Buffer Component Ratio Calculator
A buffer resists changes in pH because it holds a weak acid and its conjugate base together, and the balance between them sets the pH. The Henderson-Hasselbalch equation makes that relationship exact: the pH equals the acid's pKa plus the logarithm of the base-to-acid ratio. This calculator works both ways. Give it the pKa and the amounts of conjugate base and acid and it returns the pH and the ratio, and it also computes the base-to-acid ratio you would need to reach a target pH.
Henderson-Hasselbalch formula
pH = pKa + log10([A-] / [HA])
ratio used = [A-] / [HA]
ratio needed = 10^(target pH - pKa)
The pH is the pKa plus the base-ten logarithm of the conjugate base to acid ratio. When the two are equal the ratio is 1 and the pH equals the pKa. To hit a target pH, raise 10 to the power of the difference between the target pH and the pKa.
Buffer facts
- At a base-to-acid ratio of 1, the pH equals the pKa.
- A pH one unit above the pKa requires a ten-to-one base-to-acid ratio.
- Buffers work best within about one pH unit of the acid's pKa.
- The default pKa of 4.76 is that of acetic acid, a common buffer acid.
- Because only the ratio matters, you can enter concentrations or moles.
Buffer ratio: frequently asked questions
What is the Henderson-Hasselbalch equation?
The Henderson-Hasselbalch equation relates a buffer's pH to the pKa of its weak acid and the ratio of conjugate base to acid: pH = pKa + log10([A-]/[HA]). It lets you predict the pH of a buffer from its composition, or work out the ratio of components needed to hit a target pH.
How do I find the pH of a buffer?
Enter the pKa of the weak acid and the concentrations (or moles, since volume cancels) of the conjugate base A- and the acid HA. The pH is pKa plus the base-ten logarithm of the base-to-acid ratio. When the base and acid amounts are equal, the ratio is 1, its log is 0, and the pH equals the pKa.
What ratio of base to acid do I need for a target pH?
Rearranging the equation, [A-]/[HA] = 10^(pH - pKa). This calculator computes that required ratio from your target pH and the pKa, so you can mix the right proportion of conjugate base to acid. A pH one unit above the pKa needs a ten-to-one base-to-acid ratio.
When is the Henderson-Hasselbalch equation valid?
It works best when both buffer components are present in appreciable, comparable amounts (roughly within a factor of ten of each other) and the solution is reasonably dilute. It assumes activities equal concentrations and ignores the slight dissociation of the acid and the autoionization of water, which are negligible in a working buffer.
What inputs does this calculator use?
For pH from composition, enter the pKa, the conjugate base amount, and the acid amount in any consistent unit (the ratio is what matters). For the required ratio mode, enter the pKa and a target pH. The pH output is dimensionless and the ratio output is the dimensionless [A-]/[HA] value.
Official sources
- NIST Chemistry WebBook: Acid dissociation constant data.
- NIST: SI units and quantity definitions (SP 811).
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.