Fermi

Fermi takes its name from the physicist Enrico Fermi and the style of question he made famous: rough estimates of hard-to-know quantities built from quick reasoning. Fermi is made by Flatwhite Studios, the small studio behind Spotle and Harmonies, and its first numbered puzzle went up on 27 July 2026. The site gives you three such questions every day, numbered and dated, the same for every player.

You get one guess per question. A built-in calculator is there for napkin math; looking things up is against the spirit of the game. After each guess the real answer is revealed together with an example of how you could have reasoned your way to it, and you see where your guess fell among everyone else's.

Scoring is by ratio: hit the answer exactly and you score 1x, be off by double and you score 2x, off by ten times and you score 10x, so lower is better. Your day's score is the average of the three, shown against other players and ready to share. Players who sign in can join a leaderboard for the day, the week, all time and streaks, and earn badges. A new set arrives at midnight Eastern time, older days are in the archive, and players can submit their own questions.

It suits anyone who enjoys back-of-the-envelope reasoning, from students to consultants practising case interviews.