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Free pH to hydrogen ion concentration calculator. Enter pH to get [H⁺] in mol/L using [H⁺] = 10^(−pH), plus pOH and [OH⁻] from your pKw (default 14.00 for water at about 25 °C). Pairs with our pH calculator and other math & science tools.
Last updated: April 13, 2026
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Enter the negative log₁₀ of the hydrogen ion activity (typical dilute aqueous range about 0–14).
Leave as 14.00 for water at about 25 °C. Change only if you are working at another temperature or non-aqueous context.
Hydrogen ion [H⁺]
1.0000e-7 M
Hydroxide [OH⁻]
1.0000e-7 M
pH (input)
7
pOH
7
Quick definitions
Use [H⁺] and [OH⁻] from a measured or modeled pH when teaching logarithmic scales and the pH definition.
After estimating pH from a titration curve or indicator, convert to concentration for reporting alongside molarity workflows.
pOH and [OH⁻] follow from your chosen pKw, so you can reflect non-ambient problems without leaving the page.
For pH = 4.00 (pKw = 14): [H⁺] = 1.0000e-4 M, pOH = 10, [OH⁻] = 1.0000e-10 M.
The calculator applies the logarithmic definition of pH and the ion product of water. For a given pH and pKw, it evaluates [H⁺] = 10^(−pH), then pOH = pKw − pH and [OH⁻] = 10^(−pOH), matching standard general chemistry treatments where activity is replaced by concentration.
[H⁺] = 10^(−pH)pOH = pKw − pH[OH⁻] = 10^(−pOH)Default pKw = 14.00 corresponds to pure water at about 25 °C. Adjust pKw if your problem statement uses a different value.
Also see our dilution calculator and concentration calculator for solution preparation.
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