How Long Does It Take to Form a Habit? A Study Found 18 to 254 Days

When people repeated one behavior every day in the same context, the time it took for that behavior to become close to automatic ranged from 18 to 254 days. The gap between people was huge. The study was published online in the European Journal of Social Psychology in 2009 and appeared in the 2010 print issue.

In the Brain Mondo short video, I covered this study in about 40 seconds. In this post I go through the details the video left out, how the result relates to the popular “21 days to form a habit” idea, and what it means when you actually try to build a habit.

Video: How many days to form a habit? (Japanese)

The video is in Japanese.

What the study did

The study was run by Lally, van Jaarsveld, Potts and Wardle. Their goal was to follow how habits form in everyday life, not in a lab.

Ninety-six volunteers took part. Each one picked a single behavior to do, something to eat, drink or an activity. They then did it every day in the same context, such as “after breakfast”, for 12 weeks. That is 84 days.

Every day, participants recorded two things. One was whether they had done the behavior that day. The other was their answers to the Self-Report Habit Index (SRHI), a questionnaire where you rate feelings such as “I do this without thinking.” The researchers used a subset of those items to put a number on how automatic the behavior had become.

So the study was not measuring a single yes or no on “is it a habit yet.” It was measuring the shape of the change: how automaticity rose, day by day.

The result: 18 to 254 days

Of the 96 people, 82 left enough data to analyze. For each person, the researchers fitted a curve to their 84 days of scores, one that rises quickly at first and then levels off.

The time it took to reach 95% of each person’s own ceiling ranged from 18 to 254 days. The paper says this shows considerable variation in how long it takes people to reach their limit of automaticity, and that it can take a very long time.

Note that 254 days is longer than the 84-day study. That number comes from extending each curve forward from 84 days of data. In other words, some people were still climbing after 12 weeks.

You often hear that it takes 21 days to form a habit. The fastest person in this study did take 18 days, but that is the fastest person. Not having a habit after three weeks is completely normal.

One missed day didn’t break it

As the video mentioned, the study had one more important finding. Missing one opportunity to do the behavior did not materially affect the habit formation process.

When people build habits, they tend to focus on not breaking the streak. Many feel that one missed day means starting over. This study says a single miss does not wipe out what you have built.

The abstract also says that people who did the behavior more consistently had curves that fit better. Missing once is fine, and repeating in the same context still makes automaticity grow more smoothly overall. These two points do not contradict each other.

Not everyone fit the curve

The abstract has numbers the video didn’t cover. When the researchers fitted curves to the 82 people, the model fit for 62 of them, and fit well for 39.

For the rest, the scores did not trace a clean curve. You can imagine several reasons: some may have stopped partway, some had scores that went up and down. Habit formation does not unfold the same way for everyone.

What other research says

In 2024, Singh and colleagues published a systematic review and meta-analysis of health habit formation in the journal Healthcare.

It included 20 studies with 2,601 participants in total, with mean ages ranging from 21.5 to 73.5 years across studies. The behaviors included physical activity, drinking water, taking vitamins, flossing, healthy eating and reducing sedentary time.

Four studies reported how long it took to form a habit. The medians ranged from 59 to 66 days and the means from 106 to 154 days. Individual variation was very large, from 4 to 335 days. The authors conclude that habits can start forming in about two months, but the time varies a lot from person to person.

The review also found that frequency, timing, type of habit and whether people chose the habit themselves were linked to habit strength. Morning habits and self-chosen habits generally came out stronger. That said, most of the included studies were rated as having a high risk of bias, and the number of studies is still small.

Limits and common misreadings

A few points are worth keeping in mind when reading this study.

First, automaticity was self-rated. It measures the feeling of “I do this without thinking,” which is not the same as how often the behavior actually happened.

Second, the behaviors were fairly simple: eating, drinking, moving. We don’t know whether the same timeline applies to something heavier, like studying for a certification for two hours a day.

Third, “one miss didn’t matter” is about missing one opportunity. It does not mean skipping a whole week is fine.

Finally, the participants were volunteers, and there were fewer than 100 of them. The 18-to-254-day figure is a rough range.

Using it in daily life

Here are four ways to apply this study to daily life.

• Tie the behavior to a context. Not “when I have time,” but after something that happens every day, like “after breakfast” or “after brushing my teeth.” That is basically all the participants in this study did.
• Think in months, not days. Not having a habit after three weeks is not failure. If you expect it to take anywhere from two months to several months, you won’t blame yourself along the way.
• If you miss a day, come back the next day. One miss does not erase your progress. What matters is starting again in the same context the next day.
• Choose it yourself. Self-chosen habits tended to be stronger than ones suggested by others. Pick something you have your own reason to keep doing.

I also wrote about ways to get back on track after a lapse in Why Brain Training Habits Fail, and What Keeps Them Going.

Brain Mondo is a series of short daily videos about the brain, each based on a research paper. BrainDojo is an app for a few minutes a day of training in calculation, memory, attention and more, and it puts together Today’s Training for you every day based on your answers to the first-time setup. “This week’s goal” sets how many days a week you train, so if you miss a day, you can make it up within the same week. That fits with this study’s finding that one missed day doesn’t break a habit.

References

• Lally P, van Jaarsveld CHM, Potts HWW, Wardle J. How are habits formed: Modelling habit formation in the real world. Eur J Soc Psychol. 2010;40(6):998-1009.
• Singh B, Murphy A, Maher C, Smith AE. Time to Form a Habit: A Systematic Review and Meta-Analysis of Health Behaviour Habit Formation and Its Determinants. Healthcare (Basel). 2024;12(23):2488.