How to play Fermi Poker

Download the rules

Fermi Poker resembles poker, but instead of being dealt random cards, you make guesses about questions that seem unknowable at first, like “How many Dell laptops were sold last year?”, “How many hours did the average journey from London to San Francisco take in 1900?” or “How many dentists work in the US?” You write down a secret guess, bet on it, get helpful hints, bet again, and win by either having the best guess or by bluffing everyone else out.

Before you start

  • Everyone starts with the same number of chips.
  • Randomly pick who acts first during the betting rounds. For the next question, the player on their left goes first, and so on.
  • Don't look up answers on phones or elsewhere.
  • The organizer announces the ante and how often it goes up, for example every 20 minutes.

How a question is played

Six steps, and the pot goes to the best guess.

  1. 1. Ante

    Everyone puts in the ante, a small mandatory bet. Then the question is revealed.

  2. 2. Guess

    Everyone secretly writes down a range they think contains the answer, for example “400–1100”. Then betting round 1.

  3. 3. Hint 1

    Then betting round 2.

  4. 4. Hint 2

    Then betting round 3.

  5. 5. Answer reveal

    Then betting round 4.

  6. 6. Showdown

    The remaining players reveal their ranges, and the best one wins the pot. Revealing is voluntary for everyone who folded.

Your guess

  • Write it secretly and place it face down. Once betting round 1 starts, you can't change it.
  • A range includes both of its endpoints.
  • Write numbers however you like, for example 3,000,000, 3 × 106, 3 million or 3M, as long as they're clearly written and not ambiguous.

Example: How many dentists work in the US?

I had no idea. If I knew how many dentists work in my town, I could scale that up, but I don’t. So I broke the question into smaller ones I could guess, the way Enrico Fermi trained his students:

  1. How often do people go to the dentist? I go about once a year for a checkup. Some people go twice, some hardly ever, so once a year seems like a fair average.
  2. How long is a visit? Mine take about 45 minutes.
  3. How many patients does a dentist see? In an 8-hour day, about 10. Over 250 working days, that’s 2,500 a year.
  4. How many people live in the US? About 340 million.
  5. Put it together: 340 million people going once a year make 340 million visits, and 340,000,000 ÷ 2,500 ≈ 136,000 dentists.

Sanity check: that’s one dentist for every 2,500 people, so a town of 10,000 would have four. Sounds about right. So my best guess was about 140,000. I wasn’t sure of any single step, so my range was “100,000–200,000”.

Last chance to make your own guess…

Then I checked. The American Dental Association counted 205,088 professionally active dentists in 2025, just outside my range. Oops.

Now comes the most useful part. Because I broke the question down, I can check each assumption and see where my picture of the world was wrong:

  • How often: this is where I went wrong. About two in three adults, and most children, see a dentist each year, and many go twice for checkups, plus extra visits for fillings, crowns or braces. That’s about 1.5 visits per person, or roughly 500 million a year. I had pictured one checkup each.
  • Patients per day: 500 million visits shared by 205,000 dentists is about 2,400 each, or 10 a day. That one held up.

With 1.5 visits per person, my steps give 340 million × 1.5 ÷ 2,500 ≈ 204,000, almost exactly the real number.

That’s the point of breaking a question down. A single gut guess of 140,000 would just have been wrong. Written out step by step, my reasoning showed me exactly which part of my picture of the world to fix.

Betting

Play moves clockwise. On your turn you can:

Check

Pass the action to the next player without betting. Only possible if nobody has bet yet in this betting round.

Bet or raise

Increase the highest bet. Others must match it to stay in. The minimum bet is the current ante, and a raise must be at least the size of the last bet or raise in this betting round.

Call

Match the highest bet so far.

Fold

Drop out of this question. You give up the chips you’ve put in, but risk nothing more.

All-in

Bet all your chips. This is possible even if what you have left is less than the highest bet.

A betting round ends once everyone has acted and matched the highest bet or folded. Within a betting round, you can only raise again if someone else raised after you.

If you're all-in, you can win from each player only as much as you put in yourself. Extra bets go into a side pot that only the players who paid into it can win. Can't pay the ante? You're automatically all-in.

Who wins the pot

At the showdown

The narrowest range that contains the answer wins. Width is the high endpoint minus the low one: if the answer is 500, a range of “200–600” (400) beats a range of “400–1100” (700).

Equally narrow?

Split the pot.

No range contains the answer?

The range whose midpoint (halfway between low and high) is closest to the answer wins.

Everyone else folded?

The last remaining player wins and doesn’t have to reveal their guess.

Everyone, or all but one, is all-in?

Since no more betting is possible, read the remaining hints and answer. Then go to the showdown.

End of the game

A player with no chips left is out. The game usually ends when one player has all the chips.

Optional game variants

Single numbers or powers of ten

Guess a single number instead of a range, or allow powers of ten only (101, 102, 103…).

60-second clock

Once half the players have written their guesses, the rest have 60 seconds.

Mirror card

Usable once per player: write another player's name to copy their guess.

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