Chemistry calculator

Henderson–Hasselbalch Calculator

Compute buffer pH = pKa + log10([A−]/[HA]). Accepts pKa or Ka. Runs locally. Not ICE equilibrium and not pH = −log10[H+].

Instant result

Result

—

Enter values to calculate.

Inputs—

Mode—

Formula—

Trust summary Engine tested · Source checked · 11/11 tests · Production surface contract 4/4 · v1.0.0

Input interpretation: Enter values to calculate.
Result: —
Model: Buffer pH from Henderson–Hasselbalch. pKa may be given directly or as Ka.
Scope: Monoprotic buffer; HA and A− are the pair as given
Verification: Engine tested · 11/11 tests · Production surface contract 4/4 · Source checked · v1.0.0
Named expert review: Optional · Not performed
Sources: OpenStax Chemistry 2e — Acid-Base Buffers — Henderson–Hasselbalch equation
IUPAC Gold Book — buffer solution — conjugate acid–base pair
Sources
Evidence: 1 golden · 5 boundary · 5 property · Production surface contract 4/4 · Artifact integrity PASS
Production: Embedded snapshot: unpublished · Build schema 1.0.0 ready · Semantic contract ✓ · Attestation report not published on origin · Live production status PASS (0 stale; 164 CURRENT) @ 2026-09-16T06:44:24.909Z
Semantic contract: PASS

Formulas

Core equations used by this calculator.

Henderson–HasselbalchpH = pKa + log₁₀([A⁻]/[HA])

pKa from KapKa = −log₁₀(Ka)

iConcentrations enter only as the ratio [A−]/[HA]. This page does not solve an ICE table and does not compute pH from [H+].

How to use

Enter pKa or Ka

Give one acidity encoding. Sending both is rejected.

Enter the buffer pair

[HA] and [A−] must both be greater than zero. Units cancel in the ratio.

Read pH

pH = pKa + log10([A−]/[HA]). Equal concentrations give pH = pKa.

Example calculations

Common configurations with formula and result.

Equal concentrations

pKa=5, [HA]=0.1, [A−]=0.1

pH = pKa

pH=5

Two-to-one conjugate base

pKa=5, [HA]=0.1, [A−]=0.2

pH = 5 + log10(2)

pH≈5.30103

Henderson–Hasselbalch calculator specification

Version 1.0.0 · Engine tested

Calculation status

Engine tested 11/11 tests · Production surface contract 4/4
Named expert review Not performed
Calculation version 1.0.0

Review policy · Evidence

Definition: Henderson–Hasselbalch: pH = pKa + log10([A−]/[HA]). pKa = −log10(Ka). Monoprotic buffer pair as given.
What it calculates: Buffer pH from Henderson–Hasselbalch. pKa may be given directly or as Ka.
Inputs: pKa | Ka
HA
A
Outputs: pH
pKa
Ka
HA
A
ratio
Formula: pH = pKa + log10([A−]/[HA])
Assumptions: Monoprotic buffer; HA and A− are the pair as given
Activity coefficients ≈ 1
Not ICE equilibrium
Not pH = −log10[H+]
Not polyprotic
Units: lab; HA and A as the same concentration unit
Boundary conditions: missing pKa and missing Ka → MISSING_REQUIRED_INPUT
pKa and Ka together → MIXED_INPUT_ENCODING
HA ≤ 0 or A ≤ 0 → VALUE_MUST_BE_POSITIVE
Ka ≤ 0 → VALUE_MUST_BE_POSITIVE
Example: pKa=5 HA=0.1 A=0.2 → pH=5+log10(2)
Validation cases: 3 published on this page · 11/11 tests · Production surface contract 4/4 · View evidence
pKa=5 HA=0.1 A=0.1 → pH=5
pKa=5 HA=0.1 A=0.2 → pH=5+log10(2)
HA=0.1 A=0.1 → MISSING_REQUIRED_INPUT
Sources: OpenStax Chemistry 2e — Acid-Base Buffers — Henderson–Hasselbalch equation
Supports: pH = pKa + log10([base]/[acid]) for a conjugate pair
IUPAC Gold Book — buffer solution — conjugate acid–base pair
Supports: A buffer is a conjugate acid–base pair whose ratio sets pH
Calculation version: 1.0.0

Background

Interpretation and common distinctions.

Compute buffer pH from Henderson–Hasselbalch.

Supported and not supported

Supported — pH from pKa (or Ka) and [HA], [A−] · API chemistry.acid_base.henderson_hasselbalch

Not supported — ICE tables, activity coefficients, polyprotic, titration curves; pH from [H+] is /calc/chemistry/ph

Agent / API notes

Capability id: chemistry.acid_base.henderson_hasselbalch · tool id: henderson-hasselbalch · pin 1.0.0.

{ "pKa": 5, "HA": 0.1, "A": 0.2 }

Alternate encoding: { "Ka": 1e-5, "HA": 0.1, "A": 0.2 }. Errors: MISSING_REQUIRED_INPUT, INVALID_NUMBER, VALUE_MUST_BE_POSITIVE, MIXED_INPUT_ENCODING.

Calculator URL is /calc/chemistry/henderson-hasselbalch. Converters stay on /convert/chemistry.

Other calculators in this family: pH, Mole, Molarity .

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Frequently asked questions

Key distinctions behind the calculation.

Is this pH = −log10[H+]?

No. That identity is /calc/chemistry/ph. This page is the buffer approximation from pKa and the HA / A− pair.

Does this solve an ICE table?

No. Henderson–Hasselbalch takes the pair as given. Weak-acid ICE and polyprotic systems are out of scope.

Can I enter Ka instead of pKa?

Yes on the web form, REST, and MCP. Do not send pKa and Ka together.

Where does this run?

Locally in the browser by default. REST and MCP call the same chemistry-engine Henderson–Hasselbalch wrapper.