Still Sprouting

How long does it take to build a habit? The "21 days" claim has no research behind it

November 3, 2026 · about 6 min read · compiled by the Still Sprouting team, with sources

The short version
  • No research supports "21 days to build a habit".
  • A University College London study found it took 66 days on average to reach peak automaticity, ranging from 18 days to a projected 254 days between people.
  • Missing the occasional day made little difference; missing often is what slowed people down.
  • Instead of chasing 21 days, expect a couple of months of practice, and make the action smaller.

Where does "21 days" come from?

"21 days to build a habit" is probably the most quoted claim of all. It is often traced to plastic surgeon Maxwell Maltz and his 1960s book Psycho-Cybernetics, where he noticed that patients took about 21 days to get used to their new appearance. That was a clinical observation, not an experiment on habit formation, and we could not find academic research that supports "21 days".

What did researchers actually measure?

Lally et al. (2010) asked 96 university volunteers to pick one small daily behavior (a small food or exercise action, for example) and report every day for 12 weeks how automatic it felt. 82 of them were included in the analysis. The results:

A German 84-day randomized trial (Keller et al., 2021) painted a similar picture: among people who formed the habit, the median was 59 days, with a range from 4 to 335 days (the upper end includes projection). Among the 117 people who followed the protocol, only 23% formed the habit. (The study analyzed 135 adults, and its evidence is moderate. The design differs from the study above, so the numbers cannot be compared directly.)

So what should you do?

  1. Set your expectation at two or three months. If it still takes effort on day 21, that is normal. It doesn't mean you're not cut out for it.
  2. If you miss a day, just pick it up the next day. Don't make "never break the chain" so stressful that the habit turns into a chore. (More on the two sides of streaks in Missed a day?)
  3. Make the action smaller first. "Read one page" or "do five squats" is easier to do every day than "read for an hour". This direction comes from BJ Fogg's Tiny Habits method (a method based on experience, not the kind of experiment above). The benefit is that you don't need willpower to hold on, so it is easier to last those two or three months.
  4. Attach it to your day with "if…then…". For example, "If I finish brushing my teeth, then I read one page." This kind of plan made in advance (called an implementation intention in research) is a fairly reliable method in meta-analyses. See How to change a bad habit.

How Sprout is designed

Sprout grows by accumulated companion days (it grows at companion days 3, 10 and 21) and never asks for a streak. Days you skip are just days Sprout rests; nothing resets to zero. Sprout blooms on companion day 21, but that is game progress, not a claim that "the habit is formed". We don't make that promise.

Keep reading

References

  1. Lally, P., van Jaarsveld, C. H. M., Potts, H. W. W., & Wardle, J. (2010). How are habits formed: Modelling habit formation in the real world. European Journal of Social Psychology, 40(6). doi:10.1002/ejsp.674. Plain-language summary: British Psychological Society Research Digest.
  2. Keller, J., Kwasnicka, D., Klaiber, P., Sichert, L., Lally, P., & Fleig, L. (2021). Habit formation following routine-based versus time-based cue planning: A randomized controlled trial. British Journal of Health Psychology. Full text (UCL Discovery).
  3. Gollwitzer, P. M., & Sheeran, P. (2006). Implementation intentions and goal achievement: A meta-analysis of effects and processes. Advances in Experimental Social Psychology, 38. doi:10.1016/S0065-2601(06)38002-1
  4. Fogg, B. J. (2019). Tiny Habits: The Small Changes That Change Everything (a method based on experience, not an experimental paper).

This article is general information about habits, not medical or psychological advice. If you are struggling with addiction, an eating disorder, or emotional distress, please reach out to a professional. In Taiwan you can call the 1925 helpline (24 hours); elsewhere, please use your local helpline.