How long does it take to form a habit?
About 66 days on average, but that number hides a range from 18 to 254 days. Here is what the research actually measured, and a curve you can drag through.
The automaticity curve
Lally’s team found automaticity rises steeply at first, then levels off. Drag the day slider to see where you’d be; change the plateau to match an easier or harder habit.
74% of the way to the plateau on day 30. The steep middle: each repetition adds a lot. Keep the cue consistent.
Simplified curve of the shape reported by Lally et al.; individual data were noisy, and the model fitted well for about half of participants.
What the 66-day study did
Phillippa Lally and colleagues at University College London asked 96 volunteers to pick one eating, drinking or exercise behaviour and do it once a day in the same context, such as “after breakfast, go for a 10-minute walk”. For 84 days, participants rated how automatic it felt using items from the Self-Report Habit Index.
- For participants whose data fitted the curve well, the median time to reach 95% of their plateau was 66 days.
- The range was 18 to 254 days. Some of those figures are projections beyond the 84 days measured.
- More demanding behaviours tended to take longer than simple ones.
- Missing one day did not materially set people back.
Where “21 days” came from
Maxwell Maltz, a plastic surgeon, wrote in Psycho-Cybernetics (1960) that people seemed to need a minimum of about 21 days to adjust to a change, like a new face after surgery or the loss of a limb. It was an observation, not an experiment, and “minimum” got dropped as the idea spread. Fine for motivation; wrong as a deadline.
What to do with this
- Plan for about two months of deliberate effort, not three weeks.
- Keep the cue identical each day: same place, same preceding action. Use a habit stack.
- Start small with the 2-Minute Rule; simpler behaviours reach the plateau faster.
- If you miss a day, pick up the next. The tracker shows streaks, but the 30-day count matters more.
Sources: Lally, P., van Jaarsveld, C. H. M., Potts, H. W. W., & Wardle, J. (2010). How are habits formed: Modelling habit formation in the real world. European Journal of Social Psychology, 40(6), 998–1009. Buyalskaya, A. et al. (2023). What can machine learning teach us about habit formation? PNAS, 120(17). Maltz, M. (1960). Psycho-Cybernetics.
Estimate your date
Using a plateau around day 66 (set by the slider above), the habit should feel close to automatic by about:
…
A rough guide, not a promise. Your own curve depends on the habit, the consistency of the cue and life in general.
Keep designing
- Habit trackerDesign habits with a cue, identity and 2-minute start, then stitch off each day.
- Habit stackingAnchor a new habit to one you already do.
- 2-Minute RuleShrink any habit until starting is easy.
- Tiny Habits recipeBJ Fogg’s anchor, behaviour, celebration.
- If-then plannerDecide when and where in advance.
- Temptation bundlingPair a “should” with a “want”.
- Habit scorecardAudit what you already do each day.
- Break a bad habitInvert the four laws into a plan.
- Morning routine builderA timed stack for the first hour.
Questions
How long does it take to form a habit?
In the best-known study (Lally et al., 2010, University College London), participants repeating a daily behaviour took a median of 66 days to reach 95% of their maximum automaticity. Individual times ranged from 18 to 254 days, so expect roughly two months for a simple daily habit, and possibly much longer for harder ones.
Is it true it takes 21 days to form a habit?
No study shows a 21-day rule. The figure is usually traced to plastic surgeon Maxwell Maltz’s 1960 book Psycho-Cybernetics, where he observed that patients took a minimum of about 21 days to get used to changes such as a new face. Over time “a minimum of 21 days” became “21 days”.
Does missing a day reset a habit?
No. In Lally’s study, missing one opportunity to perform the behaviour did not materially affect the habit-formation process. What matters is getting back on track quickly.
Why do some habits take longer?
Complexity and effort. In Lally’s data, simple behaviours such as drinking a glass of water tended to become automatic sooner than more demanding ones like doing 50 sit-ups. A 2023 PNAS study using gym check-ins and hospital hand-washing data found gym habits took months to become predictable while hand-washing took weeks.
What does “automatic” actually mean here?
Researchers measured automaticity with questions from the Self-Report Habit Index, such as whether the behaviour is something you do without thinking or would find hard not to do. A habit is “formed” when those scores stop rising, not on a fixed date.