Pokémon Go introduces the appraisal system since update 0.35.0 for Android and 1.5.0 for iOS. You can get an appraisal from your team leader and roughly know the IV stats of your favorite Pokémon.
Since this update, the IV calculators just got better by incorporating the in-game appraisal results in its calculation. You can get more accurate IV and no more vague results like 10 or 20 different possibilities of IV for a single Pokémon.
Source: Unlike Aik
To calculate the IV of a Pokémon, open the IV Calculator. Then key-in the CP, HP, and Stardust required to power-up the Pokémon. What follows next is the result of appraisal from your team leader in-game. IV result from the calculation is more reliable since it matches with what your team leader says.