Matchmaking Rating

Matchmaking Rating (also known as "MMR") is a behind-the-scenes value assigned to each player that estimates their performance in comparison to other players. This value is used to place similarly rated players together in an attempt to create a balanced match, where both teams have an approximately equal chance of winning. Matching players by their effectiveness or performance helps make sure that less experienced players aren't always getting stomped by expert players, and that competitors at all levels are earning their victories by battling players of roughly equivalent ability.

Matchmaking Rating in Heroes of the Storm
It has not been revealed exactly what kind of system is used in Heroes of the Storm, but several comments from developers suggest that the Matchmaking Rating in Heroes of the Storm likely uses the same principles as more publicly known rating systems, like and. That is, calculating the changes in players' ratings is based on the skill differences between the two teams, and using a confidence value to describe a player's MMR.

Uncertainty
Uncertainty, as referred to by developers, is used to describe how confident the system is in that rating. In simple terms, this means that MMR is more of a "range" of where the player's MMR is likely to be than just a single number. What people usually think of as "MMR" is the average of this range. When the uncertainty is high and the system doesn't know exactly where they should be, the player's MMR will receive larger adjustments to get them to their correct rating faster.

New players start with high uncertainty, but as matches are played, the system gains more information and the uncertainty decreases. Uncertainty can increase over time if the player has not played any matches lately. Some observations suggest that uncertainty can also increase if the player is experiencing streaks or abnormal winrates for longer amounts of time, but this has not been confirmed by official sources.

Assigning MMR
Every Game Mode tracks its own separate MMR for the player, meaning that games played in one game mode have no effect on the ratings of the other game modes.

Only Quick Match uses a predetermined starting point for MMR. When the player later ventures into other game modes, the system will seed the player's MMR from Quick Match for a more accurate starting point. Storm League prefers to seed from the player's previous Hero League or Team League MMR or from Unranked Draft, but will opt to use Quick Match MMR if not enough matches have not been played in these game modes.

Calculating MMR
Changes in matchmaking rating are entirely based on the player's own rating and the ratings of the other players in the match and the result of the match. Any statistics or accolades, like MVP, in that match have no effect on how much MMR is gained or lost. In terms of Rank Points MMR gained from a match can vary from 1 - 600 points, but should be between 150 and 250 when MMR uncertainty is low (many recent games have been played) except in Bronze 5 or Master, where point gains may be significantly less. When MMR is updated after each match, there are three main factors that define how much your rating changes:

1. Match result
Changes in an individual player's Matchmaking Rating (MMR) are determined by which team won the match. When you win, the MMR of all players in your team goes up, while the opposite is true for when you lose. Except for the very high and low MMR, games in Storm League are observed to award on average an MMR amount equivalent of approximately 200 Rank Points for players with low uncertainty. At extreme ratings, the amount gained is dampened to a particular direction to prevent players from separating too far from the others and facing difficulties finding matches.

2. The difference of the two teams' collective MMR
Before a match the system makes a prediction based on the teams' ratings on the probability for either team to win a match. Ideally, both teams would have an equal chance to win, but many factors may prevent making a completely even match. As such, the team with higher collective MMR is considered "favored" and a minor adjustment is made to the point gains of both teams that is based on the MMR difference of the two.

In case the higher MMR team wins, they will receive slightly less rating points, as they were expected to win the match, and the lower MMR team will also lose slightly less in compensation. Correspondingly, if the team that was unfavored wins, they will gain a bit more points, and the favored team will lose a bit more. In terms of Rank Points, the effect of this is observed to almost always be between -50 to +50 Rank Points.

3. Player's personal uncertainty value
Uncertainty increases the amount of rating a player can gain or lose from the match, and is the cause of bigger MMR and rank adjustments. Usually, only new players and players that have not been playing for months can get to the higher values for a short period of time, as uncertainty decreases relatively fast when games are played.

Matchmaking Rating & Rank
In unranked game modes the player's MMR is kept hidden, and there is no way to observe changes in one's MMR. While Storm League also keeps the actual MMR value hidden, the player's MMR is visualized with Leagues, Ranks and Rank Points. Rank Points are directly mapped to certain MMR bands, meaning that for most purposes Rank Points are the same as MMR, and can only diverge if one has recently been affected by a leaver penalty.

The mapping is not, however, completely linear, especially as seen in Bronze 5, where players receive notably less Rank Points the closer they are to 0. This is mainly due to the MMR range for Bronze 5 being much wider compared to other ranks, and this wider area being "squeezed" within just 1000 Rank Points.

Decay
Players in Storm League who are ranked Diamond 5 1 RP or above are subjective to Rank and MMR decay. Players start each Ranked Season with 10 inactivity days, which can be increased by playing Ranked Matches to up to 30 days. If a player runs out of inactivity days, they will start losing Rank Points and MMR daily until they complete a ranked match or drop out to Diamond 5 0 RP.

Trivia

 * For the 2018 gameplay update the developer team made an attempt at a performance based system to improve the accuracy of the existing rating system. The idea was to use machine learning to compare around 20 different statistics to other players in similar matches to evaluate performance in a match. The addition, however, ended up quickly being indefinitely disabled. It was later revealed that while the system is still collecting data on the background, they are no longer planning to re-release it as an extension to matchmaking, but hoping to reimplement it in the future as a tool for analyzing your own gameplay.