How long to form a habit: 21 days, 66 days, or longer
Where the 21-day rule came from, what Lally's 2010 study measured instead, what a 2024 review added, and why one missed day does not reset a habit.
The short answer from the research is about two months at the median, with a very wide spread between people and between habits. The 21-day figure that most people have heard is older than any study of habits and was never measured.
Where the 21 days came from
The number traces back to Maxwell Maltz, a plastic surgeon, and his 1960 book Psycho-Cybernetics. In the preface he describes patients taking about three weeks to get used to a new face, and amputees feeling a phantom limb for about 21 days. From that he concludes that "it requires a minimum of about 21 days for an old mental image to dissolve and a new one to jell" (Maltz, 1960).
Maltz wrote about self-image, from what he saw in his practice, and he said "a minimum". Over the following decades the phrase was retold as "it takes 21 days to form a habit", with the minimum dropped.
What Lally measured
In 2010 Phillippa Lally and colleagues at University College London ran the study that is usually cited on this question (Lally et al., 2010, European Journal of Social Psychology). The design:
- 96 volunteers each chose one eating, drinking or activity behavior and did it daily in the same context, for example "after breakfast", for 12 weeks.
- Every day they rated how automatic the behavior felt, using a standard questionnaire (the Self-Report Habit Index).
- 82 people gave enough data to analyse. A growth curve fitted 62 of them, 39 of them well.
The time it took people to reach 95% of their personal plateau of automaticity ranged from 18 to 254 days. The median was 66 days, the figure most often quoted from the study; Lally has stressed the spread as much as the median (University of Surrey, 2026).
The study has two limits. It ran for 84 days, so the longer times are projections of each person's curve beyond the end of the study. And the median comes from the smaller group whose data fitted the model well.
What a 2024 review added
A systematic review and meta-analysis by Singh and colleagues pooled 20 studies with 2,601 participants (Singh et al., 2024, Healthcare). The median time to habit formation across the studies that reported it was 59 to 66 days, the mean 106 to 154 days, and individual times ran from 4 to 335 days.
The mean is higher than the median because a minority of people take much longer, which pulls the average up. For a single person, a fair expectation is two months or more, depending on the habit.
The 21/90 rule
The "21/90 rule" says 21 days build a habit and 90 days turn it into a lifestyle. It circulates in self-help writing with no known author and no study behind it. Neither number comes from habit research: 21 is Maltz's minimum with the word "minimum" removed, and 90 is a round number.
Why one missed day does not reset the habit
The most useful line in Lally's paper is short: "Missing one opportunity to perform the behaviour did not materially affect the habit formation process" (Lally et al., 2010). The curve of automaticity kept rising after a single miss.
The same study found that people who performed the behavior more consistently had curves that fitted the model better. Consistency over weeks mattered; a single gap did little harm.
A counter that drops to zero after one miss tells you that day 41 starts again from day 1, which the data does not support. Feeling that the whole run is lost after one slip can lead people to drop the habit; how streaks work covers that pattern.
What this means in practice
- Plan for months. A 30-day challenge is a good start and seldom the finish.
- Keep the cue fixed. Lally's participants tied the behavior to a moment in their day, and that link is what repetition strengthens.
- Expect the early days to move fastest. In the study, the biggest gains in automaticity came from the first repetitions, then the curve flattened.
- After a miss, do it the next day. In Lally's data, a single miss did not set the curve back.
Where Break3 fits
Break3 counts a streak for each habit and forgives one missed day automatically: the streak stays alive if your last check-in was yesterday or the day before. That rule follows Lally's finding about single misses. Milestones fall on days 3, 7, 14, 21, 30, 60, 90, 180 and 365, so day 21 is one step along the way, and day 66 falls between the 60 and 90 marks.
See also: habit trackers compared and the FAQ.