Asking how long does it take to form a habit? Often about 66 days, not 21, though you could land anywhere from 18 to 254 days. Keep going and be patient with yourself.
This page sits within the site’s wider guide to habit building, so use it for the numbers and head there for the bigger picture.
| Item | Value |
|---|---|
| Average time to form a habit in the best-known study | 66 days |
| Range of individual estimates | 18 to 254 days |
| Length of the tracking period | 84 days |
| Volunteers who started the study | 96 people |
| Median estimate for exercise (selected group) | 91 days |
| Year the 21-day idea appeared in print | 1960 |
These figures come from a UCL research blog post and a close reading of the original study by The Behavioral Scientist: see the UCL post and the close reading. Figures can change as research is reread, so check the sources before you rely on them.
Where the 21-day myth came from
The idea that a habit takes 21 days is everywhere, yet a UCL researcher wrote in a June 2012 blog post that little is known about where the claim started. The team traced a likely source to the 1960 book Psycho-cybernetics by Maxwell Maltz, a plastic surgeon turned psychologist.
Maltz wrote that it usually takes a minimum of about 21 days to change a mental image, and his examples came from patients getting used to a new face after surgery. That is habituation, or getting used to something, which is not the same as forming a habit. The UCL authors conclude that 21 days may apply to adjusting to plastic surgery but is unfounded as a basis for habit formation.
Psychologists define a habit as a learned action that is triggered automatically when you meet the situation in which you have repeatedly done it. Habits form through context-dependent repetition: each time you act in that situation, the mental link between cue and response gets stronger. So what matters is repeating the action in a consistent context, not simply counting calendar days.
What the 66-day study measured
The 66-day figure comes from Lally and colleagues, published in 2010 in the European Journal of Social Psychology. Ninety-six university volunteers each chose a healthy eating, drinking or exercise behaviour and did it daily in the same context for 84 days. Examples include drinking a glass of water after breakfast.
Each day, participants reported how automatic the behaviour felt, on a scale that ran up to 42 points. The pattern was consistent: early repetitions produced large jumps in automaticity, then the gains shrank the more often the behaviour was repeated, until automaticity plateaued.
What the researchers counted as the moment of habit formation is worth knowing:
- They treated the point where automaticity peaked as the point the habit had formed.
- Automaticity was a self-reported sense that a behaviour feels automatic, effortless and unintentional.
- On that basis, the average time to form the habit was 66 days.
Why 66 days is a median, not a deadline
A closer reading shows that 66 days was the median estimated time to reach 95% of a modeled automaticity plateau for 39 participants whose data met the researchers’ criteria. Of 96 participants, 82 provided enough data for analysis, so those 39 are 48% of the people with adequate data. The number describes a selected group, not everyone who took part.
Several details change how you should read it:
- Huge individual differences. One person reached their peak in just 18 days, while another had not got there by day 84 and was forecast to need as long as 254 days.
- Forecasts, not observations. Because the tracking lasted 84 days, any estimate beyond that point projected the fitted curve past the period when researchers were collecting reports.
- Behaviour type. The exercise median was 91 days, compared with 65 for eating and 59 for drinking. That subgroup difference was not statistically significant, so treat it as uncertain rather than a rule.
- Simple versus complex. UCL’s summary notes that habit strength for simple behaviours, such as drinking a glass of water, peaked quicker than for more complex ones, such as doing 50 sit-ups.
Habit strength varied as well as speed. For some people it peaked below the halfway point of the 42-point strength scale, and for others it peaked at the very top.
What this means for your own routine
The UCL bottom line is that 21 days is a myth and habit formation typically takes longer than that. The best estimate is 66 days, but the authors say it is unwise to attach a number to the process because it differs by person and by behaviour. As long as you keep doing the new behaviour consistently in a given situation, a habit will form.
Here is a practical way to use that:
- Choose one simple behaviour first, since simple actions tended to reach their peak sooner.
- Attach it to a once-daily cue, as the study volunteers did with cues like after breakfast.
- Repeat it in the same context each day, because the researchers noted that poorer fits were associated with fewer repetitions.
- Judge progress by how automatic the action feels, not by a day count.
For a full walkthrough, the guide on how to build a habit covers the steps in more detail. The daily-cue idea from the study also fits neatly with habit stacking, where a new action follows one you already do.
Frequently asked questions
Is the 21-day rule true?
Show answer
No. The UCL researchers describe 21 days as a myth that is unfounded as a basis for habit formation, and say habit formation typically takes longer than that.
Is 66 days a guarantee?
Show answer
No. It was the median for 39 selected participants, and individual estimates ran from 18 to 254 days. Treat it as a rough reference point, not a deadline.
Does exercise take longer to become a habit than drinking water?
Show answer
It may. The exercise median was 91 days against 59 for drinking, and UCL notes simple behaviours peaked quicker than complex ones. The overall subgroup difference was not statistically significant, though, so it remains uncertain.
What does “automaticity” mean in habit research?
Show answer
It is a person’s subjective sense that a behaviour feels automatic, effortless and unintentional. The study measured that self-reported feeling rather than directly observing a habit.
