pH CalculatorFree online science calculator — instant results, no sign-up
Calculate pH from hydrogen ion concentration.
About the pH Calculator
The pH Calculator calculates pH from the hydrogen ion concentration of an aqueous solution entered in mol/L. It also reports pOH and classifies the solution as acidic, neutral, or basic under the standard 25 °C assumption. With the default H⁺ concentration of 0.0001 mol/L, it returns pH = 4, pOH = 10, and Nature = Acidic.
- H⁺ concentration (mol/L)
- The concentration of hydrogen ions in the solution, expressed in moles per liter (mol/L), with a default value of 0.0001.
Formula and method
pH = −log₁₀([H⁺]/c°); pOH = 14 − pH. Nature = Acidic if pH < 7, Neutral if pH = 7, and Basic if pH > 7. Here c° = 1 mol/L, so entering [H⁺] in mol/L gives the familiar numerical calculation pH = −log₁₀([H⁺]).
- [H⁺]Hydrogen ion concentration in mol/L.
- c°Standard concentration of 1 mol/L, used to make the logarithm's argument dimensionless.
- log₁₀Base-10 logarithm.
- pHNegative base-10 logarithm of hydrogen ion activity, approximated here using concentration.
- pOHNegative base-10 logarithm of hydroxide ion activity, inferred here from pOH = 14 − pH.
- NatureAcidic, neutral, or basic classification based on comparison with pH 7 at 25 °C.
Use a positive hydrogen ion concentration in mol/L, take its base-10 logarithm, and reverse the sign to obtain pH. Subtract pH from 14 to obtain pOH, then compare pH with 7 to classify the solution. For the default input, −log₁₀(0.0001) = 4 and 14 − 4 = 10, so the solution is Acidic.
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- H⁺ concentration0.0001 mol/L
Results
- pH4
- pOH10
- NatureAcidic
How to use the pH Calculator
- 1Find the solution's hydrogen ion concentration and ensure it is expressed in mol/L.
- 2Enter that positive value in H⁺ concentration (mol/L), or retain the default 0.0001.
- 3Run the calculation to obtain pH; the default input gives pH = 4.
- 4Read the pOH row; for the default input, pOH = 10.
- 5Read the Nature row; the default solution is Acidic because its pH is below 7.
Reading your result
The pH row describes acidity: lower pH indicates greater hydrogen ion activity, and each one-unit decrease corresponds to a tenfold increase. The pOH row describes hydroxide ion activity on a corresponding logarithmic scale and is calculated here so that pH + pOH = 14. The Nature row labels solutions below pH 7 as Acidic, at pH 7 as Neutral, and above pH 7 as Basic. Thus, the default outputs pH = 4, pOH = 10, and Nature = Acidic describe an acidic solution under the calculator's assumptions.
Limitations
This calculator approximates hydrogen ion activity using concentration, which is most suitable for dilute aqueous solutions with activity coefficients near unity. It assumes 25 °C for pH + pOH = 14 and the neutral threshold of pH 7; both depend on temperature. Zero and negative concentrations are invalid logarithm inputs. Enter the actual equilibrium hydrogen ion concentration, not simply the amount of acid added, because dissociation, buffering, and water autoionization can affect it. Concentrated or high-ionic-strength solutions require activity corrections, and pH is not universally restricted to 0–14.
Common questions
Why does 0.0001 mol/L give pH 4?+
The default concentration is 10⁻⁴ mol/L. Its base-10 logarithm is −4, and reversing that sign gives pH = 4.
Why does the calculator report pOH 10 for the default input?+
It uses pH + pOH = 14 at 25 °C. With pH = 4, pOH = 14 − 4 = 10.
Can I enter an acid's molar concentration as the H⁺ concentration?+
Only if that value accurately represents the equilibrium hydrogen ion concentration. Weak acids dissociate only partially, and acids that can release multiple protons require additional equilibrium considerations.
Why can’t I enter zero for H⁺ concentration?+
The logarithm of zero is undefined, so no finite pH can be calculated from that input. Aqueous solutions also contain hydrogen ions from water autoionization even when no acid is added.
Does the calculator save my information?+
Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.
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Results are estimates for informational purposes and are not financial, medical or legal advice.