How Long Does It Take to Build a Habit? What the 66-Day Study Really Says

There is no fixed number. In the best-known study, daily habits took a median of 66 days to feel automatic, but it ranged from 18 to 254 days depending on the person. A 2024 review found similar medians and longer averages. So expect about two months, often more, and don't worry if yours is slower.

You've probably heard that it takes 21 days to build a habit. That number didn't come from a habit study. It goes back to Maxwell Maltz, a plastic surgeon who wrote a book called Psycho-Cybernetics in 1960. He noticed that his patients usually needed about three weeks to get used to their new face.

Maltz wrote that it usually takes 'a minimum of about 21 days' for a mental image to change. Over the years, 'a minimum of about 21 days' became 'exactly 21 days,' and 'getting used to a new face' became 'building any habit.' A 2012 article by habit researchers in the British Journal of General Practice calls the 21-day rule a myth that appears to come from those surgery patients.

The study behind the 66 days was published in 2010 in the European Journal of Social Psychology by Phillippa Lally and her colleagues. They asked 96 volunteers to pick one new eating, drinking or physical activity behavior and do it every day, in the same situation, for 84 days. Along the way, people rated how automatic the behavior felt, meaning how much it happened without having to think about it.

For each person, the researchers drew a curve of that feeling over time and looked for the point where it flattened out, when the behavior was about as automatic as it was going to get. The median time to get there was 66 days. The spread was huge: 18 days for the fastest person and an estimated 254 days for the slowest.

So 66 is the middle of a very wide range, not a deadline. Half of the people it was measured for took longer than that, and nobody's habit ran on a schedule.

It's a useful study, and also a small one with real limits. Of the 96 people who started, 82 gave usable data, a curve could be fitted for 62, and only 39 fitted it well. The 66-day figure comes from that smaller group.

The study also ended on day 84. Any number above that, including the 254, is an estimate of where the curve would have gone if it had kept running. And automaticity was self-reported: people rated their own sense of it, and no one measured it from the outside.

None of this makes the study wrong. It means 66 days is a rough guide. The honest version is: about two months on average, often longer.

A 2024 systematic review and meta-analysis in the journal Healthcare pulled together 20 studies with 2,601 people. Only four of them actually timed how long habits took to form. Across those four, the medians ranged from 59 to 66 days, the means from 106 to 154 days, and individual people from 4 to 335 days.

The gap between medians and means matters. A few people who take a very long time pull the average up, so the typical person lands around two months while the average sits between three and a half and five months. The review also flagged that 11 of the 20 studies had a high risk of bias, so treat these numbers as a starting point, not a verdict.

One of those four studies, a 2021 randomized trial with 192 people in the British Journal of Health Psychology, found a median of 59 days. It also found that only 23% of participants reached the researchers' habit threshold at all. Habits take work, and some take longer than any study has followed.

Repeating it in the same context. In Lally's study, people did their behavior daily in the same situation, and the 2024 review points the same way. That's the whole idea of a habit: after enough repeats, the situation itself starts to prompt the action.

A clear cue helps, and it doesn't need to be clever. In the 2021 trial with 192 people, tying a new habit to something you already do, like right after breakfast, worked no better than picking a fixed clock time. Both built automaticity. Pick whichever fits your day.

Deciding when and where also helps. A 2006 review of 94 tests found that if-then plans ('when X happens, I'll do Y') made people more likely to follow through, with a medium to large effect. A 2013 review focused on physical activity found a smaller effect, so a plan is a boost, not magic.

Morning habits and habits you choose yourself did better. The 2024 review found both grew stronger. In one small study of 48 people, a stretch done in the morning took about 106 days to become automatic, while the same stretch in the evening took about 154.

Phone reminders are a mixed bag. A small study presented at the CHI 2015 conference found that reminders helped people remember to do the behavior but got in the way of it becoming automatic. Let something in your day be the cue, and keep reminders as a backup.

No. In Lally's study, missing one opportunity to do the behavior did not materially affect habit formation. A 2012 article coauthored by Lally put it simply: after one missed day, the gains in automaticity soon picked up again.

What can hurt is how you read the miss. Research on goals finds that an early slip can make people feel less committed, and some give up entirely. One day off is not the end of the habit. The next day counts just as much as the ones before.

Research shows that keeping track of your progress helps you reach goals: a 2016 review of 138 studies in Psychological Bulletin found it works better when progress is recorded and reported to someone. That's why in Kairo you prove a habit every day with a photo, and your friends see it. Tap a photo and the timeline shows that person's whole history with that habit, day by day, yours included.

Two months is a long stretch, and it's easier when someone notices. Kairo keeps the circle small on purpose, up to 20 friends on the free plan, so the people who see your days are the ones who'll notice you keep going. Kairo is free.