How long does it really take to form a habit?

Somewhere along the way, "it takes 21 days to form a habit" became one of those facts everyone knows. It's tidy, it's encouraging, and it fits neatly into a month. It's also not what the best-known research found.

What the study found

In a study led by Phillippa Lally and colleagues, published in 2010, participants each chose a new behaviour, linked it to a daily cue and repeated it every day for 84 days. Each day they rated how automatic it felt. The British Psychological Society's Research Digest summarised the results: the average time to reach maximum automaticity was 66 days, with a range from 18 days to a predicted 254. Of 96 people recruited, 82 completed the study.

So: about two months on average, but with enormous variation. For some people and some behaviours, under three weeks. For others, most of a year.

A few things are worth noticing about the details:

• The behaviours were simple. The examples given include a short run before dinner or a piece of fruit with lunch. More complex habits may take longer.
• The upper figure is a projection. The study lasted 84 days, so the longest times are estimates based on how things were going.
• Everything was self-reported. People rated how automatic a behaviour felt, which is useful but not an exact measurement.

Missing a day is fine

For practical purposes, this is the most useful finding. According to the same summary, missing a single day had little effect on how automatic the habit eventually became. Many missed days, even spread out, did lower the level of automaticity people reached.

In other words, one skipped day doesn't reset the clock. What matters is the overall pattern. If a missed day makes you want to give up, our guide to what to do after you break a streak is for exactly that moment.

Why the number matters less than you think

Knowing the average is mildly interesting, but it's not much use for planning, for a few reasons:

• You won't know your number in advance. It depends on you, the behaviour and your circumstances.
• There's no switch. Habits don't flick from effortful to automatic on day 66. They get gradually easier, often so gradually you don't notice.
• Counting can make it a test. Ticking days off towards a magic number turns a habit into an exam, and exams are easy to fail.

A better question than "how long?" is "how do I make each repetition easier?"

What actually helps

The study's set-up hints at the answer: each behaviour was tied to a cue that happened every day anyway. That's a good model.

• Attach it to something fixed. After the first coffee, after brushing your teeth, after you lock your bike at work.
• Keep it small. A short walk, one page, five minutes of practice. Small habits are easier to repeat on bad days.
• Make it pleasant. A nicer route, better music, company.
• Aim for most days, not every day. The research suggests the overall pattern matters more than a perfect streak.

For a broader take on the subject, see our piece on what Atomic Habits argues.