The planning fallacy: why everything takes longer than you think

Updated: October 23, 2026 · Brizemi

The planning fallacy is the tendency to underestimate how long a task will take, even when you know similar tasks took longer before. Psychologists Daniel Kahneman and Amos Tversky named it in 1979. You can't switch it off, but you can plan around it: estimate from past experience instead of hope, break tasks into parts and leave buffers in your day.

Where the idea comes from

Kahneman and Tversky described the planning fallacy in a 1979 paper on intuitive prediction. Their point was that when we plan, we picture how this task will go, step by step, and the picture we make is usually the smooth version.

Kahneman tells a story from his own work in Thinking, Fast and Slow (2011). A textbook project his team was planning took years longer than they expected, even after a member of the team recalled that similar projects by other groups had taken far longer than planned. Knowing the pattern wasn't enough to escape it.

In 1994, Roger Buehler, Dale Griffin and Michael Ross tested the idea with students, including students writing their theses. Most took longer than they had predicted, and many took longer than even their own worst-case guess. When people explained their past delays, they tended to see them as one-off circumstances rather than as a pattern worth planning for.

Why it happens

• We plan the best case. We imagine the task going right, not the interruptions, the file we can't find or the answer we have to wait for.
• We leave out the edges. Setup, travel, switching between tasks and wrapping up rarely make it into the estimate.
• We treat past delays as exceptions. "Last time the printer jammed" feels like a fluke, not like a normal part of how things go.
• The deadline sets the guess. When something has to be done by Friday, it's tempting to estimate whatever fits before Friday.

None of this is a personal flaw. It's how human prediction works, which is why planning for it beats trying harder to guess right.

The other side: Parkinson's law

Padding every task isn't the whole answer either. In a 1955 essay, C. Northcote Parkinson observed that work expands to fill the time available for it. Give a task an open-ended afternoon and it may well take the afternoon.

So the trick has two halves: estimate each task honestly, and keep the extra time as shared buffers in the day, not hidden inside every task.

Six ways to plan for it

1. Ask "how long did it take last time?" Kahneman calls this taking the outside view: look at how similar tasks actually went, not at how this one feels.
2. Estimate in parts. Setup, the work itself, wrap-up. "Write the report" becomes 10 + 60 + 15 minutes.
3. Put buffers in the day. Leave 10 to 15 minutes after meetings and longer tasks, plus one open half hour in the afternoon. Time blocking with buffers shows how to lay this out.
4. Time a few tasks for a week. Jot down start and end times for things you do often. Your own numbers beat any rule of thumb.
5. Plan only part of your free time. A practical cap is about two-thirds. The rest absorbs what you couldn't predict. The full method is in how to make a realistic daily plan.
6. Decide in advance what moves. "If the report runs long, the inbox cleanup moves to tomorrow." Peter Gollwitzer's research on implementation intentions suggests that if-then plans like this are easier to act on in the moment.

Gut guess vs. real time

Try filling in a table like this for your own week. The gaps are often surprising, and they're useful information, not a verdict on you. (The numbers below are an illustration, not research data.)

Notice the email row. That's Parkinson's law at work: some tasks grow to fit whatever time you give them, so a firm container helps more than a bigger estimate.

A worked example

On Friday at 8:00 AM, Leah plans her day with gut estimates: the weekly report (30 min), invoices (20 min), slides for Monday (60 min) and errands (30 min), around two client calls at 10:00 and 2:00. On paper it's a light day, with plenty of slack.

Then she applies the six steps. She checks how long the report took last week, splits the invoices into setup, entry and sending, adds the call prep she always forgets to plan, and puts buffers after both calls.

Her if-then rule: the second half of the slides moves to Monday at 9:00 AM. The day looks fuller than the gut version. It's also the one that will still be standing at 4 PM.

Planning for it in Brizemi

Brizemi plans a realistic day around the fixed events in your calendar and puts breathing room between things, so overruns have somewhere to go. With Pro, it also learns how long things take you: after a few days, it sets aside time based on how your days actually go. The Today screen tells you in one line whether the day fits. And when a task runs long anyway, Rescue rebuilds the rest of the day around what's fixed; Rescue is always free.

Get Brizemi for iPhone or Android and give your estimates some breathing room.

Questions people ask

Sources

1. Kahneman, D., & Tversky, A. (1979). Intuitive prediction: Biases and corrective procedures. TIMS Studies in Management Science, 12, 313–327.
2. Buehler, R., Griffin, D., & Ross, M. (1994). Exploring the 'planning fallacy': Why people underestimate their task completion times. Journal of Personality and Social Psychology, 67(3), 366–381.
3. Kahneman, D. (2011). Thinking, Fast and Slow. Farrar, Straus and Giroux.
4. Parkinson, C. N. (1955, November 19). Parkinson's law. The Economist.
5. Gollwitzer, P. M. (1999). Implementation intentions: Strong effects of simple plans. American Psychologist, 54(7), 493–503.