How long does it take to form a habit?

The study behind "66 days"

In 2010, researchers at University College London asked 96 volunteers to pick one eating, drinking or exercise behaviour and do it once a day in the same situation — a glass of water after breakfast, a walk before dinner — for 12 weeks. Every day they rated how automatic it felt.

For the 39 people whose data fitted a clear curve, it took a median of 66 days to get 95% of the way to their own peak automaticity. The middle half took between 39 and 102 days; the full range was 18 to 254. Simple habits were faster than harder ones: drinking took a median of 59 days, eating 65 and exercise 91, though the difference wasn't statistically significant.

Limits worth knowing: the study ran for 84 days, so every figure above 84 days is extrapolated from each person's curve, and most participants were postgraduate students.

What a 2024 review adds

A systematic review published in 2024 pooled 20 studies with 2,601 participants. Only four measured how long habits took, so the headline figures are individual studies rather than one combined estimate: medians of 59 days in a 2021 study and 66 days in the 2010 one, and a range of 4 to 335 days between individuals.

Its overall conclusion: most health habits take two to five months. Habits grew stronger when they were practised more often, done in the morning, chosen by the person themselves, and simple and tied to a clear cue — habit strength grew more for flossing than for changing diet.

Timing matters even for small things: in one study, people who did a single stretch every morning were projected to reach automaticity in about 106 days, against about 154 days for those who did it in the evening.

Exercise takes longer

A study of 111 new gym members found that going at least four times a week for six weeks was the minimum to establish an exercise habit; doing it consistently and keeping it simple predicted it best. A 2023 machine-learning study of more than 30,000 gym-goers found gym habits took about two to three months to become predictable, while handwashing among hospital workers took about two weeks.

If you've seen "4 to 7 months" quoted from that 2023 study, it was corrected to 68–78 days.

Where "21 days" comes from

The number traces back to Maxwell Maltz's 1960 book Psycho-Cybernetics. Maltz, a plastic surgeon, wrote that it seemed to take "a minimum of about 21 days" for people to adjust to a change such as a new face or a new house. It was an observation about getting used to change, stated as a minimum — not a finding about how long habits take to form.

What helps, according to the research

• Same time, same place. Repeat the action in the same situation so the situation itself becomes the cue. Doctors' guidance based on this research says to expect it to feel automatic within about ten weeks.
• Plan it as "if–then". Writing "after I pour my coffee, I'll read for ten minutes" works: a meta-analysis of 94 tests with 8,461 participants found a medium-to-large effect on reaching goals (d = 0.65).
• Start small and simple. Simple, cued actions gained more habit strength than complex ones.
• Do it often. For exercise, about four sessions a week was the threshold in new gym members.
• Choose it yourself. Self-chosen habits formed more strongly than assigned ones.
• Don't panic over one missed day. Missing once barely slowed progress in the 2010 study — see what happens if you miss a day.

Using a habit tracker with this in mind

Plan for months, not weeks, and count repetitions rather than perfection. A tracker helps most by keeping the cue in front of you and showing the repetitions add up.

That's how Habiter is built: a reminder at the time you choose, a calendar and heatmap that show consistency over months, streaks that count only the days you planned, and Apple Health to tick habits off automatically. It's free for up to 5 habits. We make Habiter.