The Planning Fallacy: Why “Five Minutes” Is Never Five Minutes

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

The thesis study

In 1994, Roger Buehler, Dale Griffin and Michael Ross asked psychology students when they expected to finish their honours theses. On average, they predicted about 34 days. The average actual time was about 56 days. Only around 30% finished by the date they predicted.

Even when asked for a “worst case” estimate, many students still finished later than that.

Why it happens

• We picture the plan, not the past. When estimating, we imagine the steps going smoothly and ignore interruptions, illness and the time we lost last time.
• We think this time is different. Past delays feel like special cases.
• We want it to be quick. A short estimate feels better and is easier to commit to.

How to estimate better

1. Use the outside view. Ask “How long did this kind of task take last time, or for other people?” before you ask “How long will this take me?”
2. Break it down. Estimating smaller steps tends to surface work you forgot.
3. Multiply. If you usually run over, apply a multiplier. Many people find 1.5 to 2 times their first guess is closer to reality.
4. Book buffer time. Put the realistic estimate in your calendar, not the hopeful one.

For students

Exams are where the planning fallacy hits hardest, because the deadline is fixed. Work out how many hours you need, then subtract sleep, meals and classes from the time you have. The Can I Cram This? calculator does that maths for you. Spacing study over several days also improves long-term memory compared with one long session.

For any other task, try the Planning Fallacy Calculator. Enter your estimate and your track record, and it gives you a more honest number.