Buffer Solution Volume & Mass Calculator
Find the mass of acid and conjugate base to weigh out for a target pH and volume, work out the volumes of two stock solutions to mix, check the pH of a buffer you already made, and dilute a buffer stock without shifting its pH, all with step-by-step working.
Select a calculation, pick a buffer system if you like, then enter the known values.
Acetic acid (CH3COOH) mass to weigh
Formula used: pH = pKa + log₁₀([A⁻]/[HA]) → moles → mass
1.7378 : 1
Base : Acid ratio
0.2193 g
Acetic acid (CH3COOH) mass
0.5207 g
Sodium acetate (CH3COONa) mass
0.01 mol
Total moles
Buffering Range & Composition Visual
Where the target pH sits relative to the buffer's usable range (pKa ± 1), and the resulting acid : base composition.
Step-by-Step Buffer Solution Calculation
Here's exactly how this answer was calculated, one step at a time.
Given: pKa = 4.76, target pH = 5.0, concentration = 0.1 mol/L, volume = 100 mL
Step 1: Find the required base-to-acid ratio from the Henderson-Hasselbalch equation
Rearranging pH = pKa + log10([A-]/[HA]) for the ratio gives the exact proportion of conjugate base to acid the buffer needs to sit at the target pH.
[A⁻]/[HA] = 10^(pH − pKa) = 10^(5 − 4.76) = 1.7378Step 2: Turn the ratio into fractions of the total buffer
fraction HA = 1 ÷ (1 + 1.7378) = 0.36526; fraction A⁻ = 1.7378 ÷ (1 + 1.7378) = 0.63474Step 3: Find the total moles of buffer needed
Total buffer concentration times the final volume (in litres) gives the combined moles of acid form plus base form the recipe needs.
moles(total) = 0.1 mol/L × 0.1 L = 0.01 molStep 4: Split the total moles between the acid form and the base form
moles HA = 0.01 × 0.36526 = 0.003653 mol; moles A⁻ = 0.01 × 0.63474 = 0.006347 molStep 5: Convert each mole amount to a mass using its molar mass
mass HA = 0.003653 × 60.05 = 0.2193 g; mass A⁻ = 0.006347 × 82.03 = 0.5207 gStep 6: Report the result
Acetic acid (CH3COOH) mass to weigh = 0.2193 g (plus 0.5207 g of the conjugate base)
Calculated result:
0.2193 g
Buffer Solution Volume & Mass Calculator: Prep Any Buffer Online
This free buffer solution calculator tells you exactly how much acid and conjugate base to weigh out, or how much of two stock solutions to mix, to make a buffer at any pH and volume you need. It also checks the pH of a buffer you've already made from known masses, and it works out how to dilute a buffer stock down to a working strength without shifting its pH.
It's built for students working through Henderson-Hasselbalch problems, lab technicians who need a quick recipe before they start weighing chemicals, and anyone prepping media, mobile phases, or reagent solutions who wants the numbers checked before they touch a balance. Pick the calculation you need, choose a buffer system from the built-in list or enter your own pKa and molar masses, and the result appears instantly with a full step-by-step breakdown.
What Is a Buffer Solution, and Why Does the Recipe Matter?
A buffer solution is a mixture of a weak acid and its conjugate base (or a weak base and its conjugate acid) that resists changes in pH when small amounts of acid or base are added to it. It works because the acid form can soak up added base, and the base form can soak up added acid, so the overall pH barely moves. This is why buffers show up everywhere from blood chemistry to shampoo formulation to the mobile phase in an HPLC run.
Getting the recipe right matters because a buffer only works well close to its pKa, and only if the acid-to-base ratio and the total concentration are both correct. Get the ratio wrong and the pH lands somewhere other than intended. Get the total concentration wrong and the buffer either can't resist much pH change, or it ends up stronger (and more expensive, and more likely to interfere with a downstream assay) than it needs to be. This calculator handles both the ratio and the concentration side of the recipe in one place.
The Henderson-Hasselbalch Equation Behind Every Buffer Recipe
Every calculation on this page comes from one equation: pH = pKa + log10([A⁻] / [HA]), where [A⁻] is the concentration of the conjugate base form and [HA] is the concentration of the weak acid form. Rearranged, it gives the ratio you need: [A⁻] / [HA] = 10^(pH − pKa). Once that ratio is known, splitting a target volume or a target total concentration between the two forms is just simple proportion and unit conversion, which is exactly what each mode of this calculator automates.
The pKa is the pH at which the acid form and the base form are present in exactly equal amounts (a 1:1 ratio), and it's a fixed property of the weak acid being used, not something you choose. Every buffer system has its own pKa, which is why picking the right acid/base pair for your target pH is the first real decision in any buffer recipe — the calculator's built-in buffer library exists to make that choice easier.
Method 1: Weighing Out Solid Acid and Conjugate Base
The most direct way to make a buffer is to weigh out both the weak acid and its conjugate base as solids, dissolve them together, and top up to volume. This calculator's "mass to weigh out" mode takes your target pH, total buffer concentration, and final volume, works out the base:acid ratio from Henderson-Hasselbalch, splits the total moles between the two forms, and converts each portion into grams using the molar masses you supply (or pull from the buffer library).
As a worked example, making 100 mL of a 0.1 mol/L acetate buffer at pH 5.0 (acetic acid pKa 4.76) needs a ratio of 10^(5.0 − 4.76) ≈ 1.74. That splits 0.01 mol total buffer into about 0.00365 mol acetic acid and 0.00635 mol sodium acetate — roughly 219 mg of acetic acid and 521 mg of sodium acetate, dissolved together and made up to 100 mL. This is the standard weigh-and-dissolve method used throughout teaching labs and small-scale prep.
Method 2: Mixing Two Ready-Made Stock Solutions
In many real labs it's faster to keep two stock solutions on hand — one of the acid form, one of the conjugate base form, both at the same molarity — and simply mix them in the right proportion to hit a target pH. This is the classic method behind published phosphate buffer tables, and it avoids weighing anything on the day you need the buffer.
Because the two stocks share the same concentration, the concentration terms cancel out of the Henderson-Hasselbalch equation, leaving a simple volume ratio: Vbase ÷ Vacid = 10^(pH − pKa). The calculator's "mix stock solutions" mode splits your target final volume using that ratio directly. For example, mixing 0.1 mol/L NaH2PO4 and 0.1 mol/L Na2HPO4 (pKa 7.21) to hit pH 7.4 for 100 mL needs a ratio of about 1.55, which works out to roughly 39 mL of the monobasic stock and 61 mL of the dibasic stock — no extra water needed, since the volumes already add up to 100 mL.
Checking the pH of a Buffer You Already Made
Sometimes the question runs in the other direction: a buffer has already been weighed out or mixed, and the question is what pH it actually sits at. This calculator's "check pH" mode takes the mass (or amount) of each component actually used, converts both to moles using their molar masses, and runs the Henderson-Hasselbalch equation forward to report the resulting pH.
This is useful for checking a recipe before it's used, troubleshooting a buffer that isn't behaving as expected, or working backward from a lab notebook entry that recorded masses but not the intended pH. It's also a handy sanity check on the other two preparation modes — plug the masses or volumes they suggest back into this mode, and the reported pH should match your original target.
Diluting a Buffer Stock Without Losing the pH
Buffers are often made up as a concentrated stock and diluted down to a working strength as needed. Because dilution adds water and nothing else, it doesn't change how much acid form or base form is present relative to each other — it only changes the total volume they're dissolved in. Since the Henderson-Hasselbalch equation only cares about the ratio of the two concentrations, that ratio stays the same after dilution, and so does the pH, at least in the ideal case.
This calculator's "dilute" mode applies the standard dilution law, C1V1 = C2V2, to the buffer's total concentration to find the final volume and how much diluent to add. What does change on dilution is buffering capacity — a more dilute buffer has fewer acid and base molecules on hand to absorb an added acid or base, so it resists pH change less strongly even though its starting pH is unchanged. Real solutions also show a small ionic-strength effect on the actual pKa, so for pH-critical dilutions it's still worth checking the final pH with a calibrated meter.
Choosing the Right Buffer System and pKa
A buffer only resists pH change well within about one pH unit of its pKa (the range pKa − 1 to pKa + 1) — this calculator's visual highlights that band directly. Outside that range, one of the two forms is present in such a small amount that it can't absorb much additional acid or base before it runs out, and the pH starts moving much more freely. The first step in any buffer recipe should always be picking a weak acid whose pKa sits close to the pH you actually need.
As a rough guide: acetate buffers cover roughly pH 3.8–5.8, phosphate buffers cover roughly pH 6.2–8.2 (making them a common choice for near-neutral biological work), Tris buffers cover roughly pH 7.1–9.1, and carbonate-bicarbonate buffers cover the alkaline range around pH 9.3–11.3. Matching the system to the target pH first, then using this calculator to work out the exact recipe, is the order most textbooks and lab protocols recommend.
Common Buffer Systems Built Into This Calculator
This calculator's buffer library covers the systems used most often across teaching labs, biochemistry, molecular biology, and pharmaceutical formulation: acetate (acetic acid / sodium acetate, pKa 4.76), phosphate (monobasic / dibasic sodium phosphate, pKa 7.21), Tris (Tris-HCl / Tris base, pKa 8.06), citrate (citric acid / trisodium citrate, pKa 6.40), carbonate-bicarbonate (sodium bicarbonate / sodium carbonate, pKa 10.33), glycine (glycine hydrochloride / glycine, pKa 9.60), borate (boric acid / borax, pKa 9.24), HEPES (free acid / sodium salt, pKa 7.48), MES (free acid / sodium salt, pKa 6.15), and an ammonia buffer (ammonium chloride / ammonia, pKa 9.25).
Picking any of these from the dropdown auto-fills the pKa and both molar masses, so the only numbers left to enter are the target pH, concentration, and volume. A "custom" option is also available for any acid/base pair not on the list — just enter the pKa and molar masses from a reference source or a certificate of analysis.
Where Buffer Solution Calculations Are Used
Biology and biochemistry labs use buffers constantly — cell culture media, protein purification steps, enzyme assays, and DNA/RNA work all need a stable pH held close to a specific value, and most published protocols specify a buffer recipe that needs to be scaled or adjusted for the volume actually being made. Analytical chemistry leans on buffers just as heavily, particularly in HPLC and other chromatography methods, where the mobile phase's pH controls how well different compounds separate.
Pharmaceutical formulation uses buffers to keep a drug product's pH inside the narrow range where the active ingredient stays stable and the formulation stays comfortable to use. Food science, cosmetics, water treatment, and educational chemistry labs round out the main use cases — anywhere a solution needs to hold its pH steady despite small additions of acid or base, a buffer recipe like the ones this calculator produces is doing the work behind the scenes.
Mistakes to Avoid When Preparing a Buffer
The most common mistake is picking a buffer system whose pKa is too far from the target pH — even a perfectly calculated recipe will make a weak, poorly-resisting buffer if the pH sits outside the pKa ± 1 range. A second common mistake is confusing total buffer concentration with the concentration of just one component; the total concentration this calculator asks for is the combined acid-form-plus-base-form concentration, not either one on its own.
It's also easy to grab the wrong molar mass by forgetting that many buffer salts are sold as hydrates (for example, trisodium citrate dihydrate versus the anhydrous salt), which changes the molar mass and therefore the mass actually needed. Finally, remember that the Henderson-Hasselbalch equation is an approximation that assumes ideal, dilute behavior — for very concentrated buffers, very low pKa acids, or pH-critical work, always confirm the final pH with a calibrated pH meter rather than relying on the calculated value alone.
Buffer Solution Calculator FAQ and Quick Reference
To find the mass of acid and base to weigh out, use ratio = 10^(pH − pKa), split the total moles (concentration × volume) between the two forms using that ratio, then multiply each portion by its molar mass. To mix from equal-molarity stock solutions, use the same ratio directly as a volume ratio and split the target final volume accordingly. To check a buffer's pH, convert each weighed mass to moles and apply pH = pKa + log10(moles base ÷ moles acid). To dilute a buffer, apply C1V1 = C2V2 to the total concentration — the pH itself stays essentially unchanged.
This free buffer solution volume and mass calculator is meant for study, lab preparation, and general reference use in chemistry, biology, and pharmaceutical settings. Always confirm your buffer system's actual pKa and the molar mass of the specific salt form you're using (including any water of hydration) against a certificate of analysis, and check the final pH of any pH-critical buffer with a calibrated meter before relying on it.
Frequently Asked Questions
How do you calculate the amount of acid and base needed for a buffer?
Use the Henderson-Hasselbalch equation to find the base:acid ratio, 10^(pH − pKa). Multiply the total buffer concentration by the final volume to get total moles, split those moles between the acid and base forms using the ratio, then convert each portion to a mass with its molar mass.
What is the Henderson-Hasselbalch equation?
pH = pKa + log10([A⁻] / [HA]), where [A⁻] is the concentration of the conjugate base form and [HA] is the concentration of the weak acid form. It relates a buffer's pH to its pKa and the ratio of its two components.
How do I choose the right buffer for a target pH?
Pick a weak acid whose pKa is within about one pH unit of the pH you need (the range pKa − 1 to pKa + 1), since that's where a buffer resists pH change most effectively. This calculator's built-in library lists the pKa of ten common buffer systems to help with that choice.
What does 'total buffer concentration' mean?
It's the combined concentration of the acid form and the conjugate base form together, not either one on its own. A 0.1 mol/L buffer might, for example, split into 0.04 mol/L acid form and 0.06 mol/L base form, depending on the target pH.
Does diluting a buffer change its pH?
In the ideal case, no — diluting a buffer keeps the ratio of acid form to base form the same, and pH depends only on that ratio and the pKa. What does change is buffering capacity, since a more dilute buffer has less acid and base on hand to absorb an added acid or base.
Why is my calculated buffer mass different from a published recipe?
Published recipes sometimes use a hydrated form of a salt (like a dihydrate or decahydrate) with a different molar mass than the anhydrous form, or they may target a slightly different pKa value. Double-check the exact molar mass of the salt form you're using and the pKa source before comparing figures.
Can I use this calculator for phosphate buffer at pH 7.4?
Yes. Pick the phosphate buffer preset (pKa 7.21), select either the mass-to-weigh mode or the mix-stock-solutions mode, set the target pH to 7.4, and enter your desired concentration and volume.
What's the difference between this calculator and a Henderson-Hasselbalch pH calculator?
A Henderson-Hasselbalch pH calculator typically solves for pH from known concentrations. This calculator works the other direction as well — starting from a target pH and volume, it works out the actual masses or stock volumes needed to prepare the buffer, plus dilution and pH-check tools.