The IV Calculator is a tool for advanced Pokemon Trainers to help determine the Individual Values for each of your individual Pokemon's stats. To work this calculator, you should ensure that your Pokemon are freshly caught or hatched, meaning they have no EVs (Effort Values) yet. If you try to use this tool on Pokemon that you've trained for awhile without keeping track of their EVs, it will not work properly, as this tool is for advanced players who understand these concepts.
To work this tool, first select the Pokemon you want to determine its IVs, then input its starting level, nature, and optionally its characteristic or Hidden Power type (if you know it). Then, in the table below, input the Pokemon's stats at a level and it will tell you the range of IVs it can have. You can input multiple levels worth, too, to help narrow down the results until you have determined the Pokemon's IVs.