Hydrogen Ion Concentration Calculator

Enter a pH value to get [H+] and [OH-] in mol/L, written in scientific notation, plus the pOH.

Updated October 27, 2026

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Formula

[H+] = 10^-pH mol/L
pOH = 14 - pH at 25 C
[OH-] = 10^-pOH mol/L

Worked example

pH 3.5: [H+] = 10^-3.5 = 3.16 × 10^-4 mol/L. pOH = 10.5, so [OH-] = 3.16 × 10^-11 mol/L.

Frequently asked questions

How do you find [H+] from pH?

[H+] = 10^-pH mol/L. At pH 3.5, [H+] = 10^-3.5 = 3.16 × 10^-4 mol/L.

How do you find [OH-] from pH?

Take pOH = 14 - pH at 25 C, then [OH-] = 10^-pOH. At pH 3.5, pOH = 10.5 and [OH-] = 3.16 × 10^-11 mol/L.

Why is the answer in scientific notation?

Concentrations in water span many powers of ten, from about 1 mol/L at pH 0 to 10^-14 mol/L at pH 14. Scientific notation keeps them readable.

What happens when pH changes by 1?

[H+] changes by a factor of 10. A solution at pH 4 has ten times the hydrogen ion concentration of one at pH 5 (1.00 × 10^-4 versus 1.00 × 10^-5 mol/L).

How we calculate

Hydrogen Ion Concentration Calculator

Formula

Worked example

Frequently asked questions

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