How Long It Takes to Form a Habit and Why 21 Days Is Wrong

Day 22 is when a lot of people quit. They read somewhere that it takes 21 days to form a habit, they did the evening walk or the morning journal for three weeks, and on day 22 it still felt like effort. So they decide they must be bad at habits and stop.

The problem is not them. The 21 day figure is close to a myth, and the real answer to how long it takes to form a habit is both longer and more forgiving. Once you know what the research actually found, you can plan for the slow middle stretch instead of being ambushed by it.

Where the 21 Day Idea Came From

The number traces back to a 1960 book by Maxwell Maltz, a plastic surgeon. He noticed that patients took a minimum of about 21 days to get used to a changed face or a lost limb. Over decades of retelling, "a minimum of about 21 days to adjust" became "21 days to form any habit", and the word minimum quietly vanished.

Maltz was not studying habits, did not run an experiment and was describing the shortest cases he saw, not the typical ones. It is a neat number for a book cover. It is a poor number for planning your life around.

How Long It Takes to Form a Habit According to Research

The study most often cited was run at University College London and published in 2009. Researchers asked 96 volunteers to pick one new daily behaviour, such as drinking a glass of water with lunch or doing 50 sit ups after morning coffee, and to report every day for 12 weeks how automatic it felt.

The results:

The median time to reach a plateau of automaticity was 66 days.
The range ran from 18 days to 254 days, depending on the person and the behaviour.
Simple habits, like drinking water, became automatic far faster than effortful ones like exercise.
Missing a single day did not meaningfully set people back.

Later work points the same way. A 2023 study tracking gym members and hospital staff found that handwashing habits formed within weeks, while gym habits typically took several months.

Why Some Habits Form in Weeks and Others Take Months

The range of 18 to 254 days is not random. A few factors push a habit towards the short end or the long end.

Factor	Faster	Slower
Effort	Drink a glass of water	Run for 30 minutes
Cue	Same place and time every day	Whenever you find a gap
Frequency	Daily	Two or three times a week
Reward	Felt straight away	Only felt after weeks

A weekly habit like a Sunday meal prep gets far fewer repetitions in 66 days than a daily one. Counting repetitions instead of days is often more useful: many people find a habit settles somewhere between 40 and 80 repetitions.

The Three Phases You Will Probably Go Through

Weeks one and two: the easy start

Motivation is high and the novelty carries you. Progress feels fast. This phase is misleading because it says nothing about whether the habit will last.

Weeks three to eight: the flat middle

The novelty has gone and the habit is not automatic yet. This is where the 21 day myth does the most damage, because people expect the reward to arrive here. Expect this stretch to feel like work. Shrinking the habit to its smallest version on hard days, such as one page instead of ten, keeps the chain of repetitions going.

Month three onwards: the plateau

The behaviour starts to happen without much thought when the cue appears. It is not unbreakable. Holidays, illness and new jobs can still disrupt it, but restarting is much easier than starting.

Stop Obsessing Over Perfect Streaks

A lot of habit advice says never miss a day. Streak apps are built around that idea, with flames and counters that reset to zero the moment you slip.

The UCL data does not support that pressure. One missed day did not reset automaticity. What hurt people was missing several days in a row, which usually happened after they decided one miss had ruined everything. The all or nothing mindset breaks more habits than the miss itself. If you use a tracker, read our piece on when habit tracking backfires, and if your streak has already broken, the steps in what to do when a habit streak breaks get you moving again.

A better rule is "never miss twice". It keeps the repetitions high without turning one bad Tuesday into a reason to quit.

Planning a Habit With a Realistic Timeline

Pick a fixed cue. Tie the habit to something you already do daily, like making coffee. Our guide to habit stacking shows how.
Set a 90 day horizon, not 21. Write the date three months out in your calendar and treat that as the first checkpoint.
Define a minimum version. Decide in advance what counts on a bad day, such as two minutes of stretching instead of twenty.
Check in at day 30 and day 60. Ask how automatic it feels on a scale of one to ten. Rising scores matter more than perfect attendance.

Frequently Asked Questions

Is 66 days the number I should aim for?

Treat it as a midpoint, not a target. Your habit might settle in a month or take most of a year. Plan for about three months and be pleased if it comes sooner.

Does breaking a bad habit take the same time?

There is less research on stopping habits, but it usually takes longer, because the old cue still exists. Replacing the old behaviour with a new one tied to the same cue tends to work better than willpower alone.

Can I build two habits at once?

You can, but each one slows the other in the flat middle phase. Starting a second habit once the first feels mostly automatic is the safer approach.

Expect the Middle to Be Boring

The single most useful thing to take from the research is that the boring middle is normal. If week five feels like a chore, that is not failure. It is the exact phase where the habit is being built. Write a 90 day date in your calendar, keep a two minute version for bad days, and judge yourself by repetitions, not by a number from a 1960 book about plastic surgery.