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pH Calculator

Converts between pH, pOH, hydrogen-ion concentration and hydroxide-ion concentration at 25 °C (Kw = 1.0×10⁻¹⁴) from any one of them, and classifies the solution as acidic, neutral or basic.

When to use

You know one of pH, pOH, [H⁺] or [OH⁻] for an aqueous solution (for example the concentration of a fully dissociated strong acid or base) and need the others.

Do not use when: The solution contains a weak acid or base or a buffer, where [H⁺] is not equal to the nominal concentration (use henderson-hasselbalch for buffers), or the temperature differs a lot from 25 °C.

Formula

pH = −log10[H⁺]; pOH = −log10[OH⁻]; pH + pOH = pKw = 14.00 at 25 °C; [H⁺] × [OH⁻] = Kw = 1.0×10⁻¹⁴

Assumes an aqueous solution at 25 °C and that a strong acid or base is completely dissociated, so [H⁺] (or [OH⁻]) equals its nominal molarity; activity effects at high concentration and the autoionisation contribution below about 10⁻⁶ mol/L are ignored.

Inputs

Parameter	Type	Unit	Required	Description
`h_concentration_mol_per_l`	number	mol/L	no	Hydrogen-ion (hydronium) concentration, e.g. the molarity of a strong monoprotic acid such as HCl. Range: > 0, ≤ 100
`oh_concentration_mol_per_l`	number	mol/L	no	Hydroxide-ion concentration, e.g. the molarity of NaOH. Used when [H⁺] is not given. Range: > 0, ≤ 100
`ph`	number		no	Known pH; when given it takes precedence over the other inputs. Range: ≥ -2, ≤ 16
`poh`	number		no	Known pOH; used when pH is not given. Range: ≥ -2, ≤ 16

Outputs

Output	Type	Unit	Description
`ph`	number		−log10[H⁺].
`poh`	number		−log10[OH⁻] = 14 − pH at 25 °C.
`h_concentration_mol_per_l`	number	mol/L	Hydrogen-ion concentration = 10^(−pH).
`oh_concentration_mol_per_l`	number	mol/L	Hydroxide-ion concentration = 10^(−pOH) = Kw / [H⁺].
`h_concentration_scientific`	string		[H⁺] in scientific notation, mol/L.
`oh_concentration_scientific`	string		[OH⁻] in scientific notation, mol/L.
`classification`	string		Acidic (pH < 7), Neutral (pH = 7) or Basic (pH > 7) at 25 °C.

Example

0.001 M HCl: {"h_concentration_mol_per_l":0.001} → {"ph":3,"poh":11,"oh_concentration_mol_per_l":1e-11,"h_concentration_scientific":"1.000e-3","classification":"Acidic"}

pH 8.5 solution: {"ph":8.5} → {"poh":5.5,"h_concentration_mol_per_l":3.1623e-9,"oh_concentration_mol_per_l":0.0000031623,"h_concentration_scientific":"3.162e-9","classification":"Basic (alkaline)"}

GET https://tttkmbb.com/api/v1/calculate/ph?h_concentration_mol_per_l=0.001

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/ph (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/ph · Markdown: https://tttkmbb.com/chemistry/ph.md · JSON definition: https://tttkmbb.com/chemistry/ph.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="ph"
OpenAPI operationId: calculate_ph
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

OpenStax Chemistry 2e – 14.2 pH and pOH (textbook)
Wikipedia – pH (reference)

FAQ

Why is the pH of a 10⁻⁸ M HCl solution not 8?

Below about 10⁻⁶ mol/L the H⁺ from water's autoionisation matters and the real pH stays just under 7; this calculator ignores that contribution and reports the formal value.

Does the neutral point change with temperature?

Yes. Kw rises with temperature, so neutral water has pH 7.00 only at 25 °C (about 6.63 at 50 °C); all results here use pKw = 14.00.

Related calculators

Henderson–Hasselbalch Calculator — pH of a buffer from pKa and the acid/base ratio.
Molarity Calculator — Get the acid or base molarity that sets [H⁺] or [OH⁻].
Logarithm Calculator — General log10 calculations.