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Buffer Capacity Calculator

See how strongly your buffer resists pH change, and how close you are to its point of maximum capacity at pH = pKa.

Buffer solution values

Enter total concentration, pKa and the working pH.

Buffer capacity (β)—Enter your buffer values above.
Ka (from pKa)—
[A⁻] ÷ [HA] ratio—
Distance from pKa—
Quick insight

Buffer Capacity Calculator

Enter concentration, pKa and pH to see your buffer's resistance to pH change.

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What Is a Buffer Capacity Calculator?

Buffer capacity (β) tells you how well a solution resists a change in pH when a small amount of strong acid or base is added. This calculator uses the Van Slyke equation, a standard result in analytical chemistry, to compute β from the total buffer concentration, the weak acid's pKa, and the current solution pH.

It's the same math behind why lab buffers (like phosphate-buffered saline) and biological buffers (like blood's bicarbonate system) are chosen with a pKa close to the pH they need to maintain — buffering strength is highest right at pH = pKa.

How to Read Your Results

Buffer Capacity (β)

This is the amount of strong acid or base, in moles per liter, that would need to be added to shift the solution's pH by exactly one unit. A higher number means a stronger, more resistant buffer.

Ka (from pKa)

The acid dissociation constant, converted from your entered pKa using Ka = 10−pKa. It's shown for reference and used internally in the buffer capacity formula.

[A⁻] ÷ [HA] Ratio

The ratio of conjugate base to weak acid at your entered pH, from the Henderson-Hasselbalch equation. A ratio near 1:1 (pH close to pKa) corresponds to maximum buffer capacity.

Distance from pKa

How many pH units your working pH is above or below the buffer's pKa. Values within about ±1 unit are inside the buffer's effective working range.

Real-World Example

A biochemist prepares a 0.10 M acetate buffer (pKa = 4.76) and needs to know how resistant it will be at pH 4.76 versus pH 6.00.

ConditionpHBuffer Capacity (β)
At pKa4.76≈0.0576 mol/L per pH unit
1.24 pH units above pKa6.00≈0.0033 mol/L per pH unit

Moving just over one pH unit away from the pKa drops the buffer capacity by roughly 94% — a clear illustration of why buffers are chosen with a pKa near the pH they're meant to protect.

Tips for Choosing a Buffer

  • Pick a weak acid whose pKa is within about 1 unit of the pH you need to maintain.
  • Increase total concentration if you need more resistance to pH change — capacity scales roughly linearly with concentration.
  • Avoid relying on a buffer far outside its effective range; it will behave more like an unbuffered solution.
  • Common lab buffers: acetate (pKa 4.76), phosphate (pKa 7.21), Tris (pKa 8.06), and bicarbonate (pKa 6.35, biologically regulated to ~7.4).

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Note: This calculator uses an idealized Van Slyke approximation and assumes an ideal dilute aqueous solution. It does not account for ionic strength effects, temperature dependence of Ka, or water's own autoionization contribution near extreme pH values.

Last updated: August 2026 · Reviewed by: Simple Calculator Tools Editorial Team