How long does it take to form a habit? Not 21 days, and not exactly 66

Where the 21 days come from

The most often cited trail leads to Maxwell Maltz, a plastic surgeon who wrote in 1960 that it took “a minimum of about 21 days” to get used to a change in the image people have of themselves: a face after surgery, an amputated limb, a new house.³

Maltz was talking about getting used to a change, not about learning a new behavior, and he said “a minimum.” The researchers who have traced the origin of the claim believe that's where the rule came from, and they call it a myth.³⁴

What the 66-day study measured

The figure comes from a University College London study published in 2010.¹ It involved 96 volunteers, mostly graduate students. Each one chose a new behavior related to eating, drinking or activity—for example, having a piece of fruit with lunch—and linked it to a situation that came up every day, such as “after breakfast.” For 84 days they recorded whether they had done it and how much it came to them “without thinking.”

From those answers, the authors plotted each person's curve: it rises quickly at first and then levels off. They considered the habit formed when the curve reached 95% of its own plateau.

What other studies have found

Few studies have measured the time since then. A 2024 systematic review brought together 20 studies and 2,601 participants on the formation of health habits; only four reported how long it took.⁵

The last study is different: it doesn't ask anyone anything, but analyzes gym check-in and hand-hygiene records, and calls it a habit when the behavior becomes predictable from the context. It measures something else, but reaches the same conclusion: there's no “magic number.”⁸

What seems to speed it up or slow it down

The studies agree on a few factors, with varying degrees of support:

• Frequency. Repeating the action often is the starting point. In the gym study, the authors put the minimum for forming the habit at about four sessions a week for six weeks.⁷ The 2024 review also lists frequency among the factors that play a role.⁵
• A stable context. Always doing it after the same cue or at the same time.⁴ In the gym study, always working out at the same time of day was what was most strongly associated with the habit growing over the weeks.⁷ To set that cue, you can use an “if–then” plan.
• Simplicity. Simple actions seem to become automatic sooner than complex routines.⁴⁷ In the guide, we explain how to make the action small and specific.
• Enjoying it. Enjoying the action is associated with the habit forming.⁷⁵

What if I skip a day?

In the 2010 study, the authors looked at what happened when someone missed a day after three in a row. The sense of automaticity dropped only slightly the next day and, once the action was resumed, kept rising much as if there had been no miss; the authors found no longer-term costs.¹

That holds for an isolated miss. Drawing on other research, the authors note that missing a whole week does reduce the likelihood of continuing and makes the habit harder. And for those who repeated the action less consistently, the habit curve fit less well.¹

How a habit is measured, and why it matters

Almost all of these studies measure habit with short questionnaires in which people rate statements such as “I do it without thinking” or “I do it automatically.”⁹

There's a well-known objection: people are asked to report on something that, by definition, happens without their paying attention to it. The authors of the short version of the questionnaire think the objection may be overstated, because looking back, people usually know they weren't thinking about what they were doing; but they acknowledge that this still needs to be checked against laboratory measures.⁹ In addition, it can't be ruled out that answering the questionnaire every day influences the behavior itself.²

That's why “habit formed” doesn't mean the same thing in every study, and why the figures should be read as orders of magnitude. In the questionnaire studies it also means that the action has reached its own automaticity plateau, not that it will never take effort again.

What to do with these figures

• Plan on weeks or months, not days. In an article for family doctors, two of the authors of the original study suggest telling patients to expect about ten weeks of daily repetition.⁴
• Be prepared, too, for it not to work the first time: in the 2010 study, about half of the volunteers didn't manage to form the habit, and in the 2021 study, about three out of four.¹²
• Don't judge your attempt on day 21. If it's still hard after three weeks, that's to be expected.
• Choose a simple action and a fixed cue: that's where the studies agree.
• After a miss, pick it up again at the next opportunity. In the guide, we explain what to do after a bad week.