What is the unit to measure the loudness of sound?

Loudness is the quality of a sound that is primarily a psychological correlate of physical strength or amplitude. You can try to measure the sound pressure in decibels.

Scroll down to related links and look at "Loudness in sones - Wikipedia ", Loudness level in phon - Wikipedia ", and Subjectivly sensed loudness (volume), objectively measured sound pressure (voltage).