This app will be very much useful while you're in a Chess Tournament

This app has two kinds of Rating Calculations
Rating change Calculator
Initial Rating Calculator

Rating change Calculator
If you are already a rated player and facing another rated player in a tournament, the result of your game will affect your rating. It'll be either increased or decreased and sometimes no change. You can calculate how much exact rating will be affected in your current rating with this calculator.
A note on K-Val:
It is the development coefficient.
K = 40 for a player new to the rating list until he has completed events with at least 30 games
K = 20 as long as a player's rating remains under 2400
K = 10 once a player's published rating has reached 2400 and remains at that level subsequently, even if the rating drops below 2400
K = 40 for all players until their 18th birthday, as long as their rating remains under 2300
K = 20 for RAPID and BLITZ ratings for all players.

Initial Rating Calculator
If you don't have a rating and you played against a certain number of rated players and scored some points, then you'll get the rating. That exact rating can be calculated using this Initial Rating Calculator.
For this, you just need to provide the average rating of the rated players, the number of games played and the total points scored