BYTETOOLS

pH Calculator

Convert between pH, pOH, [H+] and [OH-], or find the pH of a weak acid or base from its Ka, Kb and concentration. Shows every step of the working.

3.000
pH
11.000
pOH
1.000e-3
[H⁺] mol/L
1.000e-11
[OH⁻] mol/L

Acidic

  • pH = −log₁₀[H⁺]
  • pOH = −log₁₀[OH⁻]
  • pH + pOH = 14.00 (water at 25 °C, Kw = 1.0 × 10⁻¹⁴)

0 acidic7 neutral14 basic

All values assume dilute aqueous solution at 25 °C, where Kw = 1.0 × 10⁻¹⁴ and pH + pOH = 14.00. Kw changes with temperature, so neutral pH is about 6.14 at 100 °C. Activity coefficients are ignored, which matters above roughly 0.1 mol/L.

What is the pH Calculator?

pH is the negative base-10 logarithm of the hydrogen ion concentration: pH = −log₁₀[H⁺]. At 25 °C pH and pOH always add up to 14, so a solution with [H⁺] = 1 × 10⁻³ mol/L has pH 3 and pOH 11.

  • Two-way conversion between pH, pOH, [H⁺] and [OH⁻]
  • Weak acid and weak base modes that solve the Ka quadratic exactly
  • Presets for acetic, formic, carbonic and other common acids and bases
  • Percent ionisation and acidic/neutral/basic classification
  • Scientific notation accepted for very small concentrations
  • Runs locally in your browser — nothing is uploaded

How to use the pH Calculator

  1. 1

    Choose convert mode, weak acid mode or weak base mode.

  2. 2

    In convert mode, pick whether you know pH, pOH, [H⁺] or [OH⁻] and enter it.

  3. 3

    In weak acid or base mode, enter the concentration and a Ka/pKa (or pick a preset).

  4. 4

    Read pH, pOH, [H⁺] and [OH⁻] together with the classification.

  5. 5

    Copy the result and the working for your notes.

About the pH Calculator

The ByteTools pH Calculator converts freely between pH, pOH, hydrogen ion concentration and hydroxide ion concentration. Enter any one of them and the other three appear immediately, along with an acidic, neutral or basic classification and a colour scale showing where the solution sits.

A weak acid mode solves the equilibrium properly rather than assuming full dissociation. Give it a concentration and either Ka or pKa and it solves the quadratic x² + Ka·x − Ka·C = 0 exactly for [H⁺], then reports the pH and the percentage ionised. A matching weak base mode does the same with Kb. Common acids and bases are available as presets.

The maths runs entirely in your browser in JavaScript, so nothing you enter is uploaded or saved. Results are estimates based on ideal dilute solutions at 25 °C — they are for study and lab planning, not for clinical or safety decisions.

Frequently asked questions

How do you calculate pH from hydrogen ion concentration?

Take the negative base-10 logarithm of the concentration in mol/L. A solution with [H⁺] = 1 × 10⁻³ has pH = −log₁₀(0.001) = 3. Going the other way, [H⁺] = 10^(−pH).

What is the pH of 0.1 M acetic acid?

About 2.88. Acetic acid is weak, with a Ka of roughly 1.8 × 10⁻⁵, so only about 1.3% of it ionises. Solving the equilibrium quadratic gives [H⁺] ≈ 1.33 × 10⁻³ mol/L, which corresponds to pH 2.88.

Why do pH and pOH add up to 14?

Because water self-ionises with Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ at 25 °C. Taking negative logarithms of both sides gives pH + pOH = 14.00. Kw rises with temperature, so at 100 °C the sum is closer to 12.3.

Is a pH of 7 always neutral?

Only at 25 °C. Neutral means [H⁺] equals [OH⁻], and because Kw changes with temperature so does the neutral point — around 6.14 in boiling water. Pure water at 100 °C is still neutral even though its pH is below 7.

How accurate is this weak acid calculation?

It solves the equilibrium expression exactly instead of using the common √(Ka·C) shortcut, so it stays accurate even when the acid is fairly strong or very dilute. It does ignore activity coefficients and the contribution of water, which matters above roughly 0.1 mol/L and below about 10⁻⁶ mol/L.

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