/estimating
How to Estimate How Long a Task Will Take (When You Always Guess Low)
Why we underestimate how long work takes — the planning fallacy — what the classic research found, and a five-step estimating method based on your own past tasks.
November 10, 2026 · 4 min read · Clockwise Desk editors
If your to-do list never fits in the day, you are not bad at time management. You are human. Psychologists have studied our habit of underestimating how long things take for decades, and they have a name for it: the planning fallacy.
Understanding why it happens points to a surprisingly simple fix. Instead of imagining how a task will go, look at how similar tasks actually went.
What the research found
The term "planning fallacy" comes from psychologists Daniel Kahneman and Amos Tversky. One of the best-known studies of it was published in 1994 by Roger Buehler, Dale Griffin and Michael Ross in the Journal of Personality and Social Psychology, "Exploring the 'planning fallacy': Why people underestimate their task completion times".
In one of their studies, psychology students predicted when they would finish their honors theses. Their average best guess was about 34 days. The average actual completion time was about 55 days, and only about 30 percent finished by the time they had predicted.
The researchers also found that people focus on their plan for the task at hand and give little weight to how long similar tasks took in the past — even when they remember those past tasks running late.
Why we guess low
Several things push estimates down:
- We imagine the smooth version. When you picture a task, you picture yourself working on it, not the interruptions, the missing file or the second round of edits.
- We treat each task as unique. Even when we know past projects ran late, we tend to explain them away as special cases.
- We want it to be quick. Wishing a task were short is easy to confuse with believing it is short.
- Small steps are invisible. Setup, waiting for replies and cleanup rarely make it into a gut estimate.
A five-step estimating method
1. Break the task into steps
"Write the report" is hard to estimate. "Collect the numbers, outline, draft, check figures, send for review, revise" is much easier. Each step should be something you can picture finishing.
2. Find similar past tasks
This is the core of the method. Instead of asking "how long will this take?", ask "how long did the last two or three tasks like this take?" If you don't remember, look at your calendar, sent emails or file history for clues.
Researchers call this taking the "outside view": starting from what usually happens to tasks like this, rather than from your plan for this one.
3. Start from the past, then adjust
Use the past number as your starting point. Adjust only for real differences — this report is twice as long, or the data is already collected — not for optimism.
4. Add a buffer for the unexpected
Add a buffer for interruptions and surprises. How much depends on your work: a quiet solo task may need little, while work that depends on other people needs more. If you have no data yet, start with an extra 25 to 50 percent and adjust as you log real times. Our time-block planner can add the buffer automatically.
5. Log the actual time
When you finish, write down how long it really took. This turns step 2 from guesswork into data. After a few weeks you will have your own personal estimate ratio — for example, "my writing tasks take about 1.5 times my first guess."
A simple estimate log
| Task | First guess | Actual | Ratio |
|---|---|---|---|
| Monthly report | 2 h | 3 h | 1.5 |
| Client proposal | 3 h | 4.5 h | 1.5 |
| Inbox cleanup | 30 min | 45 min | 1.5 |
| Slide deck | 2 h | 2.5 h | 1.25 |
The numbers above are an example layout, not data from a study. Fill the table with your own tasks; the ratio column is where the useful pattern shows up.
When you estimate for other people
Deadlines given to a manager or client deserve extra care, because missing them costs trust.
- Give a range, such as "two to three days," instead of a single number.
- State the assumptions — "assuming I get the data by Tuesday."
- Update early when something changes rather than at the deadline.
Common estimating mistakes
- Estimating only the "doing" time and forgetting setup, waiting and review
- Treating a best-case guess as the plan
- Skipping breaks and buffers to make the plan fit
- Never checking estimates against actual results
A checklist before you commit to a deadline
- Broke the task into steps
- Looked up how long two or three similar tasks took
- Started from that number, not a gut feeling
- Added a buffer for interruptions and dependencies
- Gave a range and stated assumptions if someone else depends on it
- Set a reminder to log the actual time
Frequently asked questions
What is the planning fallacy?
It is the tendency to underestimate how long a task will take, even when we know that similar tasks took longer in the past. The term comes from Kahneman and Tversky, and it has been studied in many settings since.
How much buffer should I add?
There is no universal number. Start with 25 to 50 percent if you have no data, log your actual times, and use your own ratio once you have a few weeks of results.
Does breaking a task into steps really help?
It helps you notice steps you would otherwise forget, such as setup and review. Combine it with comparing past tasks, which addresses the main cause of underestimates.