mylolmmr
What is My MMR? How MMR Works in League of Legends - Esports.net
Since Riot keeps MMR hidden, players have to rely on third-party MMR checker tools. Sites like MyLoLMMR and WhatIsMyMMR use special algorithms and collected data to provide educated guesses on players' MMRs. To use these sites, you'll need to input your region and summoner name.
What Is My MMR? League of Legends MMR Checker
These websites are, HappySmurf, MyLolMMR, and WhatIsMyMMR. To check your MMR, simply head into one of these three websites, choose your region, and type your summoner name in the search bar.