Today I’d like to discuss few key questions that a lot of you might be wondering: What is the algorithm behind this website MMR calculator? And on top of all, how accurate is it?
First thing first, How do you check your MMR? I will go through stepbystep on how to check it
How to check your MMR?
 Go to lolmmr.com, enter your summoner names and ensure your region are correct
 You will be directed to your profile page. There are several sections here, but the section of our interest is the top most parts under Summoner Skill Level. Depending on your most recent games, you might be presented with Ranked 5×5, Ranked 3×3 or Normal. If none of this exist in your recent games, the entire section would not appear at all.
 Then click the MMR calculator buttons. Your estimated MMR / ELO (or what the site used, Skill points) is the number at the bottom.
The Algorithm Behind our MMR calculator
The algorithm that we used might be slightly different from other website that you might have used. It is optimized to adopt with the new Dynamic queue. And this numbers represents your relative skills that Match Making System has scored you with.
I have to say though, the site algorithm will not represents the actual number of what Riot Games is using. It is merely of estimations.
Back in previous post where we discussed about what mmr and elo is, we discussed about how they are adopted from Chess elo points. League of Legends is not the first to adopt it, it has been used n alot of player vs player games, and its true purpose is to estimate relative skill level of a player.
Performance rating is a hypothetical rating that would result from the games of a single event only. Some chess organizations use the "algorithm of 400" to calculate performance rating. According to this algorithm, performance rating for an event is calculated in the following way:

Add 400 to the rating of each player beaten,

Subtract 400 from the rating of each player lost to,

Add the rating of each player drawn,

Divide this sum by the number of played games. (source: wikipedia)
Based on the description, in every games, each of the players will placed 400 points as a bet. Depending on the results, who ever wins the games will be able to claim the points that is bet in the game.
League of Legends might and might not used the same exact calculations. There are no publicly released algorithm that is known so far. So the key questions is, how do we figure out the score?
There are few ways we could do this,
 Try to figure out the algorithm,
 Used the League and Tier to recalculate based on tag line.
The first method is a bit tricky, especially with trying to figure out an algorithm we have not seen yet and a scoring tag that is 45 years old (elo was made visible in Season 1 and Season 2).
And the result may ends up in 2 possible ways: 1) You might be precisely correct OR 2) You might be precisely wrong.
If you look at the diagram below on Precision vs Accuracy, which one do you rather be? Roughly the right answer or Mising the points entirely?
So rather than trying to figure out what the algorithm is, the lolmmr.com used a baseline marker to estimate the points (per Tier and Division).
And this baseline marker or reference points are based on the players (individual player of 10 in each games) Tier and League and will be used to compare with reference score.
We have understood that, mmr is ‘relative’ skill level, and the key to actually put meaning into the number is to actually be consistent with the reference / baseline number. There’s no point of trying to make the number accurate, but what matter is, what does it mean?
With being consistent, it allows other possibilities, like tracking progress, and forecast.
MMR Calculator baseline per Tier and Division
BRONZE V: 0 – 940
BRONZE IV: 940 – 1010
BRONZE III:1010 – 1080
BRONZE II : 1080 1150
BRONZE I: 1150 – 1220
SILVER V: 1220 – 1290
SILVER IV: 1290 1360
SILVER III: 1360 1430
SILVER II: 1430 – 1500
SILVER I: 1500 1570
GOLD V: 1570 1640
GOLD IV:16401710
GOLD III: 17101780
GOLD II: 17801850
GOLD I: 18501920
PLATINUM V: 19201990
PLATINUM IV: 19902060
PLATINUM III: 20602130
PLATINUM II: 21302200
PLATINUM I: 21302270
DIAMOND V: 22702340
DIAMOND IV: 2340 2410
DIAMOND III: 24102480
DIAMOND II: 24802550
DIAMOND I: 25502600
MASTER I: 26002900
CHALLENGER I: 29004500