pH calculator
Calculator
The calculator needs JavaScript. The tables below give the same numbers for common cases.
How it is worked out
- pH = −log₁₀[H⁺] and pOH = −log₁₀[OH⁻].
- At 25 °C, pH + pOH = 14 (Kw = 1 × 10⁻¹⁴).
- So pH = 3.5, pOH = 10.5, [H⁺] = 10^−3.5 = 3.162 × 10^-4 mol/L.
| Inputs | Answer |
|---|---|
| You know: pH; Value: 3.5 | pH 3.5 (pOH 10.5) means [H⁺] = 3.162 × 10^-4 mol/L, and at 25 °C the solution is acidic. |
| You know: Hydrogen ion concentration [H⁺] (mol/L); Value: 0.00001 | pH 5 (pOH 9) means [H⁺] = 1 × 10^-5 mol/L, and at 25 °C the solution is acidic. |
| You know: pOH; Value: 2 | pH 12 (pOH 2) means [H⁺] = 1 × 10^-12 mol/L, and at 25 °C the solution is basic (alkaline). |
How to use it
pH is the negative base-10 logarithm of the hydrogen ion concentration in mol/L: pH = −log₁₀[H⁺]. Likewise pOH = −log₁₀[OH⁻]. At 25 °C water has pH + pOH = 14, so pH below 7 is acidic, 7 is neutral and above 7 is basic.
The calculator assumes 25 °C, where Kw = 1 × 10⁻¹⁴. At other temperatures neutral water has a different pH. Strong acid or base concentrations give the pH directly; weak acids need their dissociation constant, which this calculator does not use.
Frequently asked questions
How do you calculate pH?
pH = −log₁₀ of the hydrogen ion concentration in mol/L.
What is the pH of a solution with [H⁺] = 1 × 10⁻⁵ M?
5.
How are pH and pOH related?
At 25 °C, pH + pOH = 14.
Method: standard arithmetic shown step by step; no outside data is used. Checked 2026-11-12.
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General information only. Results are plain arithmetic on the numbers you enter; check anything important (money, health, school work) against your own records or the official source.