Greetings and welcome to this comprehensive Starcraft II ladder guide. This guide is designed to provide detailed information about the core functionality and design of the Starcraft II ladder, its leagues, and matchmaking. The information within this guide comes from empirical findings as well as Blizzard developers.
Immediately noticeable upon searching for a ladder match is your remaining Bonus Pool. The Bonus Pool is a reservoir of points that are awarded for wins, doubling your earned points per game until the Bonus Pool is exhausted. The Bonus Pool also absorbs lost points, reducing your Bonus Pool by an amount equal to the number of points you would have lost for that game.
The Bonus Pool accrues at a set rate for each league, whether playing or not.
For 1v1 or Random Team:
Grandmaster League -- 180 points per week (1 per 56 minutes)
Master League -- 180 points per week (1 per 56 minutes)
Diamond League -- 104.4 points per week (1 per 96 minutes, 33 seconds).
Platinum League -- 104.4 points per week (1 per 96 minutes, 33 seconds).
Gold League -- 104.4 points per week (1 per 96 minutes, 33 seconds).
Silver League -- 104.4 points per week (1 per 96 minutes, 33 seconds).
Bronze League -- 104.4 points per week (1 per 96 minutes, 33 seconds).
Patch 1.3 adjusted this rate such that arranged 2v2 teams accrue at a 66% rate, and arranged 3v3 and 4v4 teams accrue at a 33% rate.
For arranged 2v2 teams:
Master League -- 120 points per week (1 per 84 minutes)
Diamond League -- 69.6 points per week (1 per 144 minutes, 50 seconds)
Platinum League -- 69.6 points per week (1 per 144 minutes, 50 seconds)
Gold League -- 69.6 points per week (1 per 144 minutes, 50 seconds)
Silver League -- 69.6 points per week (1 per 144 minutes, 50 seconds)
Bronze League -- 69.6 points per week (1 per 144 minutes, 50 seconds)
For arranged 3v3 and 4v4 teams:
Master League -- 60 points per week (1 per 168 minutes)
Diamond League -- 34.8 points per week (1 per 289 minutes, 40 seconds)
Platinum League -- 34.8 points per week (1 per 289 minutes, 40 seconds)
Gold League -- 34.8 points per week (1 per 289 minutes, 40 seconds)
Silver League -- 34.8 points per week (1 per 289 minutes, 40 seconds)
Bronze League -- 34.8 points per week (1 per 289 minutes, 40 seconds)
Season 3 reduced the accrual rate for leagues below Master to 58% of the original rate in order to make spending bonus pool less daunting for casual players.
Because of this set rate and because it applies equally to everyone, this essentially acts as a global decay mechanism, separating active players from inactive ones. In a typical division, it's expected that not all players will remain active, so this means that players who consume more Bonus Pool will typically rise above most of the players in their division, because the inactive players' points are decaying relative to the active players.
We can therefore say that your displayed points minus your spent Bonus Pool are your "adjusted points", or points that account for the inflation of the ladder as a result of Bonus Pool. That is, a player who has 2500 points but spent 2400 Bonus Pool to reach it (2500 - 2400 = 100 adjusted points) would have fewer adjusted points than a player who has 2400 points but spent 1200 Bonus Pool (1200 adjusted points), indicating that the latter would likely be a more skilled player.
At the heart of the system is a hidden value known as the matchmaking rating, or MMR for short. Matchmaking rating helps to ensure you play against players around your skill level and influences how many points you stand to gain or lose per match. Your points will drift toward your hidden MMR over a period of time, but because MMR is more volatile than points, your MMR is never cemented at a fixed value. For this reason, it is extraordinarily difficult to reverse engineer MMR from points.
During the pre-game loading screen, as well as the post-game score screen, an amount of points are awarded or lost in accordance with what the system determines to be a favorable or unfavorable pairing. The Favored system compares your opponent's hidden MMR with your adjusted points and calculates an amount of points that the game will be worth if you win or lose. If you stand to gain 0-4 points or lose 20-24, you are Favored; if you stand to gain 5-9 points or lose 15-19, you are Slightly Favored; if you stand to earn or lose 10-14 points, the Teams are Even. This value is independently calculated for each team and the results will not necessarily be zero sum.
For example, let's say two players are matched together, Player A with 1500 MMR and 0 points and Player B with 1000 MMR and 0 points. Each player would see the other as Favored and would either earn many or lose few points depending on the outcome. If Player A won, because Player A's MMR is larger than Player B's by a fairly large margin, the MMRs of each player would not change very much. If Player B won, B's MMR would rise and A's MMR would fall to a greater extent than if A had won. The image below illustrates this.
The important thing to remember is that "favored" does not always mean "better" unless both players' points have approximately reached their MMRs. Until that time, the "favored" indicator only serves to determine how many points a match is worth, and is not an indicator of skill.
Minimum Division MMR
The "Favored" indicator compares your adjusted points to your opponent's MMR, but there also exists a concept known as "minimum division MMR". If your opponent's MMR is below the equivalent of 88 points in your division, that opponent will -- for the purposes of point calculation -- appear to have an MMR equivalent to 88 points in your division. This means that even if your adjusted points are below 0 and your MMR is far below your division's lower bound, you will see opponents as "Even" and earn the associated number of points for those games. This allows for all players to earn points up to a certain amount and removes the frustration associated with seeing yourself as "Favored" in every match.
Minimum Division MMR does not exist for Master and Grandmaster leagues.
The system knows that players may perform at a level above or below their MMR. This is represented as a hidden variance factor called "uncertainty". In conjunction with MMR, uncertainty determines your range of potential opponents, and that uncertainty can increase or decrease, thereby widening or narrowing that range, depending on the outcome of each game. If no player is in the queue who is within your uncertainty range, that range will gradually increase until a match is found. If two potential opponents are found and have MMRs equidistant from yours, the matchmaker will prefer the opponent from your same league. Uncertainty also determines how drastically MMR changes after a match. Generally speaking, the system becomes more uncertain about you when you beat a player whose MMR is higher or lose to one with a lower MMR, and less uncertain when you beat a player with a lower MMR or lose to one with higher MMR.
In the example above, the red line represents Player 1’s MMR. The curve colored blue represents the deviation of skill that this player may exhibit over time. Any player knows that there are external factors that may affect your play: playing on tilt, being unusually focused, getting distracted. The uncertainty curve is a graphical representation of that level of inconsistency. Players who are found to be close to your MMR (which would be near the peak of your curve) should statistically be a close match.
The example image above shows a second player that has been matched with the first. The overlapping area represents the statistical probability of a close match. Based upon the size of the overlapping area, if Player 1 is playing well and Player 2 is playing poorly, the outcome would be roughly 50-50. Because the overlapping area is small and Player 2’s MMR is higher, Player 2 would have a higher probability of winning. Player 2’s curve is wider, indicating that he is prone to more unpredictable outcomes than Player 1. If Player 2 wins, his MMR and curve will shift toward the right and his curve will become narrower, and Player 1’s MMR and curve will shift toward the left and his curve will also become narrower. If Player 1 wins, the two players’ MMRs will move closer together and their curves will overlap more. Player 1’s curve may expand because the outcome was unexpected and his curve was previously narrow, but Player 2’s curve may stay the same or even shrink because he is already unpredictable.
When starting a new team or game type, your MMR from other game types is used as a starting point for your first placement match. After the first placement match, that new team or game type's MMR is separate. This allows players to immediately start playing against players close to their skill level from the very first game.
There are seven leagues: Grandmaster, Master, Diamond, Platinum, Gold, Silver, and Bronze. It is possible to be placed in any league except for Grandmaster, which requires a promotion.
There are two types of placement matches: those where you have no prior MMR history (you play 5 placement matches) and those where your MMR is preserved from either the current or previous season (you play 1 placement match). In the former case, placement is conservative, meaning the system is reasonably confident that you are at least as good as your assigned league, or perhaps better. In the latter case, placement is more accurate using your previously-established value as a starting point.
MMR transcends leagues. That means that even when you are initially placed in a league, your MMR is not necessarily at the bottom of that league. If it takes an average of, say, 20 wins to go from the bottom of one league to the bottom of the next, and your MMR after placement puts you 95% of the way there, then it would take only a single game to reach the next league.
It is believed that when a team is first formed, the MMRs of all members are averaged to form the team's initial MMR during the first placement match. After that first placement match, the team's MMR is separate per normal. For Random Teams, it is likely that the MMRs of all members are averaged first, then an opposing team with a similar MMR (based upon a degree of uncertainty) is located.
By outperforming the rest of your league, it is possible to get promoted into a higher league. Any in-game behaviors or statistics beyond winning, losing, and the opponent's MMR are not relevant to the system. All that is required is for your MMR to cross beyond the MMR boundary of the next league. Some leagues span a wider range of MMR and therefore take more games on average to leave than others.
When you are moved from one league to another, your score is reset to 73 + the amount of bonus pool you have spent so far. This, along with the fact above about MMR transcending leagues, explains why some players early in a season may appear to have over a hundred points after only a couple of games -- they were placed just below the league boundary and were promoted, receiving the 73-point reset.
Demotions are "frozen" in the middle of a season. This means that the only way to be demoted is by dropping below the MMR boundary of your current league and playing a placement match, whether by Leave League or a new ladder season.
The Grandmaster League is a special league consisting of the top 200 players in a region as determined by moving average of MMR. After one week has elapsed in a season, the top 200 players with the highest moving average -- and who have bonus pools below 180 -- will become Grandmasters. The only way to be demoted out of Grandmaster League is to accrue 180 bonus pool.
Slots are reserved for the 200 highest moving average players, but if a player surpasses a reserved player's moving average before the reserved player can win his promotion game at the time the league opens, his slot can be taken.
Players who elect to play Unranked are matched against both Ranked and Unranked players, except in the case of the highest end of the ladder where the player pools are separate. Unranked uses a separate MMR which is seeded by your current Ranked MMR (or vice versa, if you have never played Ranked before). After your first Unranked game, your two MMRs will remain separate.
The last piece of the Starcraft II ladder puzzle is activity. Blizzard always refers to their league populations in terms of "active players", and this is because they want a relatively even spread of players across all leagues (again with the exception of Master which is designed to be smaller). Although the MMR requirements to join each league are constant, the skill level required to move into a higher league will change depending on the currently active population.
Activity in Terms of Bonus Pool
As previously mentioned, the Bonus Pool accumulates at a fixed rate for all players. This allows it to function as an activity metric, and by extension determine whether a player is active or not for the purposes of league apportionment. The lower a player's remaining bonus pool, the more accurate his MMR.
Players who meet either of these requirements are counted toward league population.
There are seven leagues, and the population of each is governed by the current active population:
Grandmaster: Top 200 players based upon moving average of MMR (not represented on the chart because its a unique league)
Master: Top 2% of active players
Diamond: Next 18% of active players
Platinum: Next 20% of active players
Gold: Next 32% of active players
Silver: Next 20% of active players
Bronze: Bottom 8% of active players
The system dynamically distributes the population of active players across a constant range of MMR values, and league boundaries are fixed in relation to MMR. The boundaries are based on MMR values selected by a prior distribution that will capture these certain percentiles. Furthermore, the league boundaries can be adjusted by Blizzard on the fly in the event that populations need to be normalized.
Your active/inactive status is not considered when evaluating you for a promotion, only your position relative to active players. The one additional requirement to getting promoted is that you must play a game, because league changes only occur after a game.
MMR now decays gradually if you go for an extended period without playing a game. The degree of decay ranges from zero to about one league's worth of rating. This allows players to warm up by facing weaker opponents when they return, though it may result in one or two demotions if the inactivity period happens across a season roll.
Seasons last about two months. During a season transition, bonus pools and points are wiped, but MMR carries over to the next season. MMR is only carried over for one season. At the end of a season, an active player's MMR will be carried over to the next. If they do not play any matches in the current season, however, and are not placed, the MMR will not carry over into a third season, effectively starting them fresh in the system.
Special Note: Random Team MMR is linked with 1v1 MMR. This means that if you did not play any 1v1 for one season, but did play a Random Team game, your 1v1 MMR would still carry over to the next season, and vice versa.
A week before the end of a season, promotion and demotion will be disabled. This is known as a "league lock," the purpose of which is to allow players to establish a final division rank for themselves. During a league lock, bonus pools will stop accumulating but players may spend their remaining bonus pools until the end of the season.
At the time of the season rollover, you will receive an end-of-season Feat of Strength based upon your final division rank in each game type and team. For example, it's possible to receive a "1v1 Top 8 Master" Feat of Strength in the same season as a "2v2 Top 16 Diamond" and "2v2 Top 50 Diamond."
Seasonal Placement Match
When the new season begins, all players with a carried over MMR must play a single placement match to rejoin the ladder. Players conduct a single match. The match is primarily to determine which players are still active. One's ranking is based primarily on one's MMR from last season. Seasonal placement matches place players in a league depending on their current MMR following the placement match, just as the initial 5 placement matches when starting a new account, and do not require crossing a confidence buffer. This can mean that some players may find themselves close to the top of their new league, and can be promoted in as little as one game after their placement in the new season.
There are some bugs in the system.
Bonus Pool Suddenly Becomes Very Large, but Unable to Spend Beyond a Certain Amount
As mentioned earlier in the guide, the Bonus Pool accumulates faster in Master league than in the lower leagues. There is a bug where occasionally, a player's Master league bonus pool amount will be shown, rather than their actual consumable pool. This is only a display bug. Relogging usually resolves it.
Pre-game and Post-game Favoritism Discrepancy
Fixed in 2.0.4
The pre-game loading screen does not always agree with the post-game score screen, and may not always be accurate. The accurate information can always be found on the score screen. Blizzard is aware of this bug, but it cannot be consistently reproduced.
The fix for this issue was to remove the pre-match "Favored" display.
Bronze Players Stuck at Zero Points
Fixed in 1.3
There is no upper or lower bound on MMR. As such, it is possible for MMR to go below the point floor. Remember that players are matched with other players who have similar MMRs, meaning that players with sub-Bronze Zero MMR will typically face opponents who also have sub-Bronze Zero MMRs. Because points have a floor of Bronze Zero (and this may not necessarily actually be zero, but rather a certain offset), and because points are compared against opponent MMR, then zero is greater than the opponent's sub-Bronze Zero MMR which means you will always be Favored. This has the effect of players winning very few or losing very many points in every match, basically anchoring them at zero points. The only way to resolve the issue is to win enough games to raise your MMR back into the positive region, and you'll know you're getting close as your points earned per game approach 12 (excluding bonus pool).
NOTE: This post will be updated with additional information as it becomes available.
Thanks to SDream and Vanick for contributions, proofreading, comments, and edits. Also thanks to everyone involved with in-depth analysis, research, and thorough data records, particularly michaelhasanalias, random user, and Mendelfist. Thanks to Shadowed for SC2Ranks.com, without which most of this analysis would not have been possible.
I'm /u/Excalibur_Z on Reddit and Excalibur_Z on Teamliquid.net.