Productivity
How long does it take to form a habit?
Short answer
About 66 days on average in the best-known study, with individuals ranging from 18 to 254 days. A 2024 review of the research found medians of 59 to 66 days. The 21-day figure comes from a 1960 self-help book, and in the research, missing one day didn't derail the process.
Start a new daily habit on January 1, 2027, and the best-known study of habit formation puts the point where it levels off around March 7. That’s day 66. The same study had volunteers who got there by mid-January and others whose curves pointed to September, so March 7 is a midpoint, and your own date could land weeks or months either side of it.
What the 66-day study measured
Phillippa Lally and colleagues at University College London ran the study the figure comes from. It was published online in 2009 and in print in 2010. They asked 96 volunteers to choose one eating, drinking or activity behavior and do it every day in the same context, such as after breakfast, for 12 weeks. Each day the volunteers logged whether they’d done it and rated how automatic it felt on a questionnaire called the Self-Report Habit Index.
Automaticity rose quickly in the first weeks and then flattened out. In a 2012 paper for family doctors, Benjamin Gardner, Lally and Jane Wardle put the plateau at an average of 66 days, with wide variation between people and between behaviors. The time each person took to reach 95% of their own plateau ran from 18 to 254 days.
What the average leaves out
Three details limit how far the 66 days can be pushed. The study lasted 84 days, so every figure past day 84, the 254 included, is a curve extended beyond the data; UCL’s own blog post about the study describes the 254 as projected. Not everyone’s data fitted the curve either: of the 82 people who gave enough data, the model fitted 62, and fitted well for 39. People who did their behavior more consistently got the better fits.
“Automatic” was also the volunteers’ own rating of how little thought the behavior took. And the behaviors differed in size. Gardner, Lally and Wardle note that simple actions (drinking water, say) reached their peak automaticity faster than elaborate routines (50 sit-ups), so the average blends easy habits with harder ones.
Missing a day
One finding gets less attention than the 66 days. Missing a single opportunity to do the behavior “did not materially affect the habit formation process,” in the study’s words, and the 2012 paper adds that gains soon resumed after one missed day.
Scroll’s streak counter is stricter than the research: it resets if you skip a day (your XP and achievements stay). By Lally’s data, one missed day barely sets the habit itself back.
What a 2024 review of 20 studies found
Singh and colleagues’ 2024 review pulled together 20 studies of health habits with 2,601 participants, whose average ages ran from 21.5 to 73.5. Only four of the studies timed habit formation. Their medians were 59 to 66 days, their means 106 to 154 days, and individuals ranged from 4 to 335 days.
Medians and means aren’t directly comparable: a few people who take most of a year pull a mean up, while a median stays with the middle person. In the healthy-eating study that produced the 59-day median and the 335-day maximum, only 23% of participants reached the researchers’ threshold for a formed habit at all.
The review rated 11 of the 20 studies at high risk of bias, and its timing figures rest on four studies of health behaviors. Within those limits, two patterns showed up. Habits people chose for themselves grew stronger than ones they were given, and in a study of daily stretching, the morning group took a mean of 106 days against 154 for the evening group.
Where the 21-day figure came from
The 21-day rule traces to Maxwell Maltz, a plastic surgeon turned psychologist. It comes from the preface to his 1960 self-help book Psycho-Cybernetics. Maltz noticed that patients took about 21 days to get used to a new face after surgery, and that amputees sensed a phantom limb for about as long. He wrote that it usually takes “a minimum of about 21 days” for a change in a person’s mental image to become perceptible.
Two things fell away as the line spread: the word “minimum,” and the subject. Maltz was describing people getting used to a change in themselves. In a 2012 post on UCL’s blog, Gardner suggested that getting used to something (habituation) may have been confused with habit formation somewhere along the way. The 2012 paper for doctors says the figure appears to have come from anecdotes about plastic surgery patients.
Twenty-one days does describe a few people. The fastest volunteer in Lally’s study took 18. For the typical volunteer, 21 days was about a third of the time needed.
What helps a habit form
The studies above point to a small set of conditions. None of them comes with a guaranteed date.
A cue you meet every day
Gardner, Lally and Wardle define habits as actions triggered automatically by cues in your surroundings that you’ve come to associate with them. That’s why Lally’s volunteers tied their behavior to a context such as “after breakfast.” The 2012 paper’s advice for patients follows from it: choose a time and place you encounter every day, and do the action each time you meet it.
Plans of this kind have their own evidence. Peter Gollwitzer and Paschal Sheeran’s 2006 meta-analysis pooled 94 tests of if-then plans, which psychologists call implementation intentions (an example of ours: “If I’ve poured my coffee, then I stretch for two minutes”). People who made such plans reached their goals more often, by an effect the authors describe as medium to large (d = 0.65).
Put another way, the average person with an if-then plan did better than about 74% of people without one. The outcome there was reaching a goal; how automatic the behavior felt wasn’t part of it.
A small version of the behavior
The 2012 paper advises choosing a simple action, toward your goal, that you can do daily. If the goal is running three times a week, the daily version might be putting your running shoes by the door after breakfast (that example is ours).
Singh’s review points the same way. For physical activity, it reports that a preparatory habit (getting exercise clothes ready each day) and enjoying the activity both helped explain who formed the habit.
Habit stacking
James Clear describes habit stacking in his 2018 book Atomic Habits: after an existing habit, you do the new one. The existing habit is the cue. The lesson below gives Clear’s formula as it appears in the Scroll app. His book is popular writing, and trials of cues like his are few.
Habit Stacking
James Clear's habit stacking formula: after I [current habit], I will [new habit]. Existing habits are neurologically strong; attaching new behaviours to them leverages existing neural pathways. After I pour my morning coffee, I will write 3 gratitudes. After I sit at my desk, I will write my top 3 priorities. After I close my laptop, I will plan tomorrow. Habit stacks build complex routines from reliable anchors, removing the need for willpower or reminders.
Source Clear, J. (2018). Atomic Habits.
Quick quiz
What is the correct format for James Clear's habit stacking formula?
The closest test we know of compared two kinds of cue. Keller and colleagues gave one group a routine-based cue (in the review’s example, a glass of water after breakfast each morning) and another a time-based one (a glass of water at 9:00 a.m.). Singh’s review reports no significant difference in how well the habits formed. Tying a new habit to an old one worked about as well as tying it to a clock time in that trial, and it hasn’t been shown to work better.
A stack that uses Scroll would read: after I pour my coffee, I read one lesson. That’s 32 to 50 seconds of reading (median 44). If the habit you’d rather change is opening a feed without deciding to, the guide on how to stop doomscrolling covers that side.
Working out your own dates
The table turns the study’s numbers into dates, counting January 1, 2027 as day 1.
| Day | Date, starting January 1, 2027 | What it marks in Lally’s study |
|---|---|---|
| 18 | January 18 | The fastest volunteer |
| 66 | March 7 | The average plateau |
| 84 | March 25 | The last day of the study |
| 254 | September 11 | The slowest volunteer, projected |
The habit formation calculator does the same arithmetic for any start date. Treat the dates as a range to expect, with the middle one as a rough guide; nobody can tell you in advance which end of the range your habit will land on.
Scroll lessons are short introductions for general interest. They aren’t medical, psychological or financial advice, and we haven’t tested whether reading them changes anyone’s habits, memory or focus.
Drafted with AI assistance and checked against the sources listed on October 17, 2026.