acalculator

What is the pH?

Type any one of pH, pOH, the hydronium ion concentration [H₃O⁺] or the hydroxide ion concentration [OH⁻]. The pH calculator finds the other three for a water solution at 25 °C and says whether it is acidic, neutral or basic.

Your numbers

pH
2.92

The pH is 2.92: the solution is acidic.

pOH
11.08
[OH⁻] (mol/L)
0.00000000000833
The solution is
acidic

pH: 2.92. The pH is 2.92: the solution is acidic.

How to calculate

Finds pH, pOH, the hydronium ion concentration [H₃O⁺] and the hydroxide ion concentration [OH⁻] of a water solution at 25 °C from any one of them, and says whether it is acidic, neutral or basic.

Example with the default inputs ([H₃O⁺] (mol/L) 0.0012): The pH is 2.92: the solution is acidic.

Formula: pH = −log₁₀[H₃O⁺]; pOH = −log₁₀[OH⁻]; pH + pOH = 14.00 at 25 °C, so [H₃O⁺] = 10^−pH and [OH⁻] = 10^−pOH.

  • A water solution at 25 °C, where Kw = [H₃O⁺][OH⁻] = 1.0 × 10⁻¹⁴ and pH + pOH = 14.00.
  • Concentrations stand in for activities, as in introductory chemistry; very strong solutions differ.
  • pH and pOH from −2 to 16; concentrations from 10⁻¹⁶ to 100 mol/L.

Machine-readable copies: Markdown, JSON.

Worked examples

Each example is checked against the calculator on every build.

  1. [H₃O⁺] (mol/L) 0.0012 gives pH 2.920819, pOH 11.079181, The solution is acidic.Source: OpenStax, Chemistry 2e, §14.2 pH and pOH (pH = −log[H₃O⁺], pOH = −log[OH⁻], pH + pOH = 14.00 at 25 °C; Example 14.4: [H₃O⁺] = 1.2 × 10⁻³ M gives pH 2.92; Example 14.5: pH 7.3 gives 5 × 10⁻⁸ M; Example 14.6: 0.0125 M KOH gives pOH 1.903 and pH 12.10), https://openstax.org/books/chemistry-2e/pages/14-2-ph-and-poh (retrieved 2026-10-02)
  2. pH 7.3 gives [H₃O⁺] (mol/L) 0, pOH 6.7, The solution is basic.Source: OpenStax, Chemistry 2e, §14.2 pH and pOH (pH = −log[H₃O⁺], pOH = −log[OH⁻], pH + pOH = 14.00 at 25 °C; Example 14.4: [H₃O⁺] = 1.2 × 10⁻³ M gives pH 2.92; Example 14.5: pH 7.3 gives 5 × 10⁻⁸ M; Example 14.6: 0.0125 M KOH gives pOH 1.903 and pH 12.10), https://openstax.org/books/chemistry-2e/pages/14-2-ph-and-poh (retrieved 2026-10-02) (5 × 10⁻⁸ M to one figure)
  3. [OH⁻] (mol/L) 0.0125 gives pOH 1.90309, pH 12.09691, The solution is basic.Source: OpenStax, Chemistry 2e, §14.2 pH and pOH (pH = −log[H₃O⁺], pOH = −log[OH⁻], pH + pOH = 14.00 at 25 °C; Example 14.4: [H₃O⁺] = 1.2 × 10⁻³ M gives pH 2.92; Example 14.5: pH 7.3 gives 5 × 10⁻⁸ M; Example 14.6: 0.0125 M KOH gives pOH 1.903 and pH 12.10), https://openstax.org/books/chemistry-2e/pages/14-2-ph-and-poh (retrieved 2026-10-02)
  4. [H₃O⁺] (mol/L) 0 gives pH 7, pOH 7, [OH⁻] (mol/L) 0, The solution is neutral.Source: OpenStax, Chemistry 2e, §14.2 pH and pOH (pH = −log[H₃O⁺], pOH = −log[OH⁻], pH + pOH = 14.00 at 25 °C; Example 14.4: [H₃O⁺] = 1.2 × 10⁻³ M gives pH 2.92; Example 14.5: pH 7.3 gives 5 × 10⁻⁸ M; Example 14.6: 0.0125 M KOH gives pOH 1.903 and pH 12.10), https://openstax.org/books/chemistry-2e/pages/14-2-ph-and-poh (retrieved 2026-10-02)

How it works

At 25 °C, for a water solution:

  • pH = −log₁₀[H₃O⁺], so [H₃O⁺] = 10^−pH
  • pOH = −log₁₀[OH⁻], so [OH⁻] = 10^−pOH
  • pH + pOH = 14.00, because Kw = [H₃O⁺][OH⁻] = 1.0 × 10⁻¹⁴

Type any one of the four values and the other three follow. Concentrations are in moles per liter (M). The solution is acidic when pH < 7, neutral when pH = 7 (within 10⁻⁹), and basic when pH > 7. The label uses the full value, so pH 7.004 shows as 7 (2 decimals) but reads basic.

Rules

  • pH and pOH are from −2 to 16; [H₃O⁺] and [OH⁻] are from 10⁻¹⁶ to 100 mol/L. A typed value outside these gets a field message; a value worked out outside them gives no answer.
  • Concentrations stand in for activities, as in introductory chemistry.

Output format. pH and pOH with up to 2 decimals; concentrations to at most 3 significant figures (for example 0.0012 or 5.01e-8).

Worked examples by hand

[H₃O⁺] = 1.2 × 10⁻³ M (stomach acid). pH = −log(0.0012) = 2.92; pOH = 14.00 − 2.92 = 11.08. Acidic.

pH 7.3 (blood). [H₃O⁺] = 10^−7.3 = 5.01 × 10⁻⁸ M; pOH = 6.7. Basic (just above 7).

[OH⁻] = 0.0125 M (KOH). pOH = −log(0.0125) = 1.903; pH = 14.00 − 1.903 = 12.10. Basic.

[H₃O⁺] = 1.0 × 10⁻⁷ M (pure water). pH = 7, pOH = 7, [OH⁻] = 1.0 × 10⁻⁷ M. Neutral.

Other questions people ask

How do I calculate pH from the hydrogen ion concentration?

Take the negative base-10 logarithm: pH = −log[H₃O⁺]. For stomach acid with [H₃O⁺] = 1.2 × 10⁻³ M, pH = −log(0.0012) = 2.92. [H⁺] and [H₃O⁺] mean the same thing here.

How do I get the concentration from the pH?

Raise 10 to the power −pH: [H₃O⁺] = 10^−pH. Blood at pH 7.3 has [H₃O⁺] = 10^−7.3 ≈ 5.0 × 10⁻⁸ M.

How are pH and pOH related?

At 25 °C, water gives [H₃O⁺] × [OH⁻] = 1.0 × 10⁻¹⁴, so pH + pOH = 14.00. A 0.0125 M KOH solution has pOH = −log(0.0125) = 1.903 and pH = 14.00 − 1.903 = 12.10.

What pH is acidic, neutral or basic?

At 25 °C a solution is acidic below pH 7, neutral at pH 7 (pure water, [H₃O⁺] = 1.0 × 10⁻⁷ M), and basic or alkaline above pH 7.

How do I find the pH of a strong acid or base?

A strong acid such as HCl gives up all its H⁺, so [H₃O⁺] equals the acid’s concentration: type it as [H₃O⁺]. A strong base such as NaOH or KOH gives [OH⁻] equal to its concentration: type it as [OH⁻]. A weak acid needs its Ka, which this page does not use.

Can pH be negative or above 14?

Yes, for very concentrated strong acids (above 1 M) or bases. This page accepts pH from −2 to 16, though at such strengths the simple concentration formula is only approximate.

Does temperature change the answer?

Yes. Kw changes with temperature, so pH + pOH = 14.00 holds only at 25 °C. This page uses the 25 °C value.