Hydrogen Ion Concentration Calculator
Hydrogen ion concentration from pH, and pH from concentration — with pOH, hydroxide concentration and how far it sits from neutral.
pH is −log₁₀[H⁺], so [H⁺] = 10^−pH.
How the hydrogen ion concentration calculator works
pH is −log₁₀[H⁺], so [H⁺] = 10^−pH. Each pH unit is a tenfold change: pH 4 has ten times the hydrogen ions of pH 5 and a hundred times pH 6.
At 25 °C, [H⁺][OH⁻] = 10⁻¹⁴, so pH + pOH = 14. That relationship is what lets one measurement give you both.
Formula: [H⁺] = 10^−pH; pH + pOH = 14
Worked examples
| Inputs | [H⁺] | Note |
|---|---|---|
| pH 3.5 | 0.000316227766 mol/L | 3.16 × 10⁻⁴ mol/L |
| Neutral water | 1.0000e-7 mol/L | 10⁻⁷ mol/L |
| From a concentration | 0.001 mol/L | pH 3 exactly |
FAQFrequently asked questions
What is the hydrogen ion concentration at pH 7?
10⁻⁷ mol/L — one ten-millionth of a mole per litre.
Why is pH logarithmic?
Because concentrations span fourteen orders of magnitude. A linear scale would be unusable.
How much stronger is pH 3 than pH 6?
A thousand times — three pH units is 10³.
Does pH + pOH always equal 14?
Only at 25 °C. The ion product of water changes with temperature, so at 50 °C the neutral point is about pH 6.6.
Can pH be negative?
Yes, for concentrated strong acids. It is awkward to measure but perfectly meaningful.
Where these figures come from
- IUPAC — Standard atomic weights (2021 conventional values) — the molar-mass table
- NIST — CODATA 2018 fundamental physical constants — Avogadro constant, gas constant, speed of light
- NIST Chemistry WebBook — thermochemical data
- Royal Society of Chemistry — the UK chemistry professional body
Last checked: September 2026. Atomic masses are the IUPAC conventional values; constants are CODATA 2018; equations are the standard textbook forms.