pH and pOH calculator
Calculate pH, pOH and the OH⁻ concentration from the H⁺ ion concentration.
pH < 7 is acidic, = 7 neutral and > 7 basic. At 25 °C, pH + pOH = 14.
How the calculation works
pH measures a solution's acidity on a 0 to 14 scale, from the concentration of H⁺ ions:
pH = −log₁₀[H⁺]
- pH < 7: acidic (lemon juice ~2, vinegar ~3)
- pH = 7: neutral (pure water)
- pH > 7: basic/alkaline (soap ~9, bleach ~12)
The pOH complements it: pOH = 14 − pH (at 25 °C).
The scale is logarithmic
Each pH point means 10× more (or less) acidity. pH 4 is ten times more acidic than pH 5.
Examples
- [H⁺] = 10⁻³ mol/L → pH 3 (acidic).
- pH 9 → pOH 5 → [OH⁻] = 10⁻⁵ mol/L.
Frequently asked questions
What is the pH of pure water?
7 (neutral), with [H⁺] = 10⁻⁷ mol/L.
Why does the scale go 0 to 14?
It is the practical range for aqueous solutions at 25 °C. Values outside exist in very concentrated solutions but are rare.
Is the gap between pH 5 and pH 3 big?
Huge: since the scale is logarithmic, pH 3 is 100 times more acidic than pH 5.
Why is the pH scale logarithmic?
Because H⁺ concentrations span from about 1 mol/L to 10⁻¹⁴ mol/L. Each point is a factor of ten: pH 5 is a hundred times more acidic than pH 7.
Is pH 7 always neutral?
Only at 25 °C. Neutrality means H⁺ equals OH⁻; at 50 °C pure water sits near pH 6.6 and is still neutral.
How do pH and pOH relate?
At 25 °C, pH + pOH = 14. A pH 3 solution has pOH 11, from water's ionic product at that temperature.
Test strip or pH meter?
Strips are accurate to about 0.5 units and suffice for aquariums and pools. A meter reaches 0.01, but only if calibrated with standard buffers first.
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See all →Updated on June 18, 2026 · by Rafael Rossi · Methodology & sources