Health Calculators
How long to form a habit.
Enter the day you started. Get the dates for the 66-day average from habit research, and the fastest and slowest cases in the study, 18 and 254 days. Runs in your browser; nothing is sent or stored.
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Habit timeline
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Where the numbers come from
In a 2010 University College London study (Lally, van Jaarsveld, Potts and Wardle), 96 volunteers chose a new daily eating, drinking or exercise behaviour and rated how automatic it felt for 84 days. The average time to reach the plateau of automaticity was 66 days, with a predicted range for individuals of 18 to 254 days. Exercise habits took about 50% longer than eating or drinking habits, so the exercise option multiplies 66 by 1.5 to give 99 days. About half the volunteers had not reached habit level by day 84.
The popular “21 days” figure is not from this research. The tool just adds the study’s day counts to your start date, counting your start date as day 1. It cannot predict how long it will take you.
Worked example. Start on January 1. Day 66 is March 7, day 18 is January 18 and day 254 is September 11. For an exercise habit the average becomes day 99, April 9.
Sources (opened 2026-10-05): Lally P, van Jaarsveld CHM, Potts HWW, Wardle J (2010), “How are habits formed: Modelling habit formation in the real world”, European Journal of Social Psychology, doi:10.1002/ejsp.674; summary by the British Psychological Society Research Digest.
FAQ
How long does it take to form a habit?
In the Lally 2010 study the average was 66 days, with individuals ranging from 18 to 254 days. It depends on the person and the behaviour.
Is it 21 days or 66 days?
The research measured an average of 66 days. The 21 day figure is a popular claim that does not come from this study.
What if I miss a day?
In the study a single missed day had minimal impact on habit formation. A pattern of missed days slows it down.
Do harder habits take longer?
Yes. Exercise habits took roughly 50% longer to become automatic than eating or drinking habits in the study, so the tool shows 99 days for them.
Does day 66 mean the habit is locked in?
No. About half of the volunteers had not reached habit level by day 84. Treat the dates as a typical timeline, not a deadline.
Track the days with the streak calculator.
Disclaimer
General information from one study of 96 volunteers, not advice for any individual. It is not a substitute for professional help with a health condition.