How Long Does It Take to Form a Habit? (66 Days, Explained)

A lot of answers to this question repeat a single number. This guide goes back to the papers instead: the 2010 study that the "66 days" figure comes from, and a 2024 review that pulled together everything published since. They give a more useful answer than a single number, because they also show what makes a habit take longer, and what happens when you miss a day.

The short version5 points

In the best-known study, it took a median of 66 days for a new daily behaviour to reach its peak of automaticity, with a range from 18 to 254 days.
A 2024 review of 20 studies found medians of 59 to 66 days and means of 106 to 154 days, with individuals ranging from 4 to 335 days.
There's no good evidence for the 21-day rule. It's usually traced to Psycho-Cybernetics, a 1960 self-help book, and its author's observations of plastic surgery patients.
Missing one day didn't materially affect habit formation in the 2010 study, but lots of missed days did add up, which is why "never miss twice" is a sensible rule.
Habits form faster when you repeat the same small action after the same cue, ideally one you picked yourself.

01How long does it take to form a habit?

It took a median of 66 days, with a range of 18 to 254 days, in the best-known study (Lally and colleagues, 2010) [1] [2]. A 2024 review found medians of 59 to 66 days, means of 106 to 154 days, and individual times from 4 to 335 days [2]. For a simple daily habit, expect about two months, and often longer.

The spread is the important part. The average hides people who got there in under three weeks and people who were still going after eight months. The review's authors concluded that people should expect "at least two to five months" for a new health habit to become automatic [2]. How long yours takes depends mostly on what the habit is, how consistently you do it, and whether it's tied to a cue you meet every day.

02Where 66 days comes from

The 66-day figure comes from a study at University College London published in 2010. It followed 96 volunteers who each chose an eating, drinking or activity behaviour and did it once a day in the same context, such as after breakfast, for 12 weeks [1]. Each day they reported whether they'd done it and how automatic it felt.

Examples people chose included eating a piece of fruit with lunch, a 15-minute run before dinner and 50 sit-ups after morning coffee [4]. The researchers measured automaticity with items from the Self-Report Habit Index, which asks how far you agree with statements like "I do it automatically" and "I do it without thinking" [4].

For most people the curve rose quickly at first and then flattened. Early repetitions added the most automaticity, and later ones added less and less until it levelled off [2] [4]. The researchers fitted a curve to each person's scores and measured how long it took to reach 95% of their personal plateau. Across the people whose data fitted, the median was 66 days and the range was 18 to 254 days [1] [2].

What the study can and can't tell you

It's a careful study, but it has limits that matter for how you use the number.

The sample was small. Of 96 volunteers, 82 gave enough data, the model fitted for 62, and only 39 showed a good fit [1].
Automaticity was self-reported, so it measures how automatic the behaviour felt, which is the researchers' own acknowledged weakness [4].
The study lasted 84 days, so the longest times, including the 254, are projections of where a still-rising curve would level off [4].
The behaviours were simple and chosen by the participants, like drinking water or eating fruit, so the results say less about complex habits.

Complexity made a clear difference within the study. Simple actions such as drinking water reached their peak sooner than more elaborate routines such as doing 50 sit-ups [3]. The British Psychological Society's summary of the paper reports that people who chose an exercise behaviour took about one and a half times as long to reach their plateau as people who chose an eating or drinking one [4].

03What the 2024 review added

In 2024, Singh and colleagues at the University of South Australia reviewed every experimental study they could find on how long health habits take to form. They included 20 studies with 2,601 participants, but only four reported how long habit formation took. Those four gave medians of 59 to 66 days and means of 106 to 154 days [2].

The four studies in the 2024 review that reported a time to habit formation [2]
Study	What people were forming	Time to habit	Note
Lally et al. 2010	An eating, drinking or activity behaviour of their choice	Median 66 days (18 to 254)	Time to 95% of each person's plateau
Keller et al. 2021	A healthy eating behaviour	Median 59 days (4 to 335)	Only 23% reached the threshold for a habit
Lally et al. 2008	Healthy eating and self-weighing	Mean 91 days	People's own estimate of how long it took
Fournier et al. 2017	Daily stretching	Mean 106 days in the morning, 154 in the evening	Morning practice formed faster

The means run longer than the medians because a few people take far longer than everyone else, which pulls the average up. That's one reason the median is the fairer single number.

The review is honest about its own weaknesses. Eleven of the 20 studies were rated at high risk of bias, there were few studies overall, and most of the evidence comes from flossing and diet [2]. Its conclusion is the useful bit. Habits can start forming within about two months, but the time "varies significantly across individuals", and 21-day challenges are unlikely to be long enough [2].

04Is the 21-day rule true?

No. There's no good evidence that habits form in 21 days, and the studies that have measured it found much longer times [2] [3]. The figure is often traced to Maxwell Maltz's 1960 book Psycho-Cybernetics, and to his observations of patients adjusting to their new appearance after plastic surgery [2] [3].

That's a long way from forming a habit. Maltz was writing about how long people took to get used to a change in how they looked, which is a different thing from a behaviour becoming automatic. Somewhere along the way, "about 21 days to adjust" became "21 days to form a habit", and it stuck because it's a tidy number.

The risk of believing it is practical. If you expect a habit to feel automatic after three weeks and it still feels like effort, it's easy to decide you've failed and stop. On the real timeline, three weeks in is still early in the curve.

05Does missing a day break a habit?

No. In the 2010 study, missing one opportunity to do the behaviour "did not materially affect the habit formation process" [1]. Automaticity gains picked up again after a single missed day [3]. One miss doesn't send you back to the start.

There's a limit to that, though. People who did the behaviour more consistently had curves that fitted the model better [1], and the BPS summary reports some evidence that many missed days, even spread out, added up and lowered the level of automaticity people eventually reached [4]. So one missed day is fine, and a pattern of missed days isn't.

Never miss twice

That's the idea behind "never miss twice", a rule James Clear sets out in Atomic Habits. His point is that "the first mistake is never the one that ruins you. It is the spiral of repeated mistakes that follows." In his words, "missing once is an accident. Missing twice is the start of a new habit" [6]. When you miss a day, the job is to get back to it the next day.

It fits the research well. The evidence says a single miss barely matters, and the rule stops a single miss turning into the run of misses that does. It also takes the pressure off, because you don't need a perfect streak, only a quick return. As I put it on the founder page, "because my problem was never one bad day but the ones that followed it, there's one rule I don't break: never miss twice." Keep moving and the 48 Laws of Money, part 2 cover the mindset side of staying consistent through bad days.

06What helps a habit form faster?

Repeat the same small action after the same cue every day, and choose the habit yourself. Those are the factors with the most support. Starting small, doing it in the morning and planning exactly when and where also have some evidence behind them, though it's thinner.

Use the same cue, in the same place

A habit is an action triggered automatically by a cue you've linked to it, such as washing your hands after using the toilet [3]. The cue can be a time of day or something you already do, like "after breakfast" or "when I get to the library" [3]. In one trial, a time cue (a glass of water at 9am) and a routine cue (a glass of water after breakfast) worked about equally well [5]. What mattered was doing it in a stable context, repeatedly [2] [5].

James Clear calls the routine version habit stacking, with a simple formula: "After [current habit], I will [new habit]" [7]. After you make your morning coffee, you write down your three tasks for the day. After you sit down on the train, you open your flashcards. The existing habit is a cue you already meet every day, so the new one has something to hang on. How to plan your day uses the same idea to anchor habits to fixed points in the day.

Start smaller than feels worthwhile

Simpler actions become automatic faster [3], so start with a version that's almost too easy, like five pages a night or one set of press-ups, and build up once it's automatic. Gardner, Lally and Wardle advise small, manageable changes partly because early failure is discouraging [3].

Pick it yourself

Habits people chose for themselves were more successful than habits assigned to them [2], and a behaviour picked for its personal value is an easier habit target than one chosen to satisfy someone else [3]. Enjoying the behaviour and planning the details of when and where you'll do it also predicted stronger exercise habits in one study [2].

Decide when and where, in advance

Plans that spell out "if situation Y happens, I'll do X" had a medium-to-large effect on people reaching their goals across 94 studies [8]. That review covered goals in general rather than habit formation. Still, writing a cue into a plan is close to what the habit studies asked their participants to do.

Morning might help too. In one study of daily stretching, the habit formed faster when people did it in the morning than in the evening [2], but that's a single study of one behaviour.

07How do you know a habit has formed?

A habit has formed when the behaviour feels automatic: you do it without deciding to, and it would feel odd not to. That's what the research measures, using statements like "I do it automatically" and "I'd find it hard not to do" [4]. In practice, you notice it when you stop needing a reminder and stop arguing with yourself about whether to do it.

Gardner, Lally and Wardle suggest a simple way to watch this happen. Keep a daily tick-sheet for the new habit and, at the end of each week, rate how automatic it feels from 1 to 10 [3]. While the rating keeps rising, keep going. When it stops climbing for a few weeks, you've probably reached your plateau.

Should you track your habits?

Tracking helps, with a limit. A meta-analysis of 138 studies with 19,951 participants found that monitoring progress towards a goal improved the chances of reaching it, and the effect was larger when progress was physically recorded or shared with other people [9]. Those studies were about goals like weight loss and diet, not habit formation as such [9].

Tracking isn't the same thing as the habit, though. The aim is for the behaviour to happen without needing the tracker to prompt it. A weekly look at how the habit went, as part of a Sunday reset, is a good place to notice which habits are becoming automatic and which still need a better cue.

08How Ritual helps

RitualWaitlist · Early access

Ritual's habits are designed around Atomic Habits, where the never-miss-twice rule comes from. Every habit is sized Quick, Steady or Big, worth £4, £7 or £10 a tick in play money, and each one you do moves your own share price. Close up days in a row and your gains multiply, x1.1 at three days, x1.25 at five and x1.5 at seven. If a day closes in the red, the next day is a recovery day, built on the rule of never missing twice: make at least that much back and your streak carries on. The weekly report shows how each week closed and which habit held, which is the right timescale for something that takes about two months to form.

Habits sized Quick, Steady or Big, the same for everyone
Streak multipliers at 3, 5 and 7 days, and a recovery day after a bad one
A weekly report every Sunday, so you see the trend across weeks

Ritual is in beta. Join the waitlist and you'll hear first when places open.

09Questions

01Is it 21 or 66 days to form a habit?

The research points to about 66 days, not 21. In Lally and colleagues' 2010 study the median time to reach peak automaticity was 66 days, with a range of 18 to 254. A 2024 review found medians of 59 to 66 days. There's no good evidence for the 21-day figure.

02Does missing one day ruin a habit?

No. In the 2010 study, missing one opportunity did not materially affect habit formation, and progress picked up again afterwards. Many missed days did add up, though, so get back to it the next day.

03What is the never miss twice rule?

It's a rule from James Clear's Atomic Habits. Missing one day is an accident, but missing two in a row starts a new pattern, so after a miss, the priority is to do the habit the next day. It fits the research, which found one missed day barely mattered.

04How long does it take to break a bad habit?

The studies here measured forming new habits, so they don't answer this. Researchers point out that you can't form a habit of not doing something, and that breaking an existing habit takes different, more effortful strategies. Replacing it with a new behaviour after the same cue is one approach.

05What's habit stacking?

Linking a new habit to one you already do, using James Clear's formula: "After [current habit], I will [new habit]." The existing habit becomes the cue, which suits the research finding that habits form through repetition in a consistent context.

06Why do some habits take longer than others?

Complexity is a big factor. In the 2010 study, simple actions like drinking water became automatic sooner than routines like doing sit-ups, and exercise took about one and a half times as long as eating or drinking habits. Consistency, the cue and whether you chose the habit also matter.

07How do I know when a habit has formed?

When the behaviour feels automatic and you'd find it hard not to do it. One practical check is to rate how automatic it feels from 1 to 10 each week. When the rating stops rising, you've probably reached your plateau.

08Should I track my habits?

It's likely to help. A meta-analysis of 138 studies found that monitoring progress towards a goal made people more likely to reach it, especially when progress was written down or shared. Those studies were about goals in general, so treat tracking as support until the habit runs on its own.

10Sources

Something here out of date, or a question the guide doesn't answer? Email hello@getritual.app. More in Guides.

How long does it take to form a habit? What the research says, and why missing a day is fine.