pH calculator from H+ concentration
Type [H+] in mol/L, such as 0.001 or 2.5e-4, to get the pH and related values at 25 °C.
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How to use it
- Enter hydrogen ion concentration [h+] (mol/l).
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
pH = −log10[H+]; pOH = 14 − pH; [OH−] = 10^-14 ÷ [H+] (ion product of water at 25 °C).
Example
Inputs: Hydrogen ion concentration [H+] (mol/L) = 2.5e-4. Result: pH: 3.6021; pOH: 10.3979; Hydroxide [OH−]: 4e-11 mol/L.
Limits and caveats
- Assumes 25 °C, where pH + pOH = 14; the sum shifts with temperature.
- Uses concentration rather than activity, so it is less accurate for concentrated solutions.
Questions about this tool
What is the pH when [H+] is 0.001 mol/L?
pH = −log(0.001) = 3, so the solution is acidic.
Can pH be negative?
Yes, when [H+] is above 1 mol/L, as in concentrated strong acids, although activity matters more there.
How do I find the pH of 0.01 M HCl?
A strong acid ionizes fully, so [H+] = 0.01 and pH = 2.