# Planning fallacy, explained

URL: https://gocutline.com/glossary/planning-fallacy/

The planning fallacy is the tendency to underestimate how long a task will take, even when you know that similar tasks took longer in the past. Daniel Kahneman and Amos Tversky named it in 1979.

## What it means
In one well-known study, students predicted how long their thesis would take; on average they took far longer than even their "worst case" estimate (Buehler, Griffin and Ross, 1994). People plan from the best-case story of the task and ignore their own track record.

## How to do it in a calendar
- Use your record, not your hope: compare planned and finished hours from past weeks.
- Add a buffer block every afternoon for overruns.
- Plan 70 to 80% of your available hours, not 100%.

## An example
Last week's record in Cutline shows 31 of 40 planned hours finished. This week, plan 34 and keep a daily 30-minute buffer.

## Common mistakes
- **Planning a perfect week.** It will not happen; plan one you can finish.
- **Ignoring the data.** The finished-hours number is the best estimate you have.

## Where it comes from
Kahneman, D., & Tversky, A. (1979). Intuitive prediction: Biases and corrective procedures. Buehler, R., Griffin, D., & Ross, M. (1994). Journal of Personality and Social Psychology, 67(3), 366–381.

## Questions

**How do I estimate better?**
Look at how long similar tasks really took (the "outside view"), and plan from that rather than from how this task feels.

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Cutline: Cutline is a weekly calendar and planner that turns time blocks into XP, levels and streaks. You plan your week in blocks, tick each one off when it is done, and see your progress on a social feed (the Floor) and a weekly leaderboard (the Arena). Planning and budgeting are free; paid plans add a badge, posting, messaging, shared calendars and lower selling fees, from $9 a month. Pro is free to try for 7 days.
Facts: https://gocutline.com/facts/ · Updated 2026-10-23