Henderson-Hasselbalch Buffer Calculator
Henderson-Hasselbalch buffer calculator. Finds buffer pH from pKa and the conjugate base / weak acid ratio, or the ratio needed for a target pH. Educational.
Henderson-Hasselbalch Buffer Calculator
How to use the Henderson-Hasselbalch calculator
Choose what to solve for
Pick "Buffer pH" to find the pH from known concentrations, or "Base/acid ratio" to find the ratio of conjugate base to weak acid needed to hit a target pH.
Enter the pKa
Use the pKa of the weak acid in your buffer — for example 4.76 for acetic acid or 7.21 for the dihydrogen-phosphate system. The buffer works best when the target pH is near this value.
Enter the concentrations or target
For pH mode, type the molar concentrations of the conjugate base and the weak acid. For ratio mode, type the pH you want to achieve.
Read the result
You get the buffer pH (or the required ratio) plus how far the pH sits from the pKa — a guide to buffering capacity. Verify against a measured pH for real preparations.
Henderson-Hasselbalch — the equation behind every buffer
pH from a ratio
A buffer resists changes in pH when small amounts of acid or base are added, and the Henderson-Hasselbalch equation is the simple relationship that describes it. It states that the pH equals the pKa of the weak acid plus the base-ten logarithm of the ratio of conjugate base to weak acid: pH = pKa + log([A⁻]/[HA]). The logic is elegant. When the concentrations of the conjugate base and the weak acid are equal, the ratio is one, its logarithm is zero, and the pH simply equals the pKa — the point of maximum buffering. Add more conjugate base and the logarithm becomes positive, raising the pH; add more acid and it becomes negative, lowering the pH. Because the relationship is logarithmic, it takes a tenfold change in the ratio to move the pH by a single unit, which is exactly why buffers are so stable: large additions of acid or base only shift the ratio modestly, so the pH barely moves.
This makes the equation a practical recipe. To prepare a buffer at a chosen pH, you pick a weak acid whose pKa is close to that pH, then set the ratio of its salt (the conjugate base) to the acid using the rearranged form, ratio = 10^(pH − pKa). A buffer is most effective within about one pH unit of the pKa, where the ratio stays between roughly 1:10 and 10:1; outside that window the buffering capacity falls off sharply. That is why biological buffers are chosen to match the pH they must hold — the phosphate and bicarbonate systems, for instance, have pKa values near physiological pH.
"A buffer is strongest where base and acid are equal — there the pH simply equals the pKa. It takes a tenfold shift in their ratio to move the pH by one unit, which is why buffers hold so steady."
What the simple equation leaves out
Henderson-Hasselbalch is an approximation, and knowing its assumptions keeps you out of trouble. It treats activities as equal to concentrations, which holds well only in dilute solutions; at higher ionic strength, activity coefficients differ and the real pH drifts from the calculated value. It also assumes the weak acid is genuinely weak and only partly dissociated, that the buffer components are present in much larger amounts than any added acid or base, and that the system is at a single temperature, since pKa itself shifts with temperature. The equation ignores the small contributions of water's own dissociation, which matter at very dilute concentrations or extreme pH. For these reasons, the calculated pH is an excellent starting point but a real buffer is finalised by measuring its pH with a calibrated meter and adjusting. Use this calculator to design buffers and build intuition for how ratio, pKa, and pH relate, and confirm the actual pH experimentally for any preparation that matters.
10 Facts About Buffers
pH = pKa + log([A⁻]/[HA]).
When base = acid, pH = pKa — peak buffering.
A tenfold ratio change moves pH by one unit.
Buffers work best within ±1 pH of the pKa.
Ratio for a target pH = 10^(pH − pKa).
Acetic acid pKa 4.76; phosphate 7.21.
Blood is buffered mainly by the bicarbonate system.
The equation assumes activities ≈ concentrations.
pKa shifts with temperature.
Always measure and adjust a real buffer's pH.
Frequently asked questions
It relates the pH of a buffer to the pKa of its weak acid and the ratio of conjugate base to acid: pH = pKa + log([A⁻]/[HA]). It lets you calculate a buffer's pH from its composition, or work out the ratio of salt to acid needed to reach a target pH. It's the foundational equation for preparing and understanding buffer solutions in chemistry and biology.
When the pH equals the pKa, the conjugate base and the weak acid are present in equal amounts, so the buffer has equal capacity to neutralise added acid or added base. Moving away from the pKa makes one component scarce, so the buffer can absorb less of one kind of addition before its pH shifts. Buffering capacity is therefore greatest at the pKa and falls off as you move away, becoming weak beyond about one pH unit on either side.
Pick a weak acid whose pKa is within about one unit of the pH you want to hold, ideally as close as possible. Then use the equation to set the ratio of conjugate base to acid: ratio = 10^(pH − pKa). For example, to make a pH 7.4 buffer you might choose a phosphate system (pKa ≈ 7.2) and adjust the salt-to-acid ratio accordingly. Matching the pKa to the target pH is the key to good buffering capacity.
A buffer is generally effective within about ±1 pH unit of its pKa, where the base-to-acid ratio stays between roughly 1:10 and 10:1. Within that window both components are abundant enough to neutralise additions. Outside it, one component becomes too scarce and the buffering capacity drops sharply, so the pH becomes unstable. That's why you select a buffer system whose pKa brackets the pH you need to maintain.
According to the simple equation, only the ratio of base to acid sets the pH, not their absolute concentrations — so a 0.1 M and a 0.01 M buffer at the same ratio give the same calculated pH. What concentration changes is the buffer capacity: a more concentrated buffer can absorb more added acid or base before its pH shifts. In reality, very dilute or very concentrated buffers also deviate from the ideal equation because of activity effects, so measure and adjust.
Because the equation is an approximation. It assumes activities equal concentrations, which only holds in dilute solutions; at higher ionic strength the real pH drifts. It also assumes a single temperature (pKa changes with temperature) and ignores water's own dissociation. Real reagents have impurities and the salt may carry water of hydration. For these reasons a calculated buffer is always finalised by measuring its pH with a calibrated meter and adjusting with small amounts of acid or base.
HA is the weak acid — the protonated form — and A⁻ is its conjugate base, the form that has lost a proton (usually added as a salt). In an acetate buffer, for example, HA is acetic acid and A⁻ is the acetate ion. The equation uses the ratio of these two forms. Adding acid converts some A⁻ to HA; adding base converts some HA to A⁻; the buffer resists pH change by shifting between them.
Yes, but you use the pKa of the specific dissociation step that brackets your target pH. Phosphoric acid, for instance, has three pKa values (about 2.1, 7.2, and 12.4); for a buffer near physiological pH you use the second one (7.2) with the dihydrogen-phosphate and hydrogen-phosphate pair. As long as your pH is near one pKa and far from the others, treating it as a single weak-acid/conjugate-base pair works well.
Use it to design buffers and learn how the variables interact, but always confirm the pH of a real preparation with a calibrated pH meter and adjust as needed. The calculation gives an excellent target, yet activity effects, temperature, and reagent purity mean the measured pH can differ. This tool is educational; for laboratory or regulated work, finalise buffers experimentally and follow your validated protocol.
No. The values you enter are processed entirely in your browser. Nothing is sent to a server, stored, or shared, and no account is required. The calculation runs on your device only.
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