pH Calculator
Quickly translate between logarithmic pH values and the underlying hydrogen or hydroxide ion concentrations.
Assumes temperature 25 °C (Kw = 1.0 × 10⁻¹⁴).
pH
7
pOH
7
[H+] (mol/L)
1.00e-7
[OH-] (mol/L)
1.00e-7
How to Use This Calculator
Choose the known quantity
Select whether you know pH, [H+], or [OH-]. The input field updates accordingly.
Enter the value
Input the measured or desired quantity. Scientific notation (e.g., 3.2e-5) is supported for concentrations.
Review computed results
The calculator outputs pH, pOH, hydrogen ion concentration, and hydroxide ion concentration at 25 °C.
Apply to your analysis
Use the values to interpret acidity/basicity, plan titrations, or validate sensor readings.
Formula
pH = -log₁₀[H+]
pOH = -log₁₀[OH-]
[H+][OH-] = 1.0 × 10⁻¹⁴ (25 °C)
Because Kw = 1.0 × 10⁻¹⁴ at 25 °C, pH + pOH = 14. Adjust Kw for other temperatures to maintain accuracy.
Example
For [H+] = 2.5 × 10⁻⁴ mol/L: pH = 3.60, [OH-] = 4.0 × 10⁻¹¹ mol/L, and pOH = 10.40.
Full Description
The pH scale expresses hydrogen ion concentration on a logarithmic scale. Because aqueous solutions obey Kw = [H+][OH-], knowing any one of pH, [H+], or [OH-] allows calculation of the others.
This calculator streamlines those conversions, letting you switch between concentrations and pH for laboratory work, environmental testing, or educational demonstrations.
Frequently Asked Questions
Can pH be negative?
Yes. Highly acidic solutions with [H+] > 1 mol/L yield negative pH values. The calculator accepts negative pH inputs.
How does temperature affect pH?
Kw changes with temperature, shifting neutral pH away from 7. Update Kw if working far from 25 °C.
Is pH valid for non-aqueous solutions?
The concept is defined for water. For other solvents, activity-based scales or different definitions may be required.
Can I input activities instead of concentrations?
Yes. Activities (unitless) can replace concentrations, giving activity pH commonly used in electrochemistry.
Why does the calculator use scientific notation?
[H+] and [OH-] span many orders of magnitude. Scientific notation preserves precision without rounding errors.