Education

pH Calculator

Last updated: June 19, 2026

Written by Blake Boege · Founder, Calculator Answers

The pH scale measures the acidity or basicity of an aqueous solution by representing the negative logarithm of the hydrogen ion concentration. A pH calculator converts between pH, pOH, hydrogen ion concentration, and hydroxide ion concentration using logarithmic equations and the constant pH + pOH = 14. Scientists, water treatment technicians, and students use these calculations to analyze chemical solutions, monitor soil quality, and evaluate chemical safety without performing manual logarithmic conversions.

Pick a mode and enter what you know. The calculator returns pH, pOH, [H+], and [OH-] for the same solution using pH = -log10[H+] and the 25 C identity pH + pOH = 14.

Quick Answer

Convert between pH, pOH, hydrogen, and hydroxide ion concentrations. Enter any one of these values to find the others.

Mode

[H+] hydrogen ion concentration

mol/L

Use scientific notation like 1e-7 for very small values. · e.g. 1e-4 or 0.0001

Assumptions

Aqueous solutions at 25 C, where Kw = 1.0 x 10^-14 and pH + pOH = 14.
Strong acid/strong base behavior unless your input already reflects the equilibrium concentration of H+ or OH-.
Activity coefficients are assumed to be 1 (ideal dilute solution).

Educational only. Not lab, medical, or water-quality advice.

Result

4.00

Acidic

FormulapH = -log10(1 x 10^-4) = 4.00

pH4.00

pOH10.00

[H+]1 x 10^-4 mol/L

[OH-]1 x 10^-10 mol/L

At 25 C, Kw = [H+][OH-] = 10^-14. So pH + pOH = 14 only at 25 C; at other temperatures the constant shifts and so does the sum.

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Examples

[H+] = 1e-4 mol/L

pH = 4 (acidic)

pH = 8.5

[H+] ≈ 3.16 x 10^-9 mol/L

[OH-] = 1e-3 mol/L

pOH = 3, pH = 11 (basic)

pH = 5

pOH = 9 (at 25 C)

How it works

The pH definition runs in both directions: take a base-10 log to go from concentration to pH, raise 10 to a negative power to go back. The pOH side uses the same relations, and at 25 C the two are tied together by the water ion product.

pH · pH = -log10[H+]

Reverse · [H+] = 10^(-pH)

25 C identity · pH + pOH = 14

Related chemistry calculators

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Molality calculator when you need a temperature-independent concentration measure.
Scientific calculator for the log and exponent steps without leaving the formula.
Log calculator for base-10 logs of any positive number.
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Frequently asked questions

pH is the negative base-10 logarithm of the hydrogen ion concentration in a solution. Lower pH means more H+ and a more acidic solution; higher pH means less H+ and a more basic (alkaline) solution. pH 7 is neutral at 25 C.

pOH is the negative base-10 logarithm of the hydroxide ion concentration. At 25 C, pH + pOH = 14 because the water ion product Kw is 1.0 x 10^-14. Knowing one of pH or pOH gives you the other.

Use scientific notation in the [H+] or [OH-] field, like 1e-4 for 0.0001 or 3.2e-9 for 3.2 x 10^-9. The calculator parses both decimal and scientific notation.

Yes, in concentrated strong acid or strong base solutions, but the standard 0 to 14 scale assumes dilute aqueous behavior at 25 C. Outside that range the activity coefficient stops being close to 1 and the simple formula loses accuracy.

Only at 25 C. The relationship comes from Kw = [H+][OH-] = 10^-14 at that temperature. Kw rises with temperature, so neutral pH drops below 7 in hot water and pH + pOH drops below 14.

Not in this version. Weak-acid calculations need Ka, the initial concentration, and an ICE table. This page converts among pH, pOH, [H+], and [OH-] given one of them.

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