency in one place instead of trying to recover it from separate padded task estimates.
How accurate can a project estimate really be?
Early estimates have more uncertainty because the scope and technical details are less settled. Use wider ranges at the start, record your assumptions, and re-estimate when you learn more about the work.
Every planning term above is defined, jargon-free, in the plain-English glossary.
Let the chart do the adding
Describe your project in plain English and ganttchart.ai lays out the tasks in dependency order, highlights the critical path, and totals the durations for you. Review the estimates before sharing the schedule.
Draw my planOr keep reading: how to plan a project · the critical path method