Standard tuning for a 5-string banjo is "G" tuning, which means that strumming all the strings "open", that is without fretting any of them, produces a G chord.

If you have a piano handy...


                     "Middle C"
                          |
                          V
           | # # | # # # | # # | # # # | # # | # # # |
           | # # | # # # | # # | # # # | # # | # # # |
           | # # | # # # | # # | # # # | # # | # # # |
           | | | | | | | | | | | | | | | | | | | | | |
          _|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_|_
              ^     ^   ^   ^     ^
              |     |   |   |     |
             4th   3rd 2nd 1st   5th String

              D     G   B   D     G

(The "1st" string is the string closest to the ground, the "5th" string is the one closest to the sky when holding the banjo normally.)

If you don't have a piano handy, but you've got a guitar that's tuned in the usual way, or you've got a pitch pipe meant for tuning guitars, the 2nd, 3rd, and 4th strings of the banjo are tuned the same as the 2nd, 3rd, and 4th strings of the guitar.

Once the 4th string is tuned, the 3rd string is tuned to the same pitch as the 4th string fretted at the 5th fret.

The 2nd string is tuned to the pitch of the 3rd string at the 4th fret.

The 1st string is tuned to the pitch of the 2nd string at the 3rd fret.

The 5th string is tuned to the pitch of the 1st string at the 5th fret.

Some other common tunings:

C tuning: From low to high: C, G, B, D, G

C tuning is the same as G tuning, except the 4th string is tuned down a whole step (two frets). So, the 3rd string will sound the same pitch as the 4th string fretted at the 7th fret.

D tuning: From low to high: D, F#, A, D, F#

The 4th string is tuned to the same D as in G tuning.

The 3rd string sounds as the 4th string at the 4th fret.

The 2nd string sounds as the 3rd string at the 3rd fret.

The 1st string sounds as the 2nd string at the 5th fret.

The 5th string sounds as the 1st string at the 4th fret.