Why some new member start with 800, 1200 or 1800 in rating?


When I registered, I was only 500 but when my friend registered, he was 1200. I don't understand why?

you start at different ratings because it depends what level you choose if you choose beginner you get 800 intermediate =1200 advanced 1600

When you register a new account on chess.com it asks you whether you are a beginner, intermediate, or advanced player and gives you a starting rank based on what you state:
beginner-800
intermediate-1200
advanced-1800
Cheers 🍻 now I learned something new 👍🏻

this is unfair, isn't it?
Yes, but not in the way you think.
Let's say you (started at 500) and your friend(started at 1200) both have a skill level of 850.
It takes about 20 games in the pool for the rating to stabilize around your skill level (+- 100).
So after 20 games, you'll both be rated between 750 and 950, likely close to 850.
But most of these 20 (or at least the first 5-10) games were encouraging wins for you, whereas your friend lost like 8 out of their first 10 games.
In the end, it doesn't matter: After 50-100 games or so, mathematically any influence of the starting rating is gone.

this is unfair, isn't it?
Yes, but not in the way you think.
Let's say you (started at 500) and your friend(started at 1200) both have a skill level of 850.
It takes about 20 games for the rating to stabilize around your skill level (+- 100).
So after 20 games, you'll both be rated between 750 and 950, likely close to 850.
But most of these 20 games were encouraging wins for you, whereas your friend lost like 8 out of their first 10 games.
In the end, it doesn't matter: After 50-100 games or so, mathematically any influence of the starting rating is gone.
for +/-100 accuracy you'll need at least 50 games
after just 20 ratings still could be like 1000 vs 600

for +/-100 accuracy you'll need at least 50 games
after just 20 ratings still could be like 1000 vs 600
The rating changes will be much larger during the first games of an account. For example, in actual reality, @Aadharshan_Sigma-GM won 71 points with his first game. Starting at 500, he reached 700 after only 3 games.

I'm not about to open a new account and find out for myself (the powers that be frown on multiples), but I thought I read that folks could choose where to start. Any recent members care to share?
You can choose where to start at when making a new account. You can choose between: New to Chess(400), Beginner(800), Intermediate(1200), and Advanced(1500), but I don't know how people start at 1800.

for +/-100 accuracy you'll need at least 50 games
after just 20 ratings still could be like 1000 vs 600
The rating changes will be much larger during the first games of an account. For example, in actual reality, @Aadharshan_Sigma-GM won 71 points with his first game. Starting at 500, he reached 700 after only 3 games.
I know. RD causes rating change to be large at first. Still 20 games ins't enough to approach 850 +/-100 with such starting rating difference between equally strong players.

I know. RD causes rating change to be large at first. Still 20 games ins't enough to approach 850 +/-100 with such starting rating difference between equally strong players.
Again, in actual reality, if he had won his fourth game, 4 games would have sufficient for @Aadharshan_Sigma-GM to be within 100 points of 850.
So I'm not sure why you think 20 or more games would be required.
Note that the two players are not playing each other, but the pool, i.e. players roughly at their rating level.

I know. RD causes rating change to be large at first. Still 20 games ins't enough to approach 850 +/-100 with such starting rating difference between equally strong players.
Again, in actual reality, if he had won his fourth game, 4 games would have sufficient for @Aadharshan_Sigma-GM to be within 100 points of 850.
So I'm not sure why you think 20 or more games would be required.
Note that the two players are not playing each other, but the pool, i.e. players roughly at their rating level.
Please do not use such words as "actual reality" when you're talking about your guesses. Demonstrate it with actual calculations first. What you are saying is impossible.

Please do not use such words as "actual reality" when you're talking about your guesses. Demonstrate it with actual calculations first. What you are saying is impossible.
I am not talking about guesses. I am talking about things that happened in the past, on chess.com.
As @Aadharshan_Sigma-GM said, he started with a rating of 500.
This is his first game, which he won: https://www.chess.com/game/live/108097658988 . Afterwards, he was rated 571.
This is his second game, which he won: https://www.chess.com/game/live/108097684874 . Afterwards, he was rated 683.
This is his third game, which he won: https://www.chess.com/game/live/108097749588 . Afterwards, he was rated 776 – within 100 points of 850.
So I was wrong before; only 3 games were necessary to get within 100 points of 850.
In his fourth game, he lost 79 rating points against a 50 point higher-rated opponent. If he had won, he would have won roughly 100 rating points, getting him to a rating of roughly 876 – pretty close to, and above 850!