How long does it take to form a habit

Where the 66-day figure comes from

The number most people quote comes from a study by Phillippa Lally and colleagues at University College London, published in the European Journal of Social Psychology in 2010. Ninety-six volunteers each chose one eating, drinking or activity behavior to do every day in the same context, for example after breakfast, for 12 weeks. Each day they recorded whether they had done it and filled in the Self-Report Habit Index, a short questionnaire about how habitual and automatic the behavior felt.

Eighty-two people gave enough data to analyze, and the researchers fitted a curve to each person's automaticity scores over the 84 days. For the 39 people whose data fit that curve well, the median time to reach 95 percent of their own plateau was 66 days. The fastest took 18 days. The slowest was projected to take 254.

So 66 days is not the moment a habit switches on. It is the middle of a very wide spread, and it measures when a behavior stops getting noticeably more automatic. In a later interview published by the University of Surrey, Lally put it plainly: what the study really showed was that how long habit formation takes is highly variable.

Why some habits take far longer than others

The type of behavior seemed to matter. Among the good-fit participants, the median time to plateau was 59 days for drinking behaviors, 65 days for eating behaviors and 91 days for exercise. The study was not large enough for that difference to be statistically significant, but the authors noted that the exercise group took about one and a half times longer and suggested that more complex behaviors may take longer to become automatic.

Consistency mattered more. Doing the behavior more consistently was associated with a better fit to the habit curve, and the authors observed that roughly half of their motivated volunteers did not perform the behavior consistently enough to reach habit status at all. Picking a habit is the easy part; repeating it in the same setting is where most attempts thin out.

A 2024 systematic review led by Ben Singh at the University of South Australia, published in the journal Healthcare, listed the factors that influenced habit strength across studies: frequency, timing, the type of habit, individual choice, how people felt about the behavior, behavioral regulation and preparatory habits. Morning practices and habits people chose for themselves generally came out stronger.

What the newer research adds

The Singh review pooled 20 studies with 2,601 participants covering physical activity, drinking water, taking vitamins, flossing, healthy eating, microwaving a dishcloth to reduce foodborne illness, and sitting less. Four of those studies reported a typical time to habit formation. Their medians ranged from 59 to 66 days and their means from 106 to 154 days, with individual results anywhere from 4 to 335 days.

The gap between the median and the mean is worth noticing. When the average is roughly twice the median, a minority of people are taking a very long time, which pulls the average up. If your habit is still taking effort at day 70, you are not an outlier in any alarming sense. The review also flagged that most of the included studies had a high risk of bias, so the authors treat the evidence as emerging rather than settled.

Where the 21-day number came from

According to the Women's Health Research Institute, now at Michigan State University, the 21-day idea traces back to the 1960 book Psycho-Cybernetics by Dr. Maxwell Maltz. Maltz was a plastic surgeon turned psychologist who noticed a roughly 21-day period in which patients got used to a new face or an amputation, and he extended the idea to other adjustments such as getting used to a new home.

That observation was about adapting to a change, not about repeating a new action until it runs on its own. It was never a measurement of habit formation, and Singh, speaking about his review, said the common claims that habits take just 21 days are not evidence-based. In the UCL study, even the fastest participant needed 18 days, and the median was about three times the 21-day figure.

Common mistakes when counting the days

Treating day 21 or day 66 as a finish line. Both numbers describe groups, not you. Plan for the habit to need attention for a couple of months and possibly much longer, and judge progress by whether it is getting easier rather than by the calendar.

Quitting after one missed day. The UCL team specifically tested this. Missing one opportunity did not materially affect the habit formation process: automaticity scores still rose afterwards, only slightly less than when the behavior was done on both days. The authors contrast this with earlier work in which missing a whole week did hurt later performance, so the practical reading is to get back on track the next day.

Changing the time and place every day. Participants in the study tied their behavior to a fixed point in the day, such as after breakfast, because habits form through repetition in a consistent context. In the Surrey interview, Lally notes that a clear plan of when and where you will do something makes you more likely to do it than a vague intention.

Choosing a habit you do not actually want. The Singh review found that self-selected habits tended to be stronger, and Singh said you are more likely to stick with a new habit if you enjoy it. A habit picked because someone else thinks you should is fighting the evidence from the start.

Making the repetitions visible

Since consistency is what moves the curve, the useful question each day is simply whether you did the thing in its usual slot. A paper calendar works. So does a short note to someone who expects to hear from you.

HabitFive is one way to make that daily record visible to other people. You pick a habit and how many days a week you will do it, post one photo from the camera on each day you do, and a small anonymous team working on the same habit can see whether you checked in and tap a cheer.

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