How long it actually takes to form a new habit — a median, not an average, over a wide range.
What the evidence says
- Median time to reach 95% of automaticity asymptote: 66 days — a *median*, not a mean; the wide range below is why "average" overstates how typical 66 days is1
- Range: 18-254 days depending on person and behavior complexity1
- Automaticity follows asymptotic curve: early repetitions yield larger gains, diminishing returns over time1
- Missing a single day does not significantly impact overall habit formation1
- Simpler behaviors (e.g., drinking water) reach automaticity faster than complex ones (e.g., 50 sit-ups) — exercise behaviors took ~1.5× longer than eating/drinking behaviors1
- The popular "21 days to form a habit" (Maltz 1960) is not supported; real habit formation takes substantially longer for most behaviors1