How to play Fermi Poker
Download the rulesFermi 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?” 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
- Make sure everyone starts with the same amount 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 use phones, calculators or the internet.
- The organizer announces the ante and how often it goes up, for example every 20 minutes.
How a question is played
The example follows three players, Alice, Bob and Carol, through one question.
1. Ante
Everyone puts in the ante, a small mandatory bet. Then the question is revealed.
Example
The ante is 100, so Alice, Bob and Carol each put in 100. That’s already 300 in the pot before anyone has seen the question. Because the ante is paid on every question and goes up over time, nobody can afford to wait an hour for a question they’re completely sure about.
Question How many new cars were sold in the US last year?
2. Guess
Everyone secretly writes down a range they think contains the answer. Then betting round 1.
Example
Bob breaks the question down. About 340 million people live in the US, but children can’t drive, many older people no longer do, and plenty of city dwellers, in New York City for example, don’t own a car. Then again, many households have two or even three cars, more than in Europe. So maybe 250 million cars are on the road. If a car stays on US roads for about 15 years, around 17 million of them have to be replaced with new ones each year. Bob writes “10–20 million”.
Alice writes “5–30 million” and Carol “1–4 million”. Alice acts first and checks, which means passing the action to the next player without betting. That’s possible as long as nobody has bet yet in this round. Bob, confident after breaking the question down, bets 200. The others now have three options: call (put in 200 as well to stay in), raise (increase the bet) or fold (drop out). Carol isn’t confident and folds. Folding means giving up whatever you’ve put in, which for Carol is just the ante so far. Alice calls.
3. Hint 1
The first hint is revealed. Then betting round 2.
Example
Hint 1 About 240 million people in the US have a driver’s license.
At first, Alice isn’t sure what to make of it. Then Alice realizes that if each driver buys a new car every 10 to 20 years, that’s 12 to 24 million a year. That fits inside “5–30 million”, so Alice bets 300. Bob calls.
4. Hint 2
The second hint is revealed. Then betting round 3.
Example
Hint 2 About 1.9 million new cars were sold in Canada last year.
Bob knows Canada has roughly a tenth of the US population, so this points to about 19 million, still inside “10–20 million”. Alice has no idea how many people live in Canada, so the hint is of little use. Still trusting the first hint, Alice bets 400. Bob, now even more confident, raises to 800. Alice calls by putting in another 400.
5. Answer reveal
The answer is revealed. Then betting round 4.
Example
Answer 16.2 million
Both Alice’s and Bob’s ranges contain it, but neither knows what the other wrote. Alice checks. Bob thinks “10–20 million” was a pretty good guess and bets 1,000. Alice hesitates, visibly uncomfortable, but with so many chips already in the pot, doesn’t want to give up now and calls.
6. Showdown
The remaining players reveal their ranges, and the best one wins the pot. Revealing is voluntary for everyone who folded.
Example
Alice shows “5–30 million”, Bob “10–20 million”. Both contain the answer, but Bob’s range is narrower: 10 million wide, against 25 million for Alice. So Bob wins the pot of 4,900. Carol is relieved: “1–4 million” would have lost anyway, players who folded don’t have to show their range, and folding early cost only the ante.
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:
- 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.
- How long is a visit? Mine take about 45 minutes.
- How many patients does a dentist see? In an 8-hour day, about 10. Over 250 working days, that’s 2,500 a year.
- How many people live in the US? About 340 million.
- 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
Each player can use one Mirror card per game. Instead of a range, you write down the name of another player and put your Mirror card in the middle of the table, so everyone can see you’ve used it. You now have the same guess as that player: their range is also yours, even though you don’t know what it is.
- When you reveal at the showdown, the player you named must show their range too, even if they folded. It’s the only time a player who folded has to show their guess.
- Mirrors can chain: if Alice names Bob and Bob names Carol, Alice and Bob both get Carol’s range.
- If a chain loops back on itself, for example when Alice names Bob and Bob names Alice, every guess in the loop is invalid. Invalid guesses lose to any valid range at the showdown, but can still win if everyone else folds. If only invalid guesses are left, they split the pot.
- While a Mirror card is on the table, nobody may erase their guess before the question is over, even after folding, because anyone could turn out to be the player who was named.
