Planning Fallacy Calculator
Enter your estimate and a few facts about the task. The calculator applies multipliers for your track record, novelty and dependencies.
How the calculator works
- Your track record sets the base multiplier (×1.2 to ×2.4).
- New tasks add up to 40%, and depending on others adds 25%.
- The result is a realistic estimate in hours and workdays.
Frequently asked questions
What is the planning fallacy?
The tendency to underestimate how long a task will take, even when you know similar tasks ran over in the past. Kahneman and Tversky described it in 1979.
Why do we underestimate?
We imagine the best-case path and ignore interruptions, mistakes and past experience.
How do I estimate better?
Look at how long similar tasks really took, break work into steps and add a buffer.
Read the research
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