What is the Henderson-Hasselbalch equation?

It is pH = pKa + log([A⁻]/[HA]), where pKa is the negative log of the acid dissociation constant, [A⁻] is the conjugate base concentration, and [HA] is the weak acid concentration. The equation accurately predicts buffer pH when both species are present in reasonable amounts.

What is buffer capacity?

Buffer capacity (β) measures how well a solution resists pH changes when acid or base is added. It is highest at pH = pKa (ratio = 1) and decreases as the ratio moves away from 1. Mathematically, β ≈ 2.303 × C × ([A⁻]/[HA]) / (1 + [A⁻]/[HA])², where C is total buffer concentration.

What is the effective buffering range?

A buffer works best within one pH unit of its pKa — that is, from pKa − 1 to pKa + 1. Outside this range the ratio [A⁻]/[HA] exceeds 10:1 or falls below 1:10, meaning one species is nearly depleted and buffering capacity is very low.

Why does the equation use a ratio instead of absolute concentrations?

Because log([A⁻]/[HA]) only depends on the ratio. Doubling both [A⁻] and [HA] keeps pH unchanged. You can express amounts in mol/L, mmol/L, or total moles (mol or mmol) — provided both components are measured in molar units within the same solution volume.

Which pKa value should I use for multi-protic acids?

Use the pKa for the specific proton transfer step that operates near your target pH. For phosphoric acid: pKa1 ≈ 2.15, pKa2 ≈ 7.20, pKa3 ≈ 12.35. The most common phosphate buffer near pH 7 uses pKa2. The presets in this tool already select the correct pKa step.

How accurate is this calculator?

It assumes ideal behavior and 25 °C. Accuracy is best when [A⁻]/[HA] is between 0.1 and 10, concentrations are 0.01–0.5 mol/L, and ionic strength is low. The calculator does not apply activity-coefficient or ionic-strength corrections; for non-ideal solutions, use an appropriate Davies or extended Debye-Hückel calculation separately.

Buffer pH Calculator

Select a common buffer or enter your own pKa and acid/base concentrations to calculate buffer pH and effective buffering range.

Buffer Parameters

Buffer System

pKa of Weak Acid

Acid dissociation constant as −log(Ka).

Weak Acid [HA] (mol/L)

Conjugate Base [A⁻] (mol/L)

Common pKa Reference

Buffer	pKa	Range
Acetate	4.74	3.7–5.7
MES	6.10	5.1–7.1
Phosphate (2)	7.20	6.2–8.2
HEPES	7.48	6.5–8.5
Tris	8.06	7.1–9.1
Ammonium	9.25	8.3–10.3

Enter buffer parameters on the left
to calculate the pH.

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Summary

The Buffer pH Calculator applies the Henderson-Hasselbalch equation (pH = pKa + log([A⁻]/[HA])) to compute the pH of any weak-acid/conjugate-base buffer system. Choose from 15 preset common buffers — acetate, phosphate, Tris, HEPES, and more — or enter a custom pKa. The tool also estimates buffer capacity (β) and shows the effective pH range (pKa ± 1). All calculations run entirely in the browser.

How it works

Select a preset buffer from the dropdown (e.g., Acetate, Phosphate, Tris) or choose "Custom" and enter your own pKa.
Enter the concentration of the weak acid [HA] and the conjugate base [A⁻] in mol/L (molar).
The buffer pH is calculated instantly using pH = pKa + log([A⁻]/[HA]).
The tool displays the base-to-acid ratio, buffer capacity estimate, and effective buffering range (pKa ± 1).
A visual scale shows where your calculated pH falls relative to the effective range.
Use the Copy button to save the full result summary to your clipboard.

Use cases

Prepare acetate or phosphate buffers for biochemistry lab experiments.
Design cell culture media buffers (HEPES, MES, PIPES) at a precise pH.
Verify buffer pH from measured component concentrations.
Quickly look up pKa values for 15 common buffer systems.
Estimate buffer capacity before mixing large-volume solutions.
Check whether a chosen buffer system is appropriate for a target pH.
Prepare for chemistry exams by practicing Henderson-Hasselbalch calculations.
Cross-check manual buffer calculations against a reliable reference.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu