The test: can you do it on your worst day?
A habit is small enough when you could still do it while tired, busy or unwell. If the version you picked only works on good days, it is a goal for good days, not a habit. Run one test: picture the worst ordinary day you had this month and ask whether you would still do it.
Shrink it with one rule
Take the habit you want and cut it until it fits in about two minutes and needs no special setup. Keep the first step, drop the rest. A two-minute version is not the finish line. It is the part you refuse to skip.
Write the shrunken version as one line in this form: After [something I already do], I will [tiny action].
Worked example
Say your habit is "exercise for 45 minutes every morning."
- Check it. On a bad day you will not manage 45 minutes, so it fails the test.
- Shrink it. The first step is getting changed. Drop the rest.
- Write the rule. After I pour my morning coffee, I will put on my workout clothes.
- Allow growth. On days you feel good, you can keep going. The rule counts as done either way.
If you do the tiny action on a day you feel awful, the day counts. If you do more on a good day, that is a bonus, not a new minimum.
Check the anchor, too
The thing you do "after" matters as much as the size. Pick an anchor that already happens every day, such as pouring coffee, brushing your teeth or closing your laptop. An anchor that only happens some days will make the habit only happen some days.
A common mistake
Do not raise the bar the moment the habit feels easy. Wait until the tiny version feels automatic, then add one small step. If a bad week arrives and the bigger version slips, drop back to the tiny version. Going back is part of the system, not a failure.
Related guides
To think about how a missed day fits into the picture, read streaks vs systems. To choose when to begin, see the best time to start a new habit.