Chemistry

pH Calculator

The pH calculator converts pH, pOH and ion concentrations using the value you already know. Enter your number, choose the concentration unit when needed and get the formulas alongside the result.

The default pKw is about 14 at 25°C. Concentration calculations assume an approximately ideal, dilute aqueous solution.

Calculate pH

How to Use the pH Calculator

You need an ion concentration, pH or pOH value. A chemical’s name alone doesn’t tell you the pH of every solution made from it.

Choose your known value. Select pH, pOH, hydrogen-ion concentration or hydroxide-ion concentration.
Enter the number and unit. Concentrations can use mol/L, mmol/L or micromol/L. You can enter 0.0000001 as 1e-7.
Check pKw. About 14 is the usual dilute-water value at 25°C. Use the value supplied with your problem for other conditions.
Calculate. The result includes pH, pOH, both ion concentrations and the formula used.

Worked Example

For a solution with [H+] = 1 × 10^-3 mol/L and pKw = 14:

pH: -log₁₀(10^-3) = 3.
pOH: 14 – 3 = 11.
[OH-]: 10^-11 mol/L.
Interpretation: the solution is acidic because pH 3 is below the neutral value of 7 under these conditions.

pH Formulas

pH measures acidity on a logarithmic scale. A decrease of 1 pH unit corresponds to a 10-fold increase in hydrogen-ion activity.

The formal IUPAC definition of pH uses hydrogen-ion activity. For dilute, approximately ideal aqueous solutions, concentration is often used as an approximation.

pH ≈ -log₁₀[H+]
pOH ≈ -log₁₀[OH-]
pH + pOH = pKw

Concentration: [H+] and [OH-] are entered in mol/L for these concentration-based formulas.
Reverse conversion: [H+] ≈ 10^-pH and [OH-] ≈ 10^-pOH.
Neutrality: equal hydrogen-ion and hydroxide-ion activities give pH = pKw / 2.
Temperature: pKw changes with temperature. A neutral solution doesn’t always have pH 7.

A calculated pH is a model. Real measurements also depend on activity, temperature, calibration and the sample. I wouldn’t substitute a concentration calculation for a properly calibrated measurement when the actual sample matters.

Understanding Your Results

The acidity label compares your pH with the neutral pH at the pKw you entered. The concentration values show the assumptions behind that label.

Below neutral pH: the hydrogen-ion concentration is greater than the hydroxide-ion concentration in this model.
Above neutral pH: the hydroxide-ion concentration is greater.
At neutral pH: the concentrations are equal.
Units change the result: 1 mmol/L is 0.001 mol/L. Treating those as the same concentration changes pH by 3 units.

Frequently Asked Questions

Can pH Be Below 0 or Above 14?

Yes. The familiar 0-14 scale isn’t an absolute limit. This calculator accepts values outside it when the resulting concentrations fit its numeric range. The dilute-solution approximation becomes less reliable for concentrated solutions.

Can I Find pH From a Chemical Formula Alone?

You normally also need concentration and information about acid-base behavior. Weak acids, weak bases and buffers require equilibrium calculations. An ion concentration can be entered here after you determine it.

Why Does Zero Concentration Give an Error?

The logarithm of 0 isn’t a finite number. You should enter a positive ion concentration. A blank field is also rejected so it can’t be mistaken for 0.

Why Is Neutral pH Sometimes Different From 7?

Neutrality means equal hydrogen-ion and hydroxide-ion activities. Neutral pH is pKw / 2, so it changes when pKw changes. The value of about 7 applies to dilute water at about 25°C.