Hearing loss is described by varying degrees, not percentages. It can be be mild, moderate, moderately-severe, severe, or profound and vary across pitches. The type and degree of hearing loss is determined by a simple hearing test, and is measured as the amount of volume loss you experience compared to normal averages.
The volume, or intensity, of sounds you hear is measured in decibels (dB), with 0 dB being the softest whisper and 120 dB being a jet engine. The softest sounds one can hear are called thresholds. Normal hearing thresholds for adults are considered 0 to 25 dB.