Paste in a song and move it to a key that suits your voice. Chord lines and anything in square brackets get transposed. The words stay exactly where they are.
What it recognises
Two formats. Either chords sitting on their own line above the lyrics, the way most song sheets are laid out, or chords in square brackets inline with the words, like [C]this [G]way. Lines that contain ordinary words are left alone, so a lyric with the word a or be in it will not be mistaken for a chord.
Sharps or flats?
The same note has two names, and which one is correct depends on the key. F sharp and G flat are the same fret. Use the flats button when the key you are moving to is a flat key, which in practice means F, B flat, E flat and A flat.
If you would rather not retune or re-learn shapes, the capo calculator does the same job with a capo.