The pH scale is estimated from the concentration of hydrogen ions.

pH = - log [H+]

For strong acids and bases, [H+] can be directly derived from the concentration of the acid or base.

For weak acids and bases, the [H+] needs to be calculated from the Ka

Ka = ( [H+] [A¯] ) / [HA]

For buffers, the pH can be calculated using the Henderson-Hasselbalch equation.