"It takes 21 days to build a habit" is repeated so often that it feels like science. It is not. The best direct evidence suggests the real number is a lot longer, and a lot more variable, which is good news for anyone who has ever felt like a failure on day 22.
Where the 21-day figure comes from
The number is usually traced to a 1960 self-help book by a plastic surgeon, who noticed that patients seemed to take about three weeks to get used to a new face. It was an observation, not an experiment on habits, and it has been stretched far beyond what it said.
What a real study found
In a well-known study at University College London, 96 volunteers each picked one new, simple behavior, such as eating a piece of fruit with lunch or running for fifteen minutes before dinner, and did it daily for 12 weeks. Every day they rated how automatic the behavior felt.1
- On average, the feeling of automaticity rose quickly at first, then leveled off.
- The median time to reach a plateau was 66 days.
- The range was huge: from 18 days to 254.
- Missing a single day did not noticeably damage the process.
- Harder behaviors, such as exercise, took longer than easy ones.
It is one study, with volunteers and self-reported ratings, so treat the numbers as a guide. The shape is the important thing: habits are not switched on at a fixed date. They accumulate, at different speeds for different people and different behaviors.
What a habit actually is
Psychologists define a habit as an association between a context and a response, learned by repetition, so that the context triggers the behavior with little deliberate thought.2 The key phrase is "context triggers". A habit is less about motivation than about a reliable cue. That is why the same person can floss every night at home and never on a trip.
If you want to see what happens when the cue is the problem and not the knowledge, read Why Insight Doesn't Change Behavior.
Five moves that fit the evidence
- Anchor it to something you already do. "After I pour my morning coffee" is a stronger cue than "in the morning".
- Start smaller than feels worthwhile. Two minutes of study is a habit you can repeat. Two hours is a plan you will abandon.
- Write an if-then plan. "If it is 7 pm and I am home, then I open my notes" turns a vague wish into a specific trigger. A large review found these plans have a medium-to-large effect on goal achievement.3
- Remove friction. Put the book on the pillow, the running shoes by the door, the app off the home screen.
- Plan for the missed day. One miss is not the end. What matters is how soon you resume.
The honest timeline
Expect to be deliberate for a couple of months, longer for something demanding. You will probably feel the behavior getting easier before it ever feels automatic. That in-between stretch is where most people quit, and it is exactly when the work is being done.
References
- Lally P, van Jaarsveld CHM, Potts HWW, Wardle J. How are habits formed: modelling habit formation in the real world. Eur J Soc Psychol. 2010;40(6):998–1009. Find it ↗
- Wood W, Rünger D. Psychology of habit. Annu Rev Psychol. 2016;67:289–314. Find it ↗
- Gollwitzer PM, Sheeran P. Implementation intentions and goal achievement: a meta-analysis of effects and processes. Adv Exp Soc Psychol. 2006;38:69–119. Find it ↗