pH and pOH Calculator
pH is the negative base-10 logarithm of the hydrogen ion concentration and is the standard measure of acidity and alkalinity in aqueous solutions. At 25 degrees C, the sum of pH and pOH equals 14 (the pKw of water), so knowing one gives you the other. This calculator converts hydrogen ion concentration to pH and pOH, and also shows you the hydroxide concentration. Enter the hydrogen ion concentration in mol/L (M) to get all related values.
pH and pOH formulas
pH = -log10([H+]) pOH = -log10([OH-]) pH + pOH = 14 (at 25 C) [OH-] = 10^(-pOH) = Kw / [H+]
At 25 degrees C, Kw (the water dissociation constant) = 1.0 x 10^(-14) mol^2/L^2. The pH scale runs from 0 (most acidic) to 14 (most basic) for common laboratory solutions, though values outside this range are possible.
Understanding the pH scale
- pH 0 to 6.99: acidic solutions (stomach acid pH 1.5-2, vinegar pH 2.5, coffee pH 5).
- pH 7.00: neutral at 25 degrees C (pure water).
- pH 7.01 to 14: basic (alkaline) solutions (blood pH 7.4, baking soda pH 8.3, bleach pH 12).
- Each unit of pH change represents a 10-fold change in hydrogen ion concentration.
- Strong acids and bases fully dissociate; their pH is calculated directly from concentration.
Frequently asked questions
What is pH?
pH is a measure of the hydrogen ion activity in a solution. It is defined as the negative base-10 logarithm of the hydrogen ion concentration: pH = -log[H+]. At 25 degrees C, pure water has pH = 7.00 (neutral); acids have pH below 7; bases have pH above 7.
What is the relationship between pH and pOH?
At 25 degrees C, pH + pOH = 14 (the pKw of water). This means if you know the pH, you can find the pOH by subtracting from 14, and vice versa.
How do I calculate [H+] from pH?
[H+] = 10^(-pH). For example, a solution with pH 3 has [H+] = 10^(-3) = 0.001 mol/L (1 mM).
Does pH + pOH always equal 14?
Only at 25 degrees C. The pKw of water changes with temperature. At 37 degrees C (body temperature), pKw is approximately 13.83, so pH + pOH = 13.83.
What does a one-unit change in pH mean?
Because pH is a logarithmic scale, a one-unit decrease in pH represents a 10-fold increase in hydrogen ion concentration. A solution at pH 4 has 10 times more H+ than a solution at pH 5.
Official sources
- IUPAC: IUPAC Gold Book - pH.
- NIST: NIST Fundamental Physical Constants.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.