pH Calculator (from [H+])
Find the pH of a solution from its hydrogen ion concentration. Enter the [H+] in moles per litre and the calculator returns pH using pH = −log₁₀[H+].
Formula
pH = -log[H+]
- pH is the negative base-10 logarithm of the hydrogen ion concentration in moles per litre.
- A lower [H+] gives a higher pH (more basic); a higher [H+] gives a lower pH (more acidic).
- Pure water at 25°C has [H+] = 10⁻⁷ M, giving a neutral pH of 7.
- Each whole pH unit represents a tenfold change in hydrogen ion concentration.
- Enter the concentration in molarity (mol/L); for example 0.001 M corresponds to pH 3.
[H+] = 0.001 M
Inputs
- [H+] Concentration (M): 0.001
pH = −log(0.001) = −(−3) = 3, a moderately acidic solution.
Frequently asked questions
How do I calculate pH from [H+]?
Take the negative base-10 logarithm of the hydrogen ion concentration: pH = −log[H+], with concentration in mol/L.
What does a pH of 7 mean?
pH 7 is neutral, corresponding to [H+] = 10⁻⁷ M, the value for pure water at 25°C.
What is the pH range?
The common scale runs 0 to 14: below 7 is acidic, 7 is neutral, and above 7 is basic. Very strong solutions can fall outside this range.
How much does one pH unit change concentration?
Each whole pH unit is a tenfold change in [H+]. A solution at pH 3 has ten times the [H+] of one at pH 4.
Can I find [H+] from pH?
Yes — reverse the formula: [H+] = 10^(−pH). For pH 3, [H+] = 10⁻³ = 0.001 M.