Use the formula: pH = -log10[H+]
Measure the hydrogen ion concentration [H+] in moles per liter
Take the base-10 logarithm of [H+]
Multiply the result by -1
If given pOH, use: pH = 14 – pOH
If given hydroxide ion concentration [OH-], use: pOH = -log10[OH-]
Then convert to pH using pH = 14 – pOH
For strong acids and bases, calculate [H+] or [OH-] from concentration first
For weak acids and bases, use the dissociation constant and an equilibrium calculation first