Enter your Pokémon's type(s), one of their moves' types, and the type(s) of the defending Pokémon to find out the multiplier of that move. It even works for Fairy-types. And the best part is that this doesn't use a million 'if' blocks for every situation. It simply uses two lists (look inside for the details).
Made using information from pokemondb.net/type. This was made for Pokémon Land and Sea, but feel free to use it for anything else (just give me credit).