How to use the pH calculator
Enter one value and the pH calculator calculates the other three: pH, pOH, hydrogen ion concentration [H+], and hydroxide ion concentration [OH-]. The result includes a classification, the calculation steps, and plain text for copying or downloading.
- Use Calculate from to choose pH, pOH, Hydrogen ion concentration [H+], or Hydroxide ion concentration [OH-].
- Enter the selected value in the field below the selector.
- When the selected value is a concentration, choose a Unit: mol/L (M), mmol/L (mM), µmol/L (µM), or nmol/L (nM).
- Click Calculate to fill the five result fields and Formula & steps.
- Use Copy result or Download result (.txt) for the plain-text output. Click Reset to clear the calculation.
What is the pH and pOH formula?
The calculator assumes a dilute aqueous solution at 25°C. Under that assumption, pH and pOH add to 14, and the ion concentrations have the product Kw = 1.0 x 10^-14.
pH = -log10[H+], so[H+] = 10^-pHpOH = -log10[OH-], so[OH-] = 10^-pOHpH + pOH = 14[H+][OH-] = Kw
For example, entering pH 4.5 produces this set of values:
pH: 4.5
pOH: 9.5
[H+]: 3.1623e-5 mol/L
[OH-]: 3.1623e-10 mol/L
Classification: AcidicHow to calculate pH from [H+]
Select Hydrogen ion concentration [H+] in Calculate from, enter a positive number, and choose its Unit. The calculator converts the concentration to mol/L before applying the logarithm. Select Hydroxide ion concentration [OH-] to start with hydroxide instead.
Calculate from: Hydrogen ion concentration [H+]
Value: 0.001
Unit: mol/L (M)
pH: 3
pOH: 11
[OH-]: 1.0000e-11 mol/L
Classification: AcidicWhich pH calculator inputs are accepted?
pH and pOH accept finite numeric values, including negative values, when the absolute value is no greater than 1e12. Concentration inputs must be greater than zero. An empty field, invalid number, or concentration that produces a non-finite pH or pOH displays an error.
The result classifies pH below 7 as acidic, exactly 7 as neutral, and above 7 as basic. The calculation keeps the 25°C value of Kw; it does not adjust for temperature, ionic strength, activity coefficients, or other non-ideal behavior. It runs in the browser, and the script does not send the entered value for calculation.
Where pH calculations come up
Start with pH or pOH when that is the value in your notes. Start with Hydrogen ion concentration [H+] or Hydroxide ion concentration [OH-] when the reported value is a concentration.
The result keeps pH, pOH, [H+], [OH-], and the classification together. Copy result places the values and steps in plain text, while Download result (.txt) saves that text as a file.
When to use another science calculator
Use Science Calculators for Physics, Chemistry and More for work outside these pH relationships. Choose Molarity Calculator for molarity from solute amount and solution volume. Choose Dilution Calculator when three concentration and volume values determine the fourth. Use Chemistry Calculator for a calculation that combines pH/pOH with molarity, dilution, molar mass, stoichiometry, or another listed chemistry calculation.
Frequently asked questions
What formula does the pH calculator use?
It uses pH = -log10[H+], pOH = -log10[OH-], and pH + pOH = 14 at 25°C.
Can I calculate pH from [H+]?
Yes, choose Hydrogen ion concentration [H+], enter a positive value, and select mol/L (M), mmol/L (mM), µmol/L (µM), or nmol/L (nM) as the Unit.
How do I calculate pOH from pH?
Choose pH in Calculate from, enter the value, and click Calculate; the result uses pOH = 14 - pH under the 25°C assumption.
What units can I use for ion concentration?
You can use mol/L (M), mmol/L (mM), µmol/L (µM), or nmol/L (nM) for Hydrogen ion concentration [H+] and Hydroxide ion concentration [OH-].
Does the pH calculator adjust for temperature?
No, it uses a dilute aqueous solution at 25°C and Kw = 1.0 x 10^-14 without temperature or ionic-strength adjustments.