Find a piano to use. Use an online piano if you don't have access to a piano.
Find a note on the piano that you can sing comfortably. Sing "La."
Play the next highest note and sing "La." Continue to play and sing higher notes until you find the highest note you can sing without straining. This is the upper limit of your "singable compass" vocal range.
Continue to play and sing higher notes until you find the highest note you can sing, even when straining. This is the upper limit of your "vocable compass" vocal range.
Take a short break to rest your voice.
Play and sing the original note you could sing comfortably.
Play and sing the next lowest note. Continue to play and sing lower notes until you find the lowest note you can sing without straining. This is the lower limit of your "singable compass" vocal range.
Continue to sing and play lower notes until you find the lowest note you can sing, even when straining. This is the lower limit of your "vocable compass" vocal range.
Write down or notate the upper and lower limits of your vocal range. Determine the notes' relation to middle C. For example, your vocal range may be from the second E below middle C to the first E above middle C.