The definitive answer is 'pH = -log(10) [H^(+)]

In Words, ' pH is the negative logarithm, to the base ten, of the hydrogen ion concentration.

As an example

If H^(+) is 0.002

Then

pH = -log(10)[0.002]

On the calculator using the 'log' button

pH = -(-2.69897...)

pH = 2.69897...

pH = 2.70

NB This would be an acidic pH

NNB pH is always quoted to 2 d.p. only.

NNNB Do NOT use the 'ln' button on a calculator.