Question

How do you measure loudness?

Answer 1

I think the correct question would be, how do you measure sound. :)
you measure sound by using amplitude. It measures how forceful the sound is.
Thats how you know whether its loud or quite :)

Answer 2

There are several instruments used to measure loudness. Loudness's unit is "decibel". You can use a microphone and a software to measure loudness. For windows : Free Decibel Reader or Audacity.