Use the Henderson-Hasselbalch calculator
The Henderson-Hasselbalch calculator relates the pH of a buffer to the acid pKa and the ratio of conjugate base to weak acid. Choose the quantity you need: buffer pH, pKa, conjugate-base amount, or weak-acid amount. The selected unknown disappears from the input area, so the remaining fields are the values that actually determine the answer.
Conjugate-base and weak-acid entries may be concentrations, moles, or another consistent amount because only their ratio enters the equation. They must use the same unit and describe the same final buffer. For example, pKa 4.76 with twice as much conjugate base as weak acid gives pH about 5.06 because log10(2) is about 0.301.
Understand the buffer equation and ratio
The acid-buffer form is pH = pKa + log10([A−]/[HA]). A− is the conjugate base and HA is the weak acid. A ratio of one makes the logarithm zero, so pH equals pKa. A ratio of ten raises pH by one unit, while a ratio of one tenth lowers pH by one unit. These reference points are quick checks for field order.
When finding pKa, the logarithm is subtracted from the entered pH. When finding one conjugate form, the calculator rearranges the ratio using 10^(pH−pKa). Every result shows the substitution, including the ratio direction, so you can verify that conjugate base is in the numerator and weak acid is in the denominator.
Check the answer by reversing the calculation
A useful reverse check is to place a calculated base or acid amount back into the pH mode. The recovered pH should match the starting value within displayed rounding. You can also swap the base and acid amounts: the new answer should be reflected across pKa because log10(1/r) is the negative of log10(r).
If the calculated amount is unexpectedly large or small, inspect the pH−pKa difference first. Each difference of one changes the required ratio by a factor of ten. Also confirm that one entry is not in millimoles while the other is in moles, or in molarity while the other is a total amount from a different volume.
Know when Henderson-Hasselbalch is suitable
The familiar concentration form is a buffer approximation. It works best when both conjugate forms are present in appreciable amounts and their formal concentrations reasonably represent equilibrium concentrations. Extreme ratios, very dilute mixtures, strong-acid or strong-base excess, polyprotic overlap, precipitation, and activity effects can make the estimate misleading even when the arithmetic is valid.
The calculator does not determine whether a mixture is chemically a buffer, choose a suitable acid pair, or model additions made after the stated composition. For a titration endpoint, concentrated electrolyte, biological matrix, or regulated formulation, use an appropriate equilibrium model and validated chemical data rather than relying on this rearranged relationship alone.
Private calculation and responsible use
Use this page for learning, homework checks, and transparent buffer estimates. Keep the selected unit and the approximation with any copied result. Experimental pH also depends on temperature, activity, calibration, and measurement technique, so a long decimal result should not be interpreted as greater laboratory certainty than the inputs provide.
The values are calculated in the current browser tab. NexaCurrent does not upload the quantities, formulas, chemical names, element choices, or other information entered here, and no account is needed. Copying is a deliberate action after the answer and working are visible. Refreshing or leaving the page clears the current calculation, so keep a copied result if it is needed for later study.