Henderson-Hasselbalch (Buffer) Calculator
Find buffer pH from pKa and the conjugate base/acid ratio, or work backwards to find the ratio needed for a target pH. A live 3D beaker and charts show how the HA/A⁻ balance shifts with the ratio.
Reviewed by the ToolNestr Editorial Team — July 2026
Tip: enter pKa and just the ratio directly in [A⁻] if you leave [HA] as 1 — or fill both concentrations, units cancel in the ratio. pKa = −log₁₀(Ka) if you only have Ka.
Two ideas that trip students up
1. Equal HA and A⁻ at the pKa point
A fixed 1:1 mix of HA (amber) and A⁻ (teal) — the ratio where pH = pKa exactly and buffering is strongest.
2. Every tenfold ratio shifts pH by one unit
Three fixed points on the pH-vs-log(ratio) line — 1:10, 1:1, and 10:1 — spaced one pH unit apart, always the same straight-line slope of 1.
Buffer graphs
How it works
The core idea in one line: a buffer's pH is set by two things — how strong its weak acid is (pKa) and how much conjugate base it has relative to remaining acid (the [A⁻]/[HA] ratio).
pH = pKa + log₁₀([A⁻]/[HA])
Henderson-Hasselbalch equation
[A⁻]/[HA] = 10(pH − pKa)
ratio needed for a target pH
pKa = −log₁₀(Ka)
from the acid dissociation constant
Because the equation is pH = pKa + log₁₀([A⁻]/[HA]), every tenfold change in the ratio shifts pH by exactly one unit, and when the ratio is 1 the log term vanishes, so pH = pKa exactly — the point where the buffer has the most reserve in both directions. Rearranged as [A⁻]/[HA] = 10^(pH − pKa), the same equation tells you how much conjugate base to mix with acid to prepare a buffer at any target pH.
Worked example 1 — equal parts acetic acid/acetate
Given: An acetate buffer has pKa = 4.76 (acetic acid) with equal concentrations of acetate ([A⁻]) and acetic acid ([HA]), so the ratio [A⁻]/[HA] = 1. Find the pH.
When [A⁻] = [HA], log₁₀(1) = 0, so pH always equals pKa exactly — this is the point of maximum buffer capacity.
Worked example 2 — phosphate buffer with a 2:1 ratio
Given: A phosphate buffer has pKa = 7.21 (HPO₄²⁻/H₂PO₄⁻ system), with [A⁻]/[HA] = 2 (twice as much conjugate base as acid). Find the pH.
log₁₀(2) ≈ 0.30103, so pH = 7.21 + 0.301 ≈ 7.511, rounded to 7.51 — close to physiological pH, which is why phosphate buffers are common in cell culture media.
Common buffer systems and their pKa
Approximate standard values at 25°C — exact pKa shifts slightly with ionic strength and temperature.
| Buffer system | Conjugate pair | pKa | Effective range |
|---|---|---|---|
| Acetic acid / acetate | CH₃COOH / CH₃COO⁻ | ≈ 4.76 | ≈ 3.8 – 5.8 |
| Carbonic acid / bicarbonate | H₂CO₃ / HCO₃⁻ | ≈ 6.1 | ≈ 5.1 – 7.1 |
| Phosphate | H₂PO₄⁻ / HPO₄²⁻ | ≈ 7.21 | ≈ 6.2 – 8.2 |
| Ammonia / ammonium | NH₄⁺ / NH₃ | ≈ 9.25 | ≈ 8.25 – 10.25 |
Effective buffering range is roughly pKa ± 1, where the ratio [A⁻]/[HA] stays between about 1:10 and 10:1. The bicarbonate system is the primary buffer of human blood.
Where buffers actually matter
🩸 Blood pH buffering
The bicarbonate system (H₂CO₃/HCO₃⁻, pKa ≈ 6.1) keeps human blood pH tightly near 7.4, working together with respiration — exhaling CO₂ shifts the equilibrium and adjusts pH within seconds.
⚗️ Biochemistry lab buffer preparation
Researchers use the Henderson-Hasselbalch equation in reverse: pick a weak acid whose pKa is close to the target pH, then calculate the [A⁻]/[HA] ratio (and hence how much of each stock solution to mix) to hit that pH precisely.
💊 Pharmaceutical formulation
Drug stability and absorption often depend on formulation pH. Buffers keep injectable and oral formulations at a target pH throughout shelf life, preventing degradation or precipitation.
🧫 Cell culture media
Cell culture media are buffered — often with phosphate or bicarbonate/CO₂ systems — to hold pH near 7.2–7.4, the narrow range most mammalian cells need to survive and grow.
Common misconceptions
"A buffer keeps pH exactly constant no matter what."
A buffer resists pH change within a limited capacity — roughly pKa ± 1. Add enough strong acid or base to use up all of the HA or A⁻ reserve, and the pH will swing rapidly, just like an unbuffered solution.
"You need equal amounts of acid and base to buffer at all."
Equal amounts ([A⁻] = [HA]) give the maximum capacity and set pH = pKa exactly, but a buffer still works at other ratios — it just resists change less well in one direction and targets a different pH (pH = pKa + log of the ratio).
"pKa is the same as pH."
pKa is a fixed property of the weak acid (from its Ka). pH is the actual acidity of the specific solution, which depends on pKa AND the ratio of conjugate base to acid present. They are only equal when the ratio is exactly 1.
"Any weak acid can buffer any pH."
A buffer only works well within about one pH unit of its pKa, because outside that window one of [A⁻] or [HA] becomes so small that the reserve to neutralise added acid or base runs out. Choose a weak acid whose pKa is close to the pH you need.
Formula sources & further reading
The formulas here are standard, traceable to:
- • OpenStax, Chemistry 2e — acid-base equilibria and buffer solutions (free, peer-reviewed). openstax.org
- • Brown, LeMay & Bursten, Chemistry: The Central Science — Chapter 17, Additional Aspects of Aqueous Equilibria (buffered solutions).
- • Zumdahl & Zumdahl, Chemistry — the Henderson-Hasselbalch equation and buffer capacity.
pH = pKa + log₁₀([A⁻]/[HA]); pKa = −log₁₀(Ka). pKa values for common buffers are standard reference approximations at 25°C. Results are rounded for display.
How to use this calculator
Pick the mode
"Find pH" solves pH from pKa and the ratio; "Find ratio" solves the ratio needed for a target pH.
Enter the values
Type pKa and the ratio (or [A⁻] and [HA] separately), or pKa and your target pH.
See it visually
The 3D diagrams show a 1:1 HA/A⁻ mix at pKa and three fixed points on the pH-vs-ratio line.
Related tools
Frequently asked questions
What is a buffer solution?
A buffer is a solution that resists changes in pH when small amounts of acid or base are added. It is made from a weak acid (HA) and its conjugate base (A⁻), usually in comparable amounts, so it can neutralise either added H⁺ or added OH⁻.
What does the Henderson-Hasselbalch equation calculate?
It calculates the pH of a buffer from the acid's pKa and the ratio of conjugate base to weak acid: pH = pKa + log₁₀([A⁻]/[HA]). Rearranged, it also gives the ratio needed to hit a target pH — the key equation for preparing buffers in the lab.
Why is buffering strongest near pH = pKa?
When [A⁻] = [HA] (ratio = 1), pH = pKa exactly, and the solution has roughly equal reserves of weak acid to soak up added base and conjugate base to soak up added acid. That balance gives the maximum resistance to pH change — buffer capacity is greatest within about one pH unit of pKa (pKa ± 1) and drops off sharply outside that range.
What are common buffer systems in biology and chemistry?
Acetic acid/acetate (pKa ≈ 4.76) is a classic lab buffer. Phosphate (H₂PO₄⁻/HPO₄²⁻, pKa ≈ 7.21) buffers near physiological pH and is used in cell culture media. Bicarbonate/carbonic acid (pKa ≈ 6.1) is the primary buffer of human blood, working alongside respiration to hold blood pH near 7.4. Ammonia/ammonium (pKa ≈ 9.25) buffers in the basic range.
Can I use Ka instead of pKa?
Yes — pKa = −log₁₀(Ka). If you know the acid dissociation constant Ka rather than pKa, convert it first. For example, acetic acid has Ka ≈ 1.8 × 10⁻⁵, so pKa = −log₁₀(1.8 × 10⁻⁵) ≈ 4.74–4.76 depending on the source.