Anna Said 100 Sounds Better
#100DaysOfCode lasts 100 days because the founder's wife thought it sounded better than three months. What that says about picking the length of your own challenge.
Why do so many challenges last 100 days? For #100DaysOfCode, the answer is small and slightly deflating. Alex Kallaway first planned to code for three months, and his wife, Anna, suggested 100 days instead because it “sounds better.” The detail comes from Quincy Larson’s freeCodeCamp history; Kallaway emailed him in June 2016.
Read that as a product decision. The number that went on to fill a hashtag, with thousands of people tweeting it hundreds of times a day by the time Larson wrote his history, was picked for how it sounds when you say it out loud to someone.
It worked anyway. Nothing in the origin story suggests anyone tested whether 100 beats 90, or 66, or 30. The habit-formation study cited most often, Lally and colleagues in 2010, found that reaching automaticity took a median of 66 days with a spread of 18 to 254 across individuals; the range around that median is the part nobody quotes. A fixed 100 fits some of those people and misses others. The rest of the rules are read line by line here.
So 100 isn’t magic. A challenge length does two jobs: it has to be short enough to survive a bad week, and memorable enough that you can announce it to another person in one breath. “One hour a day for 100 days” passes both. “Ninety-three days” invites a question.
No study I know of compares round and unround challenge lengths, so that last point is inference. The number may matter far less than the people tweeting alongside you, and Anna’s instinct about sound may be the most data-driven thing in the whole origin story.