The core idea
Use pH = −log₁₀[H⁺] with concentration in mol/L. For [H⁺] = 1.0 × 10⁻³ mol/L, pH = 3. This relationship uses hydrogen ion activity approximately equal to concentration in dilute solutions.
Try it yourself
Open the pH Calculator and enter your own values. Keep units consistent and compare the answer with the worked explanation.
Check your work
Write down your known quantities, convert units before substitution and confirm that your final units match the quantity you intended to calculate.