pH Calculator
Calculate pH from hydrogen ion concentration, or go the other way from a pH value back to [H⁺], in mol/L.
pH = -log₁₀[H⁺]. Below 7 is acidic, 7 is neutral, above 7 is alkaline.
How to calculate pH by hand
- Find the hydrogen ion concentration [H⁺] in mol/L.
- Take its base-10 logarithm.
- Flip the sign: pH = -log₁₀[H⁺].
Worked example
A solution has [H⁺] = 0.0001 mol/L (10⁻⁴). pH = -log₁₀(10⁻⁴) = 4, a fairly strong acid. Going the other way, a solution with pH 10 has [H⁺] = 10⁻¹⁰ mol/L, strongly alkaline.
Test real acids, bases and indicators and watch the colour change with pH in the Chemistry Lab.
Frequently asked questions
How do you calculate pH?
pH = -log₁₀[H⁺], where [H⁺] is the hydrogen ion concentration in mol/L. A concentration of 0.001 mol/L gives a pH of -log₁₀(0.001) = 3.
How do you find [H⁺] from pH?
Rearrange the formula to [H⁺] = 10⁻ᵖᴴ. A pH of 4 gives a hydrogen ion concentration of 10⁻⁴ = 0.0001 mol/L.
What pH values are acidic, neutral or alkaline?
Below 7 is acidic, exactly 7 is neutral (pure water at 25°C), and above 7 is alkaline (basic). The scale usually runs from 0 to 14.
Why does a lower pH mean a stronger acid?
pH is a log scale running in the opposite direction to concentration: a lower pH means a higher [H⁺], and each whole pH step is a 10-times change in concentration.
