Select an input
Choose pH, pOH, hydrogen ions, or hydroxide ions.
Acid-base chemistry calculator
pH Calculator
Calculate pH, pOH, hydrogen ion concentration, or hydroxide ion concentration with accurate formulas and a clear result breakdown.
Four input modes
Instant results
Acid-base classification
Calculation setup
Choose your known value
Select the value you already know. Concentrations must be entered in mol/L (M).
mol/L
mol/L
Core formulas
pH = −log₁₀[H⁺], pOH = −log₁₀[OH⁻], and pH + pOH = 14 at 25°C.
Calculation result
Your pH results
Enter a known value
Calculate pH, pOH, [H⁺], [OH⁻], and the solution classification.
Simple process
How the pH calculator works
The calculator converts your known acid-base value into the other three related values.
Enter the value
Use mol/L when entering an ion concentration.
Apply the formula
The tool uses logarithms and the 25°C relationship pH + pOH = 14.
Review the result
See all calculated values and whether the solution is acidic, neutral, or basic.
What is a pH calculator?
A pH calculator determines the acidity or basicity of an aqueous solution from pH, pOH, hydrogen ion concentration, or hydroxide ion concentration. It is useful for chemistry homework, laboratory checks, water testing, and acid-base calculations.
The pH and pOH formulas
pH is the negative base-10 logarithm of hydrogen ion concentration: pH = −log₁₀[H⁺]. Likewise, pOH = −log₁₀[OH⁻]. For dilute aqueous solutions at 25°C, pH + pOH = 14.
Understanding the pH scale
A pH below 7 is acidic, pH 7 is neutral, and a pH above 7 is basic under standard classroom conditions at 25°C. Because the scale is logarithmic, a one-unit pH change represents a tenfold change in hydrogen ion concentration.
Common uses
- Converting hydrogen ion concentration to pH
- Converting hydroxide ion concentration to pOH
- Checking acid-base chemistry exercises
- Estimating solution classification at 25°C
Helpful answers
pH calculator FAQs
Learn more about pH, pOH, ion concentrations, and the assumptions used by this calculator.
What is pH?
pH is a logarithmic measure of hydrogen ion concentration in an aqueous solution. A lower pH is more acidic, while a higher pH is more basic.
What formula does this calculator use?
The calculator uses pH = -log10[H+], pOH = -log10[OH-], and pH + pOH = 14 for dilute aqueous solutions at 25°C.
Can I calculate pH from hydrogen ion concentration?
Yes. Select Hydrogen Ion, enter the concentration in mol/L, and the calculator will determine pH and the related values.
How do I calculate pOH from pH?
At 25°C, subtract the pH from 14. For example, a pH of 3 gives a pOH of 11.
What concentration unit should I use?
Enter hydrogen or hydroxide ion concentration in moles per litre, written as mol/L or M.
What pH is neutral?
For a dilute aqueous solution at 25°C, pH 7 is neutral. Values below 7 are acidic and values above 7 are basic.
Can pH be below 0 or above 14?
Yes. Concentrated solutions can have pH values outside the usual 0 to 14 classroom range, although activity-based calculations are more appropriate for concentrated solutions.