pH and pOH Calculator
Convert between pH, pOH and the hydrogen and hydroxide ion concentrations.
What is pH & pOH?
pH is a logarithmic way of writing how concentrated the hydrogen ions in a solution are: pH = −log₁₀[H⁺]. Because pH and pOH always add to 14 in water at 25 °C, knowing one of the four quantities fixes all the others — which is what makes a single measurement enough.
How it works
Concentrations are converted with pH = −log₁₀[H⁺] and pOH = −log₁₀[OH⁻], and the two are linked by [H⁺][OH⁻] = 1 × 10⁻¹⁴. The tool accepts whichever value you have, computes the rest, and reports the concentrations in scientific notation because they span fifteen orders of magnitude between a strong acid and a strong base.
- Enter what you know. Every field recalculates as you type — there is no submit button. Leave the quantity you want blank and the page solves for it.
- Read the answer and the working. The result appears above the fold; underneath it is the formula with your numbers substituted, so the arithmetic can be checked rather than trusted.
- Check the units and the size. Units are shown on every value and selectable per field, so a result in kJ, kPa or km/h is converted rather than assumed. Sanity-check the magnitude against a known landmark such as 1 g/cm³ for water.
Examples
A textbook problem with a known answer
Every tool here is checked against standard example problems — the molar mass of C6H12O6 is 180.16 g/mol, and free fall from 20 m takes about 2.02 s at g = 9.8 m/s².
A result you want to reason backwards from
Leave the quantity you already know blank instead: given the final answer, most tools recover the starting value — the mass needed for a target concentration, or the height a fall must have had.
Common mistakes
Forgetting that a formula is only as good as its subscripts
H2O and H2O2 are different compounds. The formula field shows its parse — element by element — so a mis-typed subscript is visible before it reaches the arithmetic.
Mixing units in the middle of a calculation
The most common source of a wrong answer is a value in grams next to a formula expecting kilograms, or Celsius next to a gas constant expecting kelvin. This category converts on the way in and labels what it used.
Frequently asked questions
Why do pH and pOH add up to 14?
Because in water at 25 °C the ion product [H⁺][OH⁻] is 1 × 10⁻¹⁴. Taking negative logarithms of both sides turns that product into pH + pOH = 14. At other temperatures the constant changes slightly and so does the sum.
Is a pH of 7 always neutral?
Only at 25 °C. Neutral means [H⁺] equals [OH⁻], and the value at which that happens shifts with temperature — pure water at 50 °C has a pH of about 6.6 and is still neutral.
How many decimal places does a pH have?
Only as many as the significant figures in the concentration. A concentration of 1.0 × 10⁻³ M gives pH 3.00 — the digits before the point come from the exponent and are not significant.