The Interval Calculator names the interval between any two notes, with octaves. It gives the full name and short form, the number of half steps, the quality (perfect, major, minor, augmented or diminished), the inversion, whether it sounds consonant or dissonant, the just-intonation ratio, and a well-known song that opens with that interval going up or down. Play the two notes one after the other or together to train your ear.
Below the result is a reference chart of all thirteen intervals from your first note, useful for ear-training practice and theory homework alike.