Researchers Daniel Kahneman and Amos Tversky first named this pattern the ‘planning fallacy’ — the tendency people have to underestimate how long a task will take, even when they’ve done similar tasks before and know those took longer than expected. Psychologists Roger Buehler and colleagues then ran the key experiments that tested and confirmed it. It’s not just you. It’s basically everyone.
What makes this especially striking is that it persists even when you’re aware of it. Studies found that people kept underestimating future task times even when they could clearly remember finishing past tasks later than planned. Your brain knows the pattern and still repeats it. Researchers found a general tendency to underestimate task durations across many different types of tasks, including school assignments.
You may have seen claims about a specific percentage — like underestimating by 40-60%, or a ‘50% buffer’ fix. Those precise numbers aren’t directly confirmed in the research sources available, so treat them as rough rules of thumb, not established facts. What the research does clearly support is that underestimation is consistent and meaningful. The core idea of adding extra time to your estimates is a reasonable response to this bias, but the exact amount to add will vary by person and task.
Here’s the practical move: whatever time you think something will take, assume it’ll take longer — especially for anything unfamiliar or complex. If you think an essay will take an hour, plan for more. If you’re scheduling a project, build in breathing room before the deadline, not right at it. Starting earlier than you think you need to is the simplest fix for a bias your brain will keep making no matter how smart you are.
The bottom line: underestimating time isn’t a personal flaw — it’s a human brain flaw. Knowing that gives you power over it. Plan like your past self, not your optimistic self.