pH is a measure of the acidity or alkalinity of a solution. It is defined as the negative base-10 logarithm of the hydrogen ion concentration in moles per litre: pH = -log10([H+]). The scale typically runs from 0 (strongly acidic) through 7 (neutral) to 14 (strongly alkaline), though values outside this range are possible.

How are pH and pOH related?

At 25 degrees Celsius, pH plus pOH equals 14. This comes from the water dissociation constant Kw = [H+][OH-] = 1 x 10^-14. So if pH is 3, pOH is 11, and if pH is 11, pOH is 3.

What [H+] concentration gives a pH of 3?

A pH of 3 corresponds to a hydrogen ion concentration of 10^-3 mol/L, which is 0.001 mol/L. This is the default worked example in the calculator. Each whole number step on the pH scale represents a tenfold change in hydrogen ion concentration.

pH Calculator

This pH calculator converts between pH, pOH, hydrogen ion concentration [H+], and hydroxide ion concentration [OH-], and classifies the result as acidic, neutral, or basic. The pH scale is the standard way to measure how acidic or alkaline a solution is. It runs from below 0 for the most concentrated strong acids through 7 for pure neutral water to 14 and beyond for strong bases, and each whole step represents a tenfold change in hydrogen ion concentration because the scale is logarithmic. pH is defined as the negative base-10 logarithm of the hydrogen ion concentration in moles per litre: pH = -log([H+]). You can enter a value either way: type in the hydrogen ion concentration and the calculator returns pH, pOH, and the corresponding hydroxide concentration, or type in a pH value and the calculator reverses the relationship to recover all four quantities. At 25 degrees Celsius, pH and pOH always sum to 14, so once you know one you can immediately find the other. The hydroxide concentration is then 10 raised to the power of negative pOH. Solutions with pH below 7 are classified as acidic, pH 7 as neutral, and pH above 7 as basic (alkaline). The default example uses a hydrogen ion concentration of 0.001 mol/L (1 x 10^-3), which gives a pH of 3.00 and a pOH of 11.00. This tool is useful for chemistry students, water quality monitoring, aquarium and pool maintenance, food science, and any field where acidity matters. The pOH = 14 - pH relationship assumes a temperature of 25 degrees Celsius; Kw shifts slightly at other temperatures.

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Enter

[H+] (mol/L)

pH 3.00

acidic

pOH11.00

[H+] (mol/L)1.00e-3

[OH-] (mol/L)1.00e-11

Assumes temperature of 25 degrees C where pH + pOH = 14. For strong acids, [H+] equals the acid concentration; for strong bases, [OH-] equals the base concentration and pH = 14 - pOH.

How it works

From a hydrogen ion concentration [H+]: pH = -log10([H+]). From a pH value: [H+] = 10^(-pH). In both cases, pOH = 14 - pH (at 25 degrees C), and the hydroxide concentration [OH-] = 10^(-pOH). A pH below 7 is acidic, exactly 7 is neutral, and above 7 is basic. The relationship pH + pOH = 14 comes from the water ion product Kw = [H+] x [OH-] = 1 x 10^-14 at 25 degrees C.

Worked example

A hydrogen ion concentration of 0.001 mol/L (1 x 10^-3 mol/L) gives pH = -log(0.001) = 3.00. The solution is acidic. pOH = 14 - 3 = 11.00. The hydroxide concentration [OH-] = 10^-11 = 1.00 x 10^-11 mol/L. These match the default values pre-filled in the calculator above.

Related calculators

Molarity Calculator: solve M=n/V for solution concentration.
Dilution Calculator: use C1V1=C2V2 to adjust solution strength.
Molar Mass Calculator: find molar mass from a chemical formula.
Percent Error Calculator: compare a measured pH to a reference value.

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