If you've ever tried to build a new habit - whether that's exercising more, eating healthier, or going to bed earlier - you may have heard the popular claim that it only takes 21 days to form a habit.
Authors
- Ben Singh
Research Fellow, Allied Health & Human Performance, University of South Australia
- Ashleigh E. Smith
Associate Professor, Healthy Ageing, University of South Australia
It's a neat idea. Short, encouraging and full of promise. But there's just one problem: it's not true.
The 21-day myth can be traced back to Maxwell Maltz , a plastic surgeon in the 1960s, who observed it took about three weeks for his patients to adjust to physical changes. This idea was later picked up and repeated in self-help books, eventually becoming accepted wisdom.
But as psychologists and behavioural scientists have since discovered, habit formation is much more complex .
How long does it really take?
A 2010 study followed volunteers trying to build simple routines - such as drinking water after breakfast or eating a daily piece of fruit - and found it took a median of 66 days for the behaviour to become automatic.
We recently reviewed several studies looking at how long it took people to form health-related habits. We found, on average, it took around two to five months.
Specifically, the studies that measured time to reach automaticity (when a behaviour becomes second nature) found that habit formation took between 59 and 154 days. Some people developed a habit in as few as four days. Others took nearly a year.
This wide range highlights that habit formation isn't one-size-fits-all. It depends on what the behaviour is, how often it's repeated, how complex it is, and who's doing it.
What determines whether a habit will stick?
Habit strength plays a key role in consistency. A 2021 systematic review focused on physical activity and found the stronger the habit (meaning the more automatic and less effortful the behaviour felt) the more likely people were to exercise regularly.
It's not entirely surprising that easy, low-effort behaviours such as drinking water or taking a daily vitamin tend to form faster than complex ones like training for a marathon.
But whatever the habit, research shows sticking to it is not just about boosting motivation or willpower. Interventions that actively support habit formation - through repetition, cues and structure - are much more effective for creating lasting change.
For example, programs that encourage people to schedule regular exercise at the same time each day, or apps that send reminders to drink water after every meal, help build habits by making the behaviour easier to repeat and harder to forget.
Our research , which drew on data from more than 2,600 people, showed habit-building interventions can make a real difference across a range of behaviours - from flossing and healthy eating to regular exercise.
But what stood out most was that even small, everyday actions can grow into powerful routines, when repeated consistently. It's not about overhauling your life overnight, but about steadily reinforcing behaviours until they become second nature.
8 tips for building lasting habits
If you're looking to build a new habit, here are some science-backed tips to help them stick:
Give it time. Aim for consistency over 60 days . It's not about perfection - missing a day won't reset the clock.
Make it easy . Start small. Choose a behaviour you can realistically repeat daily.
Attach your new habit to an existing routine . That is, make the new habit easier to remember by linking it to something you already do - such as flossing right before you brush your teeth.
Track your progress . Use a calendar or app to tick off each successful day.
Build in rewards , for example making a special coffee after a morning walk or watching an episode of your favourite show after a week of consistent workouts. Positive emotions help habits stick, so celebrate small wins.
Morning is best. Habits practised in the morning tend to form more reliably than those attempted at night. This may be because people typically have more motivation and fewer distractions earlier in the day, making it easier to stick to new routines before daily demands build up.
Personal choice boosts success. People are more likely to stick with habits they choose themselves .
Repetition in a stable context is key. Performing the same behaviour in the same situation (such as walking right after lunch each day) increases the chances it will become automatic.
Why the 21-day myth matters
Believing habits form in 21 days sets many people up to fail. When change doesn't "click" within three weeks, it's easy to feel like you're doing something wrong. This can lead to frustration, guilt and giving up entirely.
By contrast, understanding the real timeline can help you stay motivated when things feel slow.
Evidence shows habit formation usually takes at least two months, and sometimes longer. But it also shows change is possible.
Our research and other evidence confirm that repeated, intentional actions in stable contexts really do become automatic. Over time, new behaviours can feel effortless and deeply ingrained.
So whether you're trying to move more, eat better, or improve your sleep, the key isn't speed - it's consistency. Stick with it. With time, the habit will stick with you.
Ashleigh E. Smith receives funding from the National Health and Medical Research Council, the Medical Research Future Fund and a Dementia Australia Research Foundation Henry Brodaty Mid-Career Fellowship.
Ben Singh does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
