Short answer: There’s no fixed number. In the best-known study, led by Phillippa Lally, a daily behaviour took an average of 66 days to become as automatic as it would get. Individual times ran from 18 days to an estimated 254. Simple actions settled faster than exercise, and one missed day did little harm.
You’ve probably heard that it takes 21 days to form a habit. It’s a tidy number, and it has launched a great many challenges. It’s also not what the research found.
The real answer is longer and messier, and more useful, especially if your weeks don’t run to plan.
What did the research find?
The study most people mean is by Phillippa Lally and colleagues, published in 2010. Volunteers each picked one new health behaviour, tied it to a daily cue, and did it once a day for 84 days. Examples included eating a piece of fruit with lunch and a short run before dinner. Every day they rated how automatic it felt, using statements like ‘I do it without thinking’.
According to the British Psychological Society’s summary, 96 people signed up and 82 saw it through. The average time to reach the point where extra repetitions stopped making it feel more automatic was 66 days.
Across people, that ranged from 18 days to a predicted 254 days. The top of that range is an estimate, because the study ended at 84 days and some people were still improving.
Two more findings are worth knowing. About half the participants hadn’t reached a high enough automaticity score to count as a habit by the end of the 84 days. People who picked an exercise behaviour also took longer. They took about one and a half times as long to plateau as people who picked something to eat or drink.
So is it 21 days or 66 days?
Neither, as a promise.
66 days is an average across a small group of volunteers, each doing one simple thing, once a day, in the same setting. Your habit, your setting and your week are different. Treat 66 days as a reasonable expectation that it’ll take a couple of months, not as a deadline you’re failing if it takes longer.
That expectation matters more than it sounds. If you believe three weeks is normal, week four feels like evidence that you’re bad at this. If you expect a couple of months or more, week four is the middle, and the middle is supposed to feel like nothing much.
Why does a habit take some people so much longer?
The study can’t tell you why for any one person, but the pattern points at a few things you can use.
What you pick. A 2012 paper for GPs by Benjamin Gardner, Lally and Jane Wardle covers this. Automaticity peaked more quickly for simple actions, such as drinking water, than for more elaborate routines. A glass of water is one action. A workout is a string of them.
Where and when you do it. The same paper’s advice is to repeat the behaviour in the same context until it becomes automatic. Same cue, same place, every time. A habit done after breakfast on weekdays and ‘whenever I get a chance’ at weekends is two habits, and neither gets the repetitions.
How often it happens. Once a day, every day, gives the most repetitions. Three times a week gives fewer, and each one is further from the last.
None of this is a judgement on you. It’s arithmetic about repetitions and cues.
Does missing a day reset the clock?
No. This is the finding to remember.
Missing the occasional day didn’t seriously impair habit formation. Progress picked up again after a single miss. The BPS summary adds the other half: many missed days, even spread out, did seem to lower how automatic the habit eventually became.
So one miss isn’t a disaster, and a pattern of misses is information. GROWTH by DESIGN turns that into a rule: miss once and it’s a day; miss twice in a row and it’s the emergency. After one miss, the next go is compulsory, and it’s the smallest version you have.
The bad-day version. If this week has already gone sideways, don’t start counting from zero again. Do the smallest version of the habit today, even two minutes of it, and tick it off. The clock never stopped. You’re still in the middle.
How do you make a habit stick faster on a busy week?
You can’t make it take fewer days, but you can make sure the days count.
- Pick one action, not a routine. Start with the first step of the thing, the part that takes two minutes. Let the rest grow later.
- Tie it to one cue you already have. Kettle on, school run, last email sent. An event you notice beats a time on the clock.
- Keep the setting the same. Same place and same order where you can.
- Write a bad-day version. A smaller version you’ve already agreed counts. It protects the repetitions on the days when the full version won’t happen.
The first two minutes are the part you’re building. Everything after that is a bonus, not the target.
What should you track while you wait?
Tick a box. That’s it.
A 2016 review by Benjamin Harkin and colleagues pooled 138 studies covering 19,951 people and found that monitoring progress promoted reaching goals. The effect was larger when progress was physically recorded, and also larger when it was reported to someone or made public.
For a habit, that points at the plainest possible tracker. A grid of days on paper, a tick when you do it, and one person you tell at the end of the week.
You aren’t measuring how you feel about it. You’re collecting evidence that it’s happening, because two months is a long time to go on feelings.
Frequently asked questions
Is it true that it takes 21 days to form a habit?
Not according to the best-known study. Lally and colleagues found an average of 66 days for a daily behaviour to become as automatic as it would get. Individual times ran from 18 days to an estimated 254. The 21-day figure is repeated widely, but it isn’t what that research found.
Does missing one day ruin a new habit?
No. In the same study, a single missed day had little effect on how automatic the habit became. Lots of missed days did seem to add up. So miss once without worrying, and make the next day’s go compulsory, even if it’s tiny.
Why is exercise harder to turn into a habit?
In the study, people who chose an exercise behaviour took longer to plateau. It took them about one and a half times as long as people who chose something to eat or drink. Exercise is usually several steps, not one. Starting with the first step, like putting your trainers on, helps.
How do I know when something has become a habit?
When you do it without deciding to, and not doing it feels odd. The study measured this with statements like ‘I do it without thinking’. Until then, keep the cue and the setting the same and keep ticking the box.
Read next: How to build a habit when you’re exhausted and Why can’t I stick to anything? It’s usually the plan, not you.
Two months is a long time to go on your own. The free One-Page Change Plan puts your change on one printable sheet: nine steps, in order, sized for a bad day. It comes with the GROWTH Readiness Scorecard.