You've probably heard that it takes 21 days to form a habit. It's a tidy number, it fits on a poster, and it isn't what the research says.
Where 21 days came from
The figure is usually traced back to Maxwell Maltz, a plastic surgeon who wrote in the 1960s that his patients seemed to need a minimum of about 21 days to get used to a change in their appearance. Over the decades, "a minimum of about" quietly fell off, and "getting used to a new face" became "forming any habit".
What the study found
The study most often cited on this question is by Phillippa Lally and colleagues at University College London, published in the European Journal of Social Psychology in 2010. Volunteers picked one new daily behaviour (things like drinking a glass of water after breakfast or going for a run before dinner) and reported every day for 12 weeks how automatic it felt.
- The median time to reach their personal plateau of automaticity was 66 days.
- The range was enormous: from 18 days to 254 days, depending on the person and the behaviour.
- Simple behaviours (a glass of water) became automatic faster than effortful ones (50 sit-ups).
- Missing a single day didn't meaningfully set people back. What mattered was coming back to it.
So the honest answer is: about two months for a typical habit, sometimes much less, sometimes much more, and nobody can tell you your number in advance.
What this means in practice
- Plan for months, not weeks. If you expect a habit to feel easy by day 21, day 22 feels like failure. Expecting two months or more keeps you going through the boring middle.
- Start smaller than you think. The easier the behaviour, the faster it becomes automatic. "Read one page" beats "read for an hour".
- Attach it to something you already do. Doing the habit in the same situation every day is what builds the automatic link. (More in our guide to habit stacking.)
- Don't let one miss become two. A single missed day barely matters. A missed week usually means the habit has stopped.
How to know when a habit has stuck
A habit has formed when you'd have to think about not doing it. Signs include doing it without deciding to, feeling slightly off when you skip it, and no longer needing a reminder. When that happens, keep tracking it anyway: it's a quiet record of something you built.