The planning fallacy: why everything takes longer than you think

Daniel Gavalec · Updated October 16, 2026

The planning fallacy is our tendency to underestimate how much time, money and effort a task will take – even when we know that similar tasks took longer in the past. Psychologists Daniel Kahneman and Amos Tversky named it in 1979. Below: what the research actually found, why even experienced people fall for it, and six techniques that bring your estimates closer to reality.

What is the planning fallacy?

The planning fallacy is one of the best-documented errors in how we think about the future. When we estimate how long something will take, we picture how it should go – and that picture almost never includes the flu, a surprise question from a client, a mistake in the brief or a day when nothing clicks.

Kahneman and Tversky noticed that people get it wrong even when they know similar projects took longer. Kahneman later told his own story: the team writing a curriculum textbook with him expected to finish in about two years. When he asked a colleague how long comparable projects had taken other teams, the answer was seven to ten years – and roughly 40% never finished at all. The book took eight years.

It isn’t only about time. Big construction and public projects routinely blow their budgets and deadlines. The Sydney Opera House was originally expected to cost about AU$7 million and open in 1963; it opened in 1973 at a cost of more than AU$100 million.

What the research found

The most cited study was published in 1994 by Roger Buehler, Dale Griffin and Michael Ross. They asked students when they expected to finish their senior thesis:

Only about 30% of students finished by their own estimate – and even their worst-case scenario was more optimistic than what happened. The same pattern showed up in later experiments with homework, tax returns and holiday shopping.

What the students thought about while estimating is telling: most focused on how they would do the work, and only a few recalled their past experience – usually dismissing it with “this time is different”.

Why we keep falling for it

• The inside view. We plan from the scenario “how I’ll do it”, not from the statistic “how long this usually takes”. The scenario is always cleaner than reality.
• Past delays as exceptions. Last time’s delay gets explained away (“I was ill”, “that client was difficult”), so we learn nothing from it.
• Wishful estimates. The more we want something done fast, the more optimistic the estimate. Motivation is great for working and bad for planning.
• The task as one lump. “Write the proposal” looks like an hour. Split it into “gather requirements, price it, write it, send for review, address feedback” and the estimate grows by itself.

6 ways to estimate realistically

1. Measure real time. The only reliable data about you is your own. Run a timer on your tasks, and after a few weeks you will see how far off you usually are – the time tracking app page shows how.
2. Take the outside view. Before estimating, ask: “How long did the last three similar tasks take?” This is reference class forecasting, and Kahneman recommends it.
3. Break the task into steps. Kruger and Evans (2004) found that when people unpack a task into its parts, their estimates get longer and closer to reality – especially for complex tasks.
4. Add a buffer based on your error. If your tasks usually take 50% longer, multiply estimates by 1.5. For anything you do for the first time, plan for double.
5. Plan days with free space. Leave about a quarter of the day unplanned – a broken plan costs more time than a plan with slack. See time blocking.
6. Move your own deadline forward. Promise clients a date based on the realistic estimate plus buffer, and give yourself an earlier one. A delay then eats your buffer, not your reputation.

The planning fallacy for freelancers

When you work for yourself, the planning fallacy costs money. If you quote from an optimistic estimate, every extra hour lowers your real hourly rate. A few rules from practice:

• Quote from measured hours, not gut feeling. After three similar jobs you know what they really take.
• Count the feedback rounds. Communication and revisions are often a third of the work, and they are almost always missing from the estimate.
• Watch your effective rate: billed amount divided by hours worked – the hourly rate calculator walks through it.

The planning fallacy in CORTEX

In CORTEX (free for everyone) an estimate is a normal part of every task – and the app helps you compare it with reality:

• Focus measures real time. Start it on a task and the time entry is created for you. Only measured time counts; an estimate is never logged as time worked.
• A planning-fallacy nudge. When several tasks take more than 50% longer than you expected, CORTEX tells you once, briefly, and suggests adding slack to your estimates.
• Plan my day with a buffer. The day planner fits tasks into the room you have today and keeps part of it as a buffer. What doesn’t fit is listed as “these can wait” instead of silently overflowing.

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