To get a NTRP rating, first get a USTA membership. It is $44 a year for those over 18. It is free for those under 18.
If you have never had a rating, you will need to self-rate. Information on how to do is below. You can appeal if you feel the rating is wrong.
NTRP ratings are updated every year on December 1st and players find out if they have been moved up or down based on their performance. During the first year, a player has a S next to their rating which indicates self-rating and they are at risk of being disqualified if they have not rated properly. When December 1st comes, they will receive a C and be computer rated and will be safe from disqualification.
The most classic rating system in the United States (all countries on different systems) is the NTRP system and most players who have played for a bit have some idea of their approximate rating. This is on a 7 pt scale in .5 increments where a 1.0 is brand new to the game and 7.0 is the best player in the world. Men and women are on separate scales and usually are around .5 to .7 different so a 4.0 woman would be roughly equivalent to a 3.5 man. Players have one ranking regardless of how much singles or doubles they play.
A new player should be able to get to a 2.5 within a year. It is the minimum level to play league and tournament play. Every .5 after that is difficult to achieve and can take years to attain. Many never get beyond a 3.0 or 3.5. A #1 high school varsity singles player is usually around a 4.0. A #1 high school doubles player is usually around a 3.5. A low level varsity player is usually around a 3.0. Less than 10% of players attain a 4.5. Most people playing 4.5 and above have usually played in college or high juniors. For adult recreational leagues, 5.0 is effectively the highest rating and there are few players playing at that level. Juniors (below 18) are a different system and are usually classified by the type of ball they play with, a level, or a junior ranking system.
Universal Tennis Rating (UTR) is gaining in popularity throughout the world and is primarily used to evaluate serious Junior players and by Colleges to evaluate potential players. Men and women are on the same scale so a 6 UTR man should be about the same as a 6 UTR woman. This is on a scale of 1 to 16.5 with 1 being a beginning competitive player (NTRP 2.5) and 16.5 being the best male player in the world. Players are given a separate rating for singles and doubles. You can search for any player in the world at Tennis & Pickleball Ratings and Events Platform | UTR Sports (universaltennis.com). At the time of this writing, Novak Djokovic (11/27/23) is 16.4 in singles and 14.6 in doubles. Iga Swiatek is 13.3 in singles and 12.55 in doubles. This system is used throughout the world. UTR scrapes data from USTA events and also runs their own UTR leagues and tournaments.
The International Tennis Federation's (ITF) answer to UTR was to create the World Tennis Number (WTN) as a competitor. Men and women are on the same scale so a 10 WTN man should be about the same as a 10 WTN woman. This is on a scale of 1 to 40 with 1 being a top professional and 40 being a beginner. Players are given a separate rating for singles and doubles. You can search for players at World Tennis Number - Powered by ITF. This number is used by ITF events and is increasingly be used by the USTA.