Life

How long a habit takes to form: 66 days, not 21

In the best-known study, 96 volunteers repeated a new daily behaviour for 12 weeks, and it took a median of 66 days for the action to feel automatic, with individuals ranging from 18 to 254 days. A 2024 review of 20 studies with 2,601 participants put the typical figure at 59 to 66 days (median) and 106 to 154 days (mean), with a spread of 4 to 335 days between people. The popular claim of 21 days has no support in any of these trials.

Where the 66 days come from

The number most people have heard comes from a study by Phillippa Lally and colleagues at University College London, published in the European Journal of Social Psychology in 2010. Each of 96 volunteers chose one eating, drinking or activity behaviour, such as eating a piece of fruit with lunch or going for a walk, and agreed to do it once a day in the same context, for example after breakfast, for 12 weeks. Every day they recorded whether they had done it and answered a short questionnaire, the Self-Report Habit Index, which asks how automatic the action feels.

Of the 96, 82 provided enough data to analyse. The researchers fitted a curve to each person's automaticity scores across the 84 days, and the model fitted for 62 people, 39 of them well. The curve had the same shape in almost everyone: fast early gains, then a slowing, then a plateau. The median time to reach 95% of that plateau was 66 days. The individual figures ran from 18 to 254 days, and because the study lasted only 84 days, the longer values are projected from the shape of the curve rather than observed.

Two further findings matter more than the headline. Consistent repetition gave a better fit to the curve, and missing a single opportunity did not materially affect the process. What did slow things down was the complexity of the action: automaticity peaked more quickly for simple behaviours such as drinking a glass of water than for elaborate ones such as 50 sit-ups.

What the newer studies add

In 2024, a team at the University of South Australia published a systematic review and meta-analysis in the journal Healthcare. They searched 6 databases and included 20 experimental studies with 2,601 participants, with mean ages from 21.5 to 73.5 years. The behaviours were physical activity (8 studies), healthy eating (8), flossing (3), drinking water (2), microwaving a dishcloth to cut the risk of food poisoning (2), taking a vitamin (1) and reducing sitting time (1). The interventions lasted from 3 weeks to 2 years.

Only 4 of the 20 studies reported how long habit formation took. Besides Lally's 66 days, a 2021 study of healthy eating found a median of 59 days with a range of 4 to 335 days, but only 23% of participants reached the threshold the authors had set for a habit. An earlier study by Lally in 2008 on healthy eating and self-weighing found a mean of 91 days with a standard deviation of 55. A 2017 study of daily stretching found a mean of 106 days when the stretch was done in the morning and 154 days in the evening. Across the 12 studies that could be pooled, habit scores rose by a standardised mean difference of 0.69 (95% confidence interval 0.49 to 0.88), a moderate to large change; the effect was largest for flossing (1.11) and smaller for diet (0.57).

The review also names what predicts a stronger habit: frequent repetition, especially early on, morning rather than evening practice, choosing the behaviour yourself, enjoying it, planning exactly when and where it happens, and a preparatory habit such as laying out exercise clothes. On the 21-day figure, which the authors trace to the 1960 book Psycho-Cybernetics by Maxwell Maltz, they are blunt: the results clearly refute the notion that habits form in about 21 days.

Why the gym takes months and hand washing takes weeks

The studies above rely on people rating their own automaticity. A 2023 paper in PNAS took a different route. Researchers including Katy Milkman, Angela Duckworth and Colin Camerer applied machine learning to 2 sets of objective records: more than 12 million check-ins by 60,277 members of a large gym chain, and more than 40 million hand-washing opportunities among 5,246 healthcare workers at 30 hospitals. For each person, the model learned which context variables best predicted the behaviour, and treated the point at which behaviour became highly predictable as the moment a habit had formed.

The answer was not one number. Hand washing at work became habitual within weeks; going to the gym typically took months. The authors say this directly contradicts the belief in a magic number of days. It fits the earlier finding that complex actions take longer: a review in the American Journal of Lifestyle Medicine notes that in Lally's data, physical activity took about 1.5 times as long to become automatic as eating or drinking around a mealtime. One more result from the gym data: members whose attendance was highly predictable responded less to an intervention designed to increase visits, which is what you would expect once a behaviour no longer depends on rewards.

How to shorten the road

A 2012 paper in the British Journal of General Practice, written by Benjamin Gardner, Phillippa Lally and Jane Wardle for family doctors, turns the research into advice you can hand to a patient, or to yourself. Decide on a goal. Choose one simple action that moves you towards it and that you can do every day. Plan a time and a place, tied to something that already happens: after brushing your teeth, or when the kettle boils. Do it there every time, expect to need around 10 weeks before it runs without much thought, and only then add the next action.

The details from the studies sharpen this. Pick a cue that occurs once a day, because the 66-day figure came from once-daily behaviours in a fixed context. Choose the behaviour yourself; assigned habits formed less reliably in the 2024 review. Put it in the morning if your day allows, which was linked to stronger habits and, in the stretching study, a mean of 106 rather than 154 days. Start small, with 1 glass of water or a 10-minute walk, because simple actions become automatic first. Keep a record: a tick on a calendar gives the daily feedback the study participants had, and the 30-Day Habit Challenge on this site does the same for one month, roughly the first half of the road for a typical daily habit. If the habit you want is more movement, our guide on daily steps gives sensible targets.

A missed day is a data point, not a verdict. In Lally's study, one skipped opportunity had no measurable effect; it was people who missed many days who fitted the curve poorly. If you miss 2 days in a row, do the smallest possible version on day 3 at the planned time and place, so that the cue keeps its meaning.

Limits of the evidence and when to ask for advice

The 66-day figure deserves its fame less than its precision suggests. It is a median from 82 people, 39 of whom fitted the model well, in a study that ended on day 84, so any value above 84 is a projection. Every self-report study measures how automatic an action feels, not whether it is done, and the 2024 review judged 11 of its 20 studies to be at high risk of bias, with only 4 reporting a time to habit at all. The PNAS study is large and objective but covers 2 specific behaviours in 2 specific settings. What the studies agree on is the shape: fast early gains, a long tail, big differences between people, and months rather than 3 weeks for anything demanding.

Habits are a tool, and the behaviour you attach them to needs to be safe for you. If you have diabetes and take insulin or a sulfonylurea, a new daily exercise or fasting habit can change your blood sugar, so agree the plan with your care team first. The same applies to a new exercise habit if you have heart disease or have not exercised for years, and to any restrictive eating habit if you have a history of an eating disorder, where daily self-weighing or tracking can do harm. If you are under 18, pregnant, or on medication such as blood pressure tablets, check with your doctor before starting a strict daily routine. None of this argues against habits; it argues for choosing the right one.

Six steps for a new daily habit

Frequently asked

Is it true that a habit takes 21 days?

No. The 21-day figure traces back to a 1960 self-help book, and the 2024 review of 20 studies with 2,601 participants states that its results clearly refute it. Medians in the studies run from 59 to 66 days and means from 106 to 154 days.

Does missing a day ruin a habit?

Not according to the data. In the 2010 study of 96 volunteers, missing one opportunity did not materially affect habit formation, and gains in automaticity resumed afterwards. Long runs of missed days did weaken the fit to the habit curve.

Why do some habits take longer than others?

Complexity and context. Drinking a glass of water became automatic faster than 50 sit-ups in the 2010 study, physical activity took about 1.5 times longer than eating or drinking, and the 2023 PNAS analysis found hand washing took weeks while going to the gym took months.

Is a 30-day challenge long enough?

It is a good start and covers roughly the first 30 of a typical 66 days, when gains in automaticity are fastest. Plan to carry on for around 10 weeks in total, the timeframe the 2012 GP paper gives, and longer for exercise.

Does the time of day matter?

The evidence points to mornings. In the 2024 review, morning practice was linked to stronger habits, and in the stretching study the morning group averaged 106 days to a habit against 154 days for the evening group.

How do I know when something has become a habit?

Research uses the Self-Report Habit Index, which asks whether you do the action without thinking, would find it hard not to do, and start before you realise. When most of those statements feel true and the action survives a busy week, it has become a habit.

Sources

• Lally et al. 2010, European Journal of Social Psychology: How are habits formed. Modelling habit formation in the real world · 96 volunteers, 12 weeks, 82 analysed, 62 fitted the model; median 66 days, range 18 to 254; one missed day did not materially affect the process. Publisher page answers 403 to automated requests; abstract verified through Crossref metadata
• Gardner, Lally and Wardle 2012, British Journal of General Practice: Making health habitual, the psychology of habit-formation and general practice · Advice paper for GPs: around 10 weeks, a once-daily cue, simple actions such as drinking water form faster than 50 sit-ups, one missed performance does not impair the process
• Singh et al. 2024, Healthcare: Time to form a habit, a systematic review and meta-analysis of health behaviour habit formation and its determinants · 20 studies, 2,601 participants, mean ages 21.5 to 73.5; medians 59 to 66 days, means 106 to 154 days, range 4 to 335; SMD 0.69 (95% CI 0.49 to 0.88); 11 studies at high risk of bias; refutes the 21-day claim
• Buyalskaya et al. 2023, PNAS: What can machine learning teach us about habit formation? Evidence from exercise and hygiene · Over 12 million gym check-ins (60,277 members) and over 40 million hand-washing opportunities (5,246 workers, 30 hospitals); months for the gym, weeks for hand washing. Journal page answers 403 to automated requests; abstract verified through Europe PMC
• Arlinghaus and Johnston 2019, American Journal of Lifestyle Medicine: The importance of creating habits and routine · Review of routines in lifestyle medicine; physical activity took about 1.5 times longer than eating or drinking to become automatic in the 2010 data

Read on

Tools that fit

This guide is for general information. It does not replace medical advice or an examination.