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How do you calculate the number of rounds for a Swiss tournament?

Pretty much what the title says. I want to have a tournament for my school, and Swiss style tournaments seem pretty great, but I don't know how many rounds to make it. I imagine I will have between fifteen and twenty participants.
That's rather small for a Swiss. Sure you can't make it a round robin? Or how about 2 round robins--with the two winners to face each other in a match?
The equation now involves covid-19 positivity, which is completely unpredictable, so an answer is impossible ...
@StoryMonkey said in #1:
> Pretty much what the title says. I want to have a tournament for my school, and Swiss style tournaments seem pretty great, but I don't know how many rounds to make it. I imagine I will have between fifteen and twenty participants.

More than 5. Less than 12.
@StoryMonkey
That's pretty easy.
Find x such that 2^x = number of Players.
And x is the number of rounds.
Here, 4 rounds will be played as in 5th one rematches will take place which is against the rules of Swiss.
4 or 5 rounds.

General approach is to take log(number_of_players, 2), rounded up. With 15 or 16 players, that would mean 4 rounds, with 17 to 20 players, 5 rounds.

The idea is that you want a smaller number of rounds because otherwise the tournament takes too long and a larger number of rounds to make sure that the top contenders play against each other. Log(number_of_players, 2) is the smallest numnber of rounds that you can have while guaranteeing that if two players dominate the field (i.e., win against everybody else), they will play against each other.
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The number of rounds in a Swiss Tournament should be odd, in order to have more balanced colour distributions. With 15-20 players, 5 rounds or 7 rounds work well. If the number of participants may drop below 14, you should have only 5 rounds, because the Swiss algorithm makes strange pairings sometimes, when the number of rounds exceeds half the number of players.
For example up to 64 players 5 rounds, up to 256 7 rounds. With more players some could „survive“ with 100% without having played each other.

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