Help playing piano by ear?

[anster98]

Hey so if I try to play piano by ear, it seems I can find the right hand melody fairly simply, but only by individual notes. How can I find out which chords or intervals to play in the melody and then the harmony also?

[schwartzer]

If I understood right, you want to know how to play the left hand chords? If so, that's called harmonic progression. Take a look at articles on wikipedia, they explain a lot there. You can also learn how to read sheet, and I assure you that it is the easiest way. Or if it's a popular song or piece, you can google "song/piece name chords".

[indianajo]

Ear training to hear chord numbers is my guess at the fastest path.  
I backed into training my ear by working through half a "learn to play guitar" book, which is mostly about chords.  I had the chord theory work book supplements as  a child piano student, as far as identifying the chords on a page, but that did nothing for training your ear.  I asked my teacher then to help me play something by ear, (Riders in the Sky. sung by Vaugn Monroe)  and her response was "you don't really want to learn that old thing, do you?"  As if Beethoven Bach and Mozart weren't also old.  So now I am learning to play by ear in my sixties, without a teacher.
When you can hear III, IV, V, VI, and VIIdim cords, you can start to analyze songs you hear on media, (CD's, LP's, internet, MP3) and attempt to play them. I don't have perfect pitch, and still can't pick out what key a song is in just by hearing it.  But I'm beginning to hear the chord numbers, so I can start in an easy key (usually C) and try to reproduce a song by ear. I played a hymn by ear last Sunday in church, in fact.  The arrangement in the hymnal was too tricky and nothing like I remembered the song from summer Bible school.   It worked, another guy in the congregation started telling me about 4th grade parochial school where he learned the song from the teachers (mostly nuns).  
As one gets more experienced, one can try to play in D or A by ear, which are the preferred key signatures of guitar bands.
As far as picking the actual key, your ear can get some idea  from how sweet or vanilla a recording is.  Key signatures with a lot of sharps or flats are very sweet, compared to vanilla key C, due to the effect of even tempering.  For example, a lot of songs by Elton John and George Harrison are written in keys with a lot of sharps, IMHO. Tiny Dancer, Blue Canoe, Good Day Sunshine come to mind as songs with many black notes in them.  

[hmrichter]

To determine what key, try to figure out what the tonic is. You don't need to think of the note, but think of what the home pitch is. Then, you should try to learn each notes pitch because of a piece where that piece is repetitively used (e.g. the e-f motif in the beginning of Winter Wind, or f-a-f-c-f-F motif from the beginning of Bach's Invention 8 in F), and put the two pieces together.