How Long Habits Take to Form: What the Research Really Found

Ask around and you will hear that a new habit takes 21 days. It is a tidy number, easy to remember and easy to put on a poster. It is also not what the research found.

The most widely cited real-world study of habit formation found something messier and more useful. Habits strengthen along a curve rather than switching on at a deadline, the time involved varies enormously from person to person and behavior to behavior, and one missed day does not send you back to the start.

This guide walks through that study: what it measured, what it found, what it cannot tell you, and how to turn it into a realistic plan for your own habits.

Where the 21-day idea came from

In a 2012 article in the British Journal of General Practice, psychologists Benjamin Gardner, Phillippa Lally and Jane Wardle note that some people have heard that habits take 21 days to form. They write that this myth appears to have originated from anecdotal evidence about patients who had received plastic surgery and typically adjusted psychologically to their new appearance within 21 days.

Getting used to a change in your appearance is a different thing from making a behavior automatic. The 21-day figure was never a measurement of habit formation, which is why it so often disappoints the people who rely on it. When day 22 arrives and the new habit still takes effort, it is easy to conclude that you have failed, when in fact you may be right on schedule.

What the Lally study actually did

The study behind most modern habit timelines was published in the European Journal of Social Psychology in 2010 by Phillippa Lally and colleagues van Jaarsveld, Potts and Wardle.

The researchers asked 96 volunteers to choose one eating, drinking or activity behavior and carry it out daily in the same context, for example "after breakfast," for 12 weeks. Each day, participants recorded whether they had done the behavior and completed the Self-Report Habit Index, a short questionnaire. The researchers used a subset of its items to track how automatic the behavior felt.

Of the 96 volunteers, 82 provided enough data for analysis. The team fitted a curve to each person's automaticity scores over the 84 days of the study. The model fitted for 62 people, and 39 of them showed a good fit. Performing the behavior more consistently was associated with a better fit.

What it found

• Automaticity rose along an asymptotic curve. Gains came quickly at first, then slowed as each person approached their own plateau.
• The time participants took to reach 95 percent of that plateau ranged from 18 to 254 days. The authors describe this as considerable variation and say it highlights that habit formation "can take a very long time."
• The widely quoted 66-day figure is the study's median time to reach that 95 percent level: the middle value across participants, not an average or a deadline. A 2024 systematic review by Singh and colleagues reports Lally's result this way, as a median of 66 days with a range of 18 to 254 days. Gardner, Lally and Wardle's 2012 summary describes the same figure more loosely as an average.
• Missing one opportunity to perform the behavior did not materially affect the habit formation process.
• The 2012 summary also reports that automaticity peaked more quickly for simple actions, such as drinking water, than for more elaborate routines, such as doing 50 sit-ups.

One detail is worth noticing. The study ran for 84 days, so any estimate longer than that, including the 254-day upper end, comes from projecting each person's fitted curve forward rather than from watching anyone for eight months.

What the study cannot tell you

The Lally study is valuable precisely because it tracked ordinary people doing ordinary behaviors in daily life. That design also sets its limits:

• It is one study, with a modest number of volunteers who chose their own fairly simple behaviors. It does not tell you how long it takes to master a complex skill or to stop doing something.
• Automaticity was self-reported. Participants rated how automatic the behavior felt rather than being measured by an outside observer.
• The 66-day figure is the median of a wide spread. Treating it as a new deadline repeats the mistake of the 21-day rule with a bigger number.
• Fewer than half of the original volunteers produced a good curve fit, so the tidy curve describes some people's experience better than others'.

None of this weakens the core lesson. If anything, it strengthens it: plan for variation instead of promising yourself a date.

Plan for a curve, not a deadline

Gardner and colleagues suggest it may be helpful to expect habit formation based on daily repetition to take around 10 weeks. They also describe the advice behind habit formation as simple: repeat an action consistently in the same context. That fits a broader point from Wendy Wood and Dennis Rünger's 2016 review in the Annual Review of Psychology: habits form as people repeat the same responses in a given context. Here is one way to put that into practice:

1. Choose one simple behavior. Simpler actions became automatic faster in the Lally data, so start smaller than feels impressive.
2. Tie it to one daily cue you meet every day, such as after breakfast, when you get to work or when you sit down on the train.
3. Set a review point instead of a finish line. Ten weeks is a reasonable first check-in, not a test you pass or fail.
4. Expect the early weeks to take the most effort. The curve says the work gets easier over time, but it does not say when that happens for you.
5. Treat a missed day as a missed day. One miss did not materially affect habit formation in the study, so pick up again at the next cue.
6. If progress stalls, simplify. Make the behavior smaller or the cue clearer before deciding the habit is not for you.

For example, imagine someone who wants to stretch every day. Instead of a 20-minute session at a different time each day, they choose two minutes of stretching right after they brush their teeth at night. By week three it still takes a conscious reminder. By week eight they notice they are reaching for the floor before they have thought about it. Their timeline is theirs alone; someone else's might be faster or much slower.

How to tell when a habit has formed

Gardner and colleagues describe three phases. In the initiation phase you choose the behavior and the context. In the learning phase you repeat the behavior in that context so the link between cue and action strengthens. In the stability phase the habit has formed and its strength has plateaued, so it persists with minimal effort or deliberation.

The practical signal is not a date on the calendar but how the behavior feels. The patient tool in their article suggests rating, at the end of each week, how automatic the behavior feels on a scale from 1 (not at all) to 10 (completely). When your weekly ratings stop climbing and stay high, you are probably near your own plateau. For a fuller approach to recording this, see our guide Habit Tracking That Helps: Record Progress Without the Nagging.

Key takeaways

• The 21-day rule appears to come from anecdotes about plastic surgery patients, not from habit research.
• In the Lally study, the time to reach near-peak automaticity ranged from 18 to 254 days; the often-quoted 66 days is the median, not a rule.
• Simple behaviors tied to one consistent daily cue are the most realistic starting point.
• Missing one opportunity did not materially affect habit formation, so a single slip is not a reset.
• Judge progress by how automatic the behavior feels, reviewed weekly, rather than by a fixed deadline.

Sources

• Lally, van Jaarsveld, Potts and Wardle, How are habits formed: Modelling habit formation in the real world, European Journal of Social Psychology (October 2010)
• Gardner, Lally and Wardle, Making health habitual: the psychology of 'habit-formation' and general practice, British Journal of General Practice (December 2012)
• Wood and Rünger, Psychology of Habit, Annual Review of Psychology (2016)
• Singh, Murphy, Maher and Smith, Time to Form a Habit: A Systematic Review and Meta-Analysis of Health Behaviour Habit Formation and Its Determinants, Healthcare (December 2024)