pH Calculator
pH, pOH and hydrogen ion concentration.
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pH measures how acidic or basic a solution is on a logarithmic scale. Enter a hydrogen ion concentration, a pH, or a pOH and this calculator converts between all of them, telling you at a glance whether the solution is acidic, neutral, or basic.
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pH — The Quick Answer
pH = −log₁₀[H⁺]. A pH below 7 is acidic, exactly 7 is neutral (pure water at 25 °C), and above 7 is basic. pH and pOH always add up to 14 at room temperature.
Shortcut
pH = −log₁₀[H⁺] | [H⁺] = 10^(−pH) | pH + pOH = 14
pH values of common substances
| Substance | pH |
|---|---|
| Battery acid | ~1 |
| Lemon juice | ~2 |
| Black coffee | ~5 |
| Pure water | 7 |
| Baking soda | ~9 |
| Soapy water | ~12 |
| Bleach | ~13 |
The scale is logarithmic, so a change of one pH unit means a tenfold change in hydrogen ion concentration. Stomach acid at pH 1 has a million times more H⁺ than water at pH 7.
How It Works
The Formulas
Logarithms of concentration
Formula
pH = −log₁₀[H⁺] [H⁺] = 10^(−pH) pH + pOH = 14
Variables
pH
Negative base-10 logarithm of the hydrogen ion concentration, in mol/L. Runs from about 0 (strongly acidic) to 14 (strongly basic).
Hydrogen ion concentration
Moles of hydrogen ions per litre. Pure water has about 1 × 10⁻⁷ mol/L, giving pH = 7.
pOH
The same idea for hydroxide ions, [OH⁻]. At 25 °C, pH and pOH sum to 14.
Note: The '14' comes from the ion product of water, Kw = 1 × 10⁻¹⁴ at 25 °C. Kw changes with temperature, so the sum shifts a little at very high or low temperatures.
Last updated: October 6, 2026
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Frequently Asked Questions
How do I calculate pH from hydrogen ion concentration?
Take the negative base-10 logarithm of the concentration: pH = −log₁₀[H⁺]. For [H⁺] = 0.001 mol/L (1 × 10⁻³), pH = −log₁₀(10⁻³) = 3, a fairly acidic solution.
What is the relationship between pH and pOH?
At 25 °C they always sum to 14: pH + pOH = 14. So a solution with pH 4 has a pOH of 10, and vice versa.
Is pH 7 always neutral?
Only around 25 °C. Neutral means [H⁺] equals [OH⁻], which happens at pH 7 at room temperature. At higher temperatures water ionises more, so neutral shifts slightly below 7 — pH 7 is still neutral for everyday purposes.
Why is the pH scale logarithmic?
Because hydrogen ion concentrations span many orders of magnitude — from about 10 mol/L in strong acid down to 10⁻¹⁴ mol/L in strong base. A log scale compresses that enormous range into the handy 0–14 band.