Individual values, abbreviated as IVs, are Pokémon's equivalent of genes, and help determine its stats. IVs range from 0 to 31. This tool will calculate the possible range of IVs of a Pokémon given its Species, Level, Nature, and EVs (Effort Values).
More accurate results will be possible with a higher levelled Pokémon, though Level 20 is usually sufficient. However, correct EV values are essential. A Pokemon that has just been caught has all its EVs at 0. If you don't know the EVs of your Pokémon, consider using Berries or the Sweets Set and Secret Meals from high ranked Food Stalls in Festival Plaza to reduce your EVs to 0.