pH Calculator
pH calculator: convert hydronium concentration to pH and back, and get pOH and hydroxide concentration at 25 °C.
pH = −log10[H3O+]
How to use this calculator
- Choose the quantity you want under “Solve for”.
- Type the known values and pick their units: any combination works.
- The result updates as you type. Open “Show the steps” to see the substitution.
pH is the negative base-10 logarithm of the hydronium ion concentration in mol/L: pH = −log₁₀[H₃O⁺]. Each pH unit is a tenfold change in acidity.
At 25 °C water’s ion product is Kw = 10⁻¹⁴, so pH + pOH = 14. For strong acids such as HCl, [H₃O⁺] equals the acid concentration; for weak acids use the buffer calculator or the Ka expression.
Formula
pH = −log10[H3O+]
| Symbol | Quantity | Units |
|---|---|---|
| [H3O+] | Hydronium concentration | mol/L, mmol/L, µmol/L, nmol/L, mol/m³ |
| pH | pH | — |
Worked example
Given: [H3O+] = 0.001 mol/L
pH = −log10[H3O+]
Result: pH = 3
Frequently asked questions
What is the pH of 0.01 M HCl?
HCl dissociates completely, so [H₃O⁺] = 0.01 M and pH = 2.
Can pH be negative?
Yes. Very concentrated strong acids, above 1 mol/L, give pH below zero.