How Long Does It Take to Form a Habit?

If you’ve ever tried to build a new habit, you’ve probably heard that it takes 21 days. Another popular answer is 66 days. The research gives us a more useful answer: there is no fixed number of days it takes to form a habit.

One influential study found that participants took between 18 and 254 days to reach 95% of their estimated maximum automaticity. A more recent systematic review found individual habit-formation times ranging from 4 to 335 days across the studies that measured them. So if a behaviour still requires conscious effort after three weeks, or even three months, that doesn’t necessarily mean the habit isn’t developing.

How long does it really take to form a habit?

One of the most frequently cited studies on habit formation was published by Phillippa Lally and colleagues in the European Journal of Social Psychology. The researchers asked 96 people to choose an eating, drinking or activity behaviour and repeat it in the same context each day for 12 weeks. For participants whose habit-development patterns could be modelled, the time required to reach 95% of their estimated maximum automaticity ranged from 18 to 254 days ⁽¹⁾.

More recent research reinforces just how much the timeline can vary. A 2024 systematic review and meta-analysis included 20 studies involving 2,601 participants, although only four studies directly reported time to habit formation. Across those studies, median habit-formation times ranged from 59 to 66 days, while reported means ranged from 106 to 154 days. At an individual level, reported habit-formation times ranged from 4 to 335 days.

The important takeaway isn't that we should replace 21 days with another magic number. Habit formation varies considerably depending on the behaviour, the person and the circumstances.

Where did the 21-day habit idea come from?

The 21-day rule is generally traced back to plastic surgeon Maxwell Maltz, who wrote in the 1960s about his observations of how long people took to adjust to changes. Over time, those observations were simplified into the much stronger claim that it takes 21 days to create a habit.

Modern habit research doesn't support 21 days as a universal rule. In fact, the authors of the 2024 systematic review concluded that their findings challenge popular claims that habits can reliably be formed within 21 days ⁽²⁾.

Does it really take 66 days to form a habit?

The 66-day figure has more evidence behind it, but it can still be misleading when presented as a rule. It comes from the Lally study, which measured how repeated behaviours became increasingly automatic over time. The wide 18-to-254-day range is arguably more useful than the famous 66-day figure because it shows just how different the process can be from one person and behaviour to another.

So, 66 days isn't the new 21 days. It tells us that habit formation often takes longer than popular advice suggests, not that everyone should expect a behaviour to become automatic on day 66.

What happens when a behaviour becomes a habit?

If you think about putting on your seat belt when you get into a car or picking up your keys before leaving home, you probably don't stop to make a completely new decision every time. The behaviour has become closely associated with a familiar situation.

This is an important part of automaticity. Repeating a behaviour in a consistent context can gradually make it easier to initiate without the same level of conscious deliberation. In the Lally study, automaticity generally increased with repetition before gradually approaching a plateau. Habit formation was a process rather than a switch that suddenly flipped on a particular day.

Why do some habits take longer to form than others?

A glass of water after breakfast is very different from a 45-minute exercise routine. One requires seconds and very little preparation, while the other involves time, planning and several separate actions. This helps explain why searching for a single habit-formation timeline can be misleading.

The 2024 review identified several factors associated with habit formation and strength, including frequency, timing, the type of behaviour, personal choice and preparatory routines. These findings are useful for understanding why habit formation varies, but they shouldn't be turned into another universal formula.

Does missing one day break a habit?

Missing one opportunity doesn’t automatically send you back to day one. In the Lally study, missing a single opportunity to perform the chosen behaviour did not materially affect the habit-formation process.

That doesn’t mean repetition is unimportant. It means consistency and perfection aren’t the same thing. If you miss Tuesday, Wednesday is another opportunity to practise the behaviour, not necessarily a complete restart.

How can you make a new habit stick?

Instead of focusing on reaching a particular number of days, it’s more useful to create conditions that make repetition easier. Start with a specific behaviour that you can realistically repeat. “Be healthier” is an intention. “Take a 10-minute walk after lunch” is something you can actually practise.

It also helps to connect the behaviour to a consistent cue, such as doing something after breakfast, when you arrive at work or before starting a particular task. Reducing friction can make repetition easier too. Preparing what you need in advance or placing an object where you’ll see it can remove some of the effort involved in remembering and starting.

Use reminders and nudges strategically

When a behaviour is new, it's easy to forget. Reminders and digital nudges can help prompt action while the behaviour still requires conscious effort. The reminder isn't the habit itself. Its role is to create another opportunity to practise the behaviour consistently.

Digital tools can also help with tracking, prompts and accountability. If you're looking for some practical options, we've reviewed our recommended habit-building apps for professionals, including tools designed for different habit-building goals.

What does habit formation look like at work?

Workplaces are full of repeated behaviours. Some support safety and performance, such as carrying out a check before starting equipment, looking in the direction of movement or following a consistent preparation routine. Others can introduce risk, such as automatically skipping a familiar step because nothing went wrong the last hundred times.

Habits aren't a substitute for safe equipment, procedures, risk assessments, supervision or training. But they can influence how reliably people apply what they know, particularly when attention is affected by rushing, fatigue, frustration, distraction or familiarity. SafeStart explores this relationship further in The Habit of Safety.

From knowing what to do to doing it consistently

Knowing the correct behaviour and reliably performing it aren't always the same thing. Someone can understand a procedure and still make an unintended error when attention is elsewhere or conditions make the task harder. This is why effective human error reduction training looks beyond knowledge alone and considers the factors that influence decisions and actions in the moment.

The aim isn't to automate every workplace decision. Some tasks require careful thought, judgement and deliberate checks. But practising useful behaviours can help them become easier to perform consistently, adding another layer to existing systems and controls.

Building stronger habits with YOUFactors

Building a habit requires repetition, and the research shows that this process can take weeks or months. That matters for learning too. Giving someone information once is different from creating opportunities to practise and apply it over time.

YOUFactors uses short digital learning activities, practice and nudges to support the development of skills and habits related to human performance. The goal isn't to promise that a behaviour will become automatic in 21, 66 or any other fixed number of days. It's to help people keep important skills front of mind and create opportunities to practise them until they become more established.

So, how long does it take to form a habit?

There is no universal answer. Research has found substantial variation between people and behaviours, including individual formation times ranging from 4 to 335 days in studies covered by a recent systematic review.

Rather than asking when a habit will finally “click,” a more useful question is: how can I make this behaviour easy and consistent enough to keep practising it? Choose a clear behaviour, connect it to a useful cue and keep repeating it. Missing a day isn't the end of the process. What matters is coming back and practising again.

Want to turn learning into lasting practice? Explore how YOUFactors uses short learning activities, nudges and continued practice to support stronger habits for safety and performance.