How do you calculate the pH of HCl?
pH = -log[H+]
This means you take the negative log of the hydrogen ion concentration to find the pH.
The hydrogen ion concentration is the same as the concentration of the acid because HCl is a strong acid and dissociates as follows:
A 1M HCl solution has a pH of 0
A 0.1M HCl solution has a pH of 1
a 0.01M HCl solution has a pH of 2
This video discusses additional examples