The planning fallacy
The planning fallacy is our persistent tendency to underestimate how long a task will take, how much it will cost, and how likely it is to go wrong — even when we’ve been burned by identical tasks before. We plan for the best case and are surprised by the ordinary one.
An impossibly over-scoped construction, already tilting and structurally doomed yet still being built, is the archetype of a grand project whose planners never reckoned with how it would actually run over and fail.
Pieter Bruegel the Elder, The Tower of Babel (1563) · Public domain
How does the planning fallacy work?
We build estimates from the inside: imagining the specific steps of this project going smoothly, while ignoring the outside view of how similar projects actually turned out. Optimism, the wish to commit, and the invisibility of unknown obstacles all pull the estimate toward a best case that rarely arrives.
Stop looking at your plan and look at how the last ten like it actually went.
How do you use the planning fallacy?
- Estimating from the outside view — how long did similar projects really take? — not the imagined smooth path.
- Adding explicit buffers for the unknown obstacles you can’t yet name.
- Distrusting your own confident timeline, especially when it assumes nothing goes wrong.
What does the planning fallacy look like in practice?
Students asked when they’d finish a thesis were beaten by reality even in their pessimistic “if everything went wrong” estimates. The classic remedy — asking how long past similar projects took — produces far better forecasts than imagining this one.
Where does the planning fallacy fail?
Padding every estimate blindly just moves the bias, and some deadlines really are met. The reliable correction isn’t generic pessimism but the outside view: anchoring on the track record of a reference class rather than the story of this particular plan.
- Reference-class data barely exists for genuinely novel projects, leaving the outside view with nothing to anchor on.
- Optimistic estimates aren't only cognitive error — they win approval and funding, so incentives keep regenerating them even after debiasing.
- It addresses time and cost, not worth — a project can overrun badly and still have been the right bet.
The counter-model: Parkinson's law — Parkinson's law is the opposing force — work expands to fill whatever time you allot — so padded estimates create their own overruns.
How do you apply the planning fallacy, step by step?
- Before committing to an estimate, find several comparable past projects — yours or others'.
- Record how long they actually took versus their initial plans.
- Set your estimate from that track record, not from this project's story.
- Adjust modestly for genuine differences, and only with stated reasons.
- Commit to the outside-view number and track actuals to improve the next estimate.
The deeper point
Its stubbornness comes from a trick of perspective: from inside a plan, every step looks manageable and the obstacles are invisible because they are unknown. The one move that reliably fixes it is to stop looking at your plan and look instead at how the last ten like it actually went.
Frequently asked
- What is the planning fallacy?
- The tendency to underestimate the time, cost, and risk of a task, even when we’ve done similar tasks before and know they usually overrun.
- Why does it happen?
- We estimate from the “inside view” — imagining this project going smoothly — and ignore the “outside view” of how similar projects actually turned out.
- How do you fix it?
- Use the outside view: base your estimate on the track record of comparable past projects, and add buffers for the obstacles you can’t yet name.
Related
Keep reading
Bias blind spot
You inspect your reasoning, find no felt bias, and conclude there is none.
The books behind better thinking
Listen to any of these free. Start a free Audible trial and get your first audiobook on the house.
Prefer to read? The canonical picks:
- Thinking, Fast and Slow — Daniel Kahneman
- The Art of Thinking Clearly — Rolf Dobelli
- The Great Mental Models, Volume 1 — Shane Parrish
- Poor Charlie’s Almanack — Charlie Munger
- Super Thinking — Gabriel Weinberg & Lauren McCann
- Seeking Wisdom — Peter Bevelin
As an Amazon Associate, ReadGlobe earns from qualifying purchases and Audible trials — at no extra cost to you.
Cite this page
ReadGlobe. (2026). The planning fallacy. https://readglobe.com/model/planning-fallacy/
"The planning fallacy." ReadGlobe, 1 July 2026, readglobe.com/model/planning-fallacy/.
Primary source: Wikipedia
Editorial synthesis © ReadGlobe 2026, drawing on the mental-models tradition (Charlie Munger, Farnam Street) and the primary sources for each model. · Last reviewed 2026-07-01.