What is the planning fallacy?

The tendency to underestimate the time, cost, and risk of a task — even when you know similar tasks overran — because you plan for the best case and ignore the track record.

Who discovered the planning fallacy?

Daniel Kahneman and Amos Tversky coined it in 1979; Kahneman later framed the fix as the "outside view" — basing estimates on similar past cases, not the plan in your head.

How do you avoid the planning fallacy?

Use the outside view (how long did comparable tasks really take?), add a buffer, and break the work into sub-steps where hidden effort lurks.

The planning fallacy

Estimation & judgement

The planning fallacy is the tendency to underestimate how long a task will take, how much it will cost, and how likely it is to go wrong — even when you know similar past tasks ran over. We plan for the smooth best case, not the messy real one.

Why it happens

We build estimates from an idealised, step-by-step "inside view" of the specific plan, picturing everything going right, and ignore the "outside view" — the track record of how similar projects actually went.

Examples

Home renovations and software projects routinely running double their estimate.
The Sydney Opera House: estimated at 4 years and $7M, it took 14 years and $102M.
"I’ll finish this in an hour" for a task that reliably takes three.

How to counter it

Take the outside view: base the estimate on how long similar tasks actually took, not the plan in your head.
Multiply optimistic estimates by a reality factor (often 1.5–3×).
Break the task down — hidden sub-steps are where overruns hide.

The deeper point

It survives experience because each project feels uniquely controllable — "last time had problems, but this time I’ve accounted for them." You haven’t; you’ve just moved the unknown unknowns somewhere you can’t see them yet.

Frequently asked

What is the planning fallacy?: The tendency to underestimate the time, cost, and risk of a task — even when you know similar tasks overran — because you plan for the best case and ignore the track record.
Who discovered the planning fallacy?: Daniel Kahneman and Amos Tversky coined it in 1979; Kahneman later framed the fix as the "outside view" — basing estimates on similar past cases, not the plan in your head.
How do you avoid the planning fallacy?: Use the outside view (how long did comparable tasks really take?), add a buffer, and break the work into sub-steps where hidden effort lurks.

Optimism bias Anchoring bias Margin of safety Second-order thinking

Editorial synthesis © ReadGlobe 2026, drawing on Kahneman’s Thinking, Fast and Slow, the Tversky–Kahneman research program, and the primary cognitive-science literature. · Last reviewed 2026-05-29.