League of Legends MMR checkers are third-party systems that allow you to check your MMR, which is an invisible number associated with your rank.
While League of Legends gives you a rank inside their competitive mode, your MMR tells you how many points you will win or lose depending on the ranking in which you are placed.
League of Legends MMR Checker: Where You Can Check Your MMR
Just like most third-party systems, there are a few from which to choose. Whichever one you choose, they do the same thing, estimate your MMR. It will help you understand whether you're in a ranking too high for your MMR or too low. So relax, if you're losing a lot of points after losses, it's because the system thinks you don't belong!
Photo courtesy of Riot Games