Look at any CS2 event with a Swiss stage and you will see the same thing: the opening best-of-one rounds throw up results nobody expected, and the best-of-three matches later on mostly go the way the rankings say. That is not luck running out. It is arithmetic, and it should change how you read the odds on every match.
The formats
- BO1: one map, picked through a ban-only veto. Common in early Swiss rounds and some online qualifiers.
- BO3: first to two maps. Each team picks one map, and the third is a decider. The standard format for most of tier 1 and tier 2 CS2.
- BO5: first to three maps, mostly used for grand finals.
- BO2: two maps, and 1-1 is a draw. Used in some group stages and leagues.
At recent Majors, early Swiss rounds have been played as best-of-one while matches that decide advancement or elimination are best-of-three. Playoffs are best-of-three with a best-of-five grand final. Always check the format of the specific event on Liquipedia before you bet, because organizers change it between stages.
The maths behind it
Give the stronger team a chance p of winning any single map, and treat each map as independent. Then:
- BO1 win chance = p
- BO3 win chance = p² × (3 − 2p)
- BO5 win chance = p³ × (10 − 15p + 6p²)
The BO3 formula is just the two ways to get there added together: win the first two maps (p²), or split them and win the decider (2 × p × (1 − p) × p).
Series win probability by format
Here is what that does to a favorite. The left column is the chance of winning one map. The other columns are the chance of winning the series.
| Per-map win chance | BO1 | BO3 | BO5 |
|---|---|---|---|
| 50% | 50.0% | 50.0% | 50.0% |
| 55% | 55.0% | 57.5% | 59.3% |
| 60% | 60.0% | 64.8% | 68.3% |
| 65% | 65.0% | 71.8% | 76.5% |
| 70% | 70.0% | 78.4% | 83.7% |
| 75% | 75.0% | 84.4% | 89.6% |
| 80% | 80.0% | 89.6% | 94.2% |
Read it from the underdog's side. Against a team that wins 70% of maps, the underdog wins 30% of BO1s but only 21.6% of BO3s and 16.3% of BO5s. That is about 40% more upsets in the BO1 than in the BO3.
In price terms, a 65% per-map favorite is fairly worth about 1.54 in a BO1 but only 1.39 in a BO3. If a book offers 1.45 on that team in a BO1, the price is too short. The same 1.45 in a BO3 would be generous.
Why single CS2 maps are so volatile
CS2 plays MR12: the first team to 13 rounds wins the map, with 24 rounds in regulation and overtime after 12-12. CS:GO used MR15, first to 16. Fewer rounds per map means each round carries more weight, and a few rounds swing a map more easily.
- Pistol rounds: each map has two, one per half. Win both and the economy that follows can decide most of the map.
- Side bias: some maps favor CT or T sides, so the knife round or side choice matters more in a short format.
- Hot hands: one player having a career game is enough to steal a single map. Doing it twice in a row is much harder.
Where the simple model breaks
The table assumes every map is the same coin with the same bias. Real series are messier:
- Map picks: in a BO3 each team plays its own pick, where it is usually strongest. That narrows the gap on the underdog's pick and widens it on the favorite's. The decider is often a map neither team loves.
- BO1 vetoes: with bans only, the map that survives is often a compromise, which can help or hurt the favorite depending on how deep its map pool is. Our map veto guide walks through the ban order.
- Momentum and fatigue: a team that lost a long overtime map sometimes collapses on the next one. Late-night playoff days and travel make this worse.
- Three maps happen more than the model says: under the simple model a BO3 reaches a third map at most 50% of the time, when the teams are dead even. Because teams tend to win their own picks, real series go the distance more often. That matters for over 2.5 maps bets.
What it means for betting
- Price the format, not just the teams. Start from a per-map estimate, then convert it with the table. A BO1 favorite should be priced longer than the same favorite in a BO3.
- Be careful laying short prices in BO1s. A string of 1.30 to 1.45 favorites in Swiss BO1 rounds is one of the fastest ways to give back a month of profit.
- Handicaps follow the same maths. In a BO3, the favorite wins 2-0 with chance p². For a 65% team that is 42%, so the underdog +1.5 maps is fairly priced around 1.73 before margin.
- Use map stats, not just rankings. Per-map win rates over the last few months, against similar opposition, are the input this whole method depends on. Our guide to CS2 stats sites lists where to find them.
If you want to check whether a price clears your estimate, the odds edge calculator does the conversion, and the odds guide explains implied probability and margin. You can also see how our own picks have done across formats on the results page.