The rule of thumb: halve the field until one is left
In a Swiss tournament everyone plays every round, and each round you pair players on the same score. If nobody draws, each round halves the number of players who are still unbeaten. With 16 players, 8 are unbeaten after round one, 4 after round two, 2 after round three and 1 after round four. So 16 players need 4 rounds to produce one player with a perfect score.
In general: the number of rounds is log2 of the number of players, rounded up. With 20 players that's 4.3, so 5 rounds.
| Players | Rounds | Note |
|---|---|---|
| 2 | 1 | Just play a match |
| 3 to 4 | 2 | A round robin may be better |
| 5 to 8 | 3 | A round robin of 7 rounds is the alternative |
| 9 to 16 | 4 | |
| 17 to 32 | 5 | |
| 33 to 64 | 6 | |
| 65 to 128 | 7 | |
| 129 to 256 | 8 | |
| 257 to 512 | 9 |
If you'd rather not count, the Swiss rounds calculator does it for any number of players.
What the rules bodies say
Chess's governing body doesn't set a number. FIDE's basic rules for Swiss systems say only that "the number of rounds to be played is declared beforehand", that two participants shall not play each other more than once, and that with an odd number one player gets a bye worth as many points as a win, unless the tournament's rules say otherwise. The choice is the organizer's, as long as it's announced in advance.
Magic: The Gathering's tournament rules are more specific. Appendix E of the Magic Tournament Rules recommends the number of Swiss rounds and the size of the playoff by attendance, and requires it for Premier events:
| Players | Swiss rounds | Playoff |
|---|---|---|
| 5 to 8 | None: 3 rounds of single elimination | None |
| 9 to 16 | 5 (4 for Limited with a booster draft playoff) | Top 4 (top 8 for that Limited case) |
| 17 to 32 | 5 | Top 8 |
| 33 to 64 | 6 | Top 8 |
| 65 to 128 | 7 | Top 8 |
| 129 to 226 | 8 | Top 8 |
| 227 to 409 | 9 | Top 8 |
| 410 and up | 10 | Top 8 |
Two things stand out. With 8 or fewer players, Magic's rules skip Swiss entirely and play a single-elimination bracket. And at the top end the table adds rounds faster than log2 would (9 rounds from 227 players rather than 257). With a top-8 cut, what matters isn't one undefeated player but a clear set of eight to go through, and an extra round helps separate them.
When to play more rounds
The log2 number finds a winner. It doesn't rank everyone else well. After 4 rounds with 16 players, there will usually be several players on 3 wins and 1 loss, and tiebreaks decide who's second. If second and third place matter (prizes, qualification, a league table), add a round or two. Each extra round separates the players on the same score a bit more.
There's a ceiling: you can't play more than one fewer round than you have players without pairing someone twice. Near that point a round robin is simpler: everyone plays everyone once, and there's nothing to argue about.
When to play fewer rounds, with a top cut
If time is short, play fewer Swiss rounds and then a single-elimination playoff among the top 4 or top 8. The Swiss part sorts the field; the playoff decides the winner. This is what Magic's table does for every event above 16 players. A rough guide: play enough Swiss rounds that the players making the cut have clearly better records than the ones just missing it, which is usually log2 of the players, minus one or two.
Byes, draws and tiebreaks
- Byes. With an odd number of players, one sits out each round and scores a win. Give it to the lowest-placed player who hasn't had one, so it doesn't decide the top.
- Draws. Draws slow down the halving, so more players finish on the same score. If your game draws often, as chess does, play an extra round.
- Tiebreaks. Decide them before you start and publish them. Common choices are head-to-head result, game or score difference, and Buchholz (the sum of your opponents' scores, which rewards having played stronger opponents).
How long will it take?
Multiply the rounds by the length of a round, including the time to post pairings and for the slowest match to finish. For a game with 50-minute rounds plus 10 minutes between them, 5 rounds is 5 hours before any playoff. That's often what decides the number of rounds in the end: plan backwards from when the venue closes.
Doing this in Programmable Talk
Tournaments run as part of the event: entrants register through the event page, and you choose single or double elimination, round robin with groups and playoffs, Swiss, or judged rounds. For Swiss, the number of rounds defaults to log2 of the entrants, rounded up, and you can set it anywhere from 1 to 20. Each round is paired from the standings: players on the same score where possible, never the same opponent twice while another pairing exists, and a bye for the lowest-placed player who hasn't had one, counted as a win. Points for a win, draw and loss are yours to set (3, 1 and 0 by default), and standings break ties by head-to-head, score difference and then Buchholz. Seeding can be random, by rating or by hand. Talk doesn't allocate colours, so for a FIDE-rated chess event use FIDE-endorsed pairing software. See tournaments.