pH & Buffer Calculator
pH, pOH, [H⁺] and [OH⁻] from any one of them, and buffer pH or the base/acid ratio by Henderson–Hasselbalch with your pKa and pKw.
All four of pH, pOH, [H⁺] and [OH⁻] from the one you know (pKw yours, 14.00 at 25 °C), or a buffer pH / the [A⁻]/[HA] ratio for a target pH — definitions only, no pKa table.
Example: [H⁺] = 0.001 mol/L is pH 3.00, pOH 11.00, [OH⁻] = 1 × 10⁻¹¹; a buffer at pKa 4.76 needs [A⁻]/[HA] = 1.74 for pH 5.00 — about 63 % in the base form.
Definitions,
not a table of acids.
What pH, pOH and pKw are, what Henderson–Hasselbalch assumes, and what is left to you.
pH and pOH
pH = −log10 [H⁺] and pOH = −log10 [OH⁻]; because water dissociates, [H⁺][OH⁻] = Kw, so pH + pOH = pKw. Given any one of the four, the other three follow. pKw is 14.00 at 25 °C (13.995 to three decimals) and falls as water warms — about 13.0 at 60 °C (CRC Handbook) — so it is a field, and neutral is pKw/2 rather than always 7.
Buffers
For a weak acid HA and its conjugate base A⁻, pH = pKa + log10 ([A⁻]/[HA]). Enter the two concentrations to get the pH, or a target pH to get the ratio you need. The buffer works best within one pH unit of pKa, where both forms are present in comparable amounts; the page says so when you leave that range.
What is assumed
Concentrations stand in for activities — the dilute, ideal-solution approximation of every introductory course; at high ionic strength the real pH differs. No pKa table is embedded: pKa depends on temperature and ionic strength and belongs to your data source. Nothing leaves the browser; the same four anonymous usage counts as the rest of the site apply.
SOURCES
- pH = −log10 [H⁺], pH + pOH = pKw; Henderson–Hasselbalch pH = pKa + log10 ([A⁻]/[HA]) (textbook definitions; concentrations taken as activities, no pKa table embedded)
Last reviewed 20 September 2026. How results are checked: How we verify.