Buffer Calculator

Q: Can I make a 10x or 50x stock and dilute it later?

Yes. For TAE, TE, TBST, and TEN, type the multiple in the Concentration Multiple box. The tool scales every component. Very strong stocks can crash out of solution when cold, so warm and mix before use. A 5x TBE stock, for example, keeps better than a 10x stock because its solutes do not settle out.

Introduction

A buffer is a solution that keeps its pH steady. Even when you add a little acid or base, the pH barely moves. Labs use buffers every day for cell work, protein tests, DNA gels, and more.

This buffer calculator tells you exactly how much of each chemical to weigh out. Pick your buffer, type in the volume, the concentration, and the pH you want. The tool then gives you the mass of every component in grams, milligrams, or other units you choose.

You can pick from more than 30 buffer systems, such as HEPES, Tris-HCl, PBS, phosphate, citrate, acetate, MES, MOPS, and TAE. Each one shows its pKa and its useful pH range, so you can see at a glance if it fits your target pH.

The calculator also does the harder chemistry for you. It corrects the pKa for temperature, adjusts for ionic strength using the Debye-Hückel equation, and works out how much salt to add to hit a set ionic strength. It shows your buffer capacity, warns you if your pH is too far from the pKa, and predicts how the pH will shift when you move the buffer from the bench to a warm incubator or a cold room.

The ionic-strength correction uses the extended Debye–Hückel equation, with the empirical constant B taken as 1.6 kg^1/2 mol^−1/2.¹

Along with the numbers, you get a step-by-step recipe, the full math behind each answer, and charts of buffer capacity and component mass. Everything updates as soon as you change an input.

How to use our Buffer Calculator

Pick your buffer, type in your volume, strength, and target pH, and the calculator tells you how much of each chemical to weigh out. You also get the buffer capacity, the ionic strength, a step-by-step recipe, and the full math behind it.

Filter by category: Click a category chip, like Good's Biological or Phosphate-Based, to shorten the buffer list. Click "Show All" to see every buffer again.

Select Buffer: Choose the buffer system you want to make. Each choice shows its pKa and its useful pH range, so pick one with a pKa close to your target pH.

Display masses in: Pick the unit for your results, such as grams, milligrams, micrograms, kilograms, ounces, or pounds.

Final Volume: Enter how much buffer you need and pick the unit (µL, mL, L, fl oz, or gal). You can also tap a preset button like 500 mL or 1000 mL. Changing the unit converts your number for you.

Buffer Concentration: Type the strength you want and choose nM, µM, mM, or M. This box only shows for buffers where you set the concentration yourself.

Concentration Multiple (×): For stock recipes like TAE, TE, or TBST, enter how strong you want the stock, such as 1×, 10×, or 50×. To dilute that stock down to a working strength, use the Dilution Calculator.

Target pH: Enter the pH you want. Some buffers give you a drop-down list of tested pH values instead, and a few, like TBST, have a fixed pH you cannot change. The colour bar shows if your pH sits in the buffer's best range.

Adjust Ionic Strength: Tick this box if you want to add salt to hit a set ionic strength. Untick it to skip the salt.

Target Ionic Strength (mM): Enter the total ionic strength you need. 154 mM matches normal saline.

Adjustment Salt: Pick NaCl, KCl, or Na₂SO₄ to make up the ionic strength. Choose "Other salt" to type your own molar mass and charge type. The Molecular Weight Calculator gives you the molar mass of any formula you enter.

Preparation Temperature: Enter the temperature of the room where you mix and pH the buffer. Choose °C, °F, or K.

Use Temperature: Enter the temperature where the buffer will be used. The tool shows how much the pH will drift between the two temperatures.

Form / Hydration: In the results table, some chemicals let you switch between anhydrous and hydrated forms. Pick the one on your shelf and the mass updates.

Calculate and Reset: Results update as you type, but you can press Calculate any time. Press Reset to start over.

What Is a Buffer Solution?

A buffer is a water solution that keeps its pH almost the same, even when you add a little acid or base. It works because it holds two partners: a weak acid and its matching base (called the conjugate base).² The acid part soaks up extra base. The base part soaks up extra acid. This keeps the pH steady.

Buffers matter because most living things and many lab tests only work in a narrow pH window. Blood, cells, enzymes, DNA gels, and medicines all need a stable pH. Labs use buffers in studies of enzymes and other parts of cells to keep the pH from changing.² That is why buffers are used every day in biology, chemistry, food science, and medicine.

pKa and Buffer Range

Each weak acid has a number called the pKa. At that pH, half the buffer is acid and half is base. This is the strongest point of the buffer. As a rule, a buffer stops working well once one partner falls below about 10% of the other, which happens about one pH unit below or above its pKa.² Outside that range, it gets weak and cannot hold the pH.

So pick a buffer whose pKa is close to the pH you want. At 25 °C, HEPES has a pKa of 7.564, phosphate 7.198 and acetate 4.756.¹ So for pH 7.4 work, HEPES or phosphate are good picks, and for pH 4.8, acetate fits better.

The Henderson–Hasselbalch Equation

This simple formula links pH, pKa, and the mix of acid and base:

pH = pKa + log ( [base] / [acid] )

If you know the pH you want and the pKa of your buffer, you can solve for how much acid form and how much base form you need.² Then you multiply by the volume and the molar mass to get the grams to weigh out. The log term is a base-10 logarithm.

Buffer Capacity (β)

Buffer capacity tells you how much acid or base the buffer can take before the pH moves. It is written as β, in moles per liter per pH unit. Two things raise it:

Higher concentration: more buffer means more capacity.²
pH close to the pKa: capacity is highest right at the pKa and drops fast as you move away.

Why Temperature Changes pH

The pKa of a buffer shifts when the temperature changes. Each buffer has its own rate, called dpKa/dT. Tris is a big one: its ionization enthalpy is 47.45 kJ/mol.¹ That makes its pKa fall by about 0.028 for each °C of warming, so a Tris buffer set to pH 8.00 in a cold room at 4 °C will read about pH 7.08 at 37 °C. Phosphate, with an enthalpy of only 3.6 kJ/mol, barely moves.¹ Always note the temperature you set the pH at, and check it again at the temperature you will use it.

Ionic Strength and Salt

Ionic strength is half the sum, over all the ions in a solution, of each ion's concentration times the square of its charge.³ For a 1:1 salt such as NaCl, the ionic strength equals the salt's molar concentration.³ Each ion gathers a cloud of oppositely charged ions around it, which screens it and changes how it acts.³ This shifts the real pKa of a buffer. At about 150 mM, the shift is about 0.1 pH unit for HEPES or Tris, about 0.4 for phosphate near pH 7, and about 0.6 for phosphate near pH 12. Many recipes, such as PBS, add a neutral salt such as NaCl or KCl, which sets the ionic strength. For cell and protein work, people often aim for about 150 mM, close to the level inside the body. Normal saline (0.9% sodium chloride) is 154 mM.

Common Buffer Families

Some ranges in this table reach more than one pH unit past the pKa. Near those ends, one partner is below about 10% of the other, so the buffer holds the pH only weakly.²

Family	Examples	Useful pH
Good's biological buffers	MES, PIPES, MOPS, HEPES, Tricine, CHES, CAPS	5.5 to 11.1
Phosphate	Sodium phosphate, KH₂PO₄ with NaOH, PBS	5.8 to 8.0
Citrate	Citric acid with sodium citrate, citrate with HCl	2.2 to 6.6
Phosphate and citrate	McIlvaine	2.2 to 8.0
Carbonate and borate	Borax, bicarbonate with carbonate	7.4 to 10.8
Amino acid	Glycine with HCl, glycine with NaOH	2.0 to 3.6 and 8.6 to 10.6
Molecular biology stocks	TE, TAE, TBST, TEN	7.4 to 8.0

Hydrates Matter When You Weigh

Many buffer salts come with water locked in the crystal. Sodium phosphate dibasic can be anhydrous (142 g/mol) or dodecahydrate (358 g/mol). The same number of moles needs a very different mass. Always check the label on your bottle and use the matching molar mass, or your buffer will be the wrong strength.

Tips for Good Buffers

Dissolve the solids in about 80% of the final water first, then set the pH, then top up to volume. Adding water last keeps the concentration right.
Calibrate your pH meter at the same temperature you are working at.
Pick a buffer that does not bind metals or react with your sample. Phosphate, for example, forms solids with calcium and magnesium, and it binds metal ions with two or more charges, which can slow reactions that need those metals.
Store buffers cold and covered. Tris and phosphate can grow mold or bacteria over time, so throw out any that turn cloudy.
Carbonate buffers pick up CO₂ from the air, so keep them closed.

Formulas used

pKa corrected to preparation temperature

pK_a(T) = pK_a(25^\circ\mathrm{C}) + \frac{dpK_a}{dT}\,(T - 25)

Effective pKa with extended Debye-Huckel ionic strength correction

pK_a' = pK_a(T) - A\,(z_B^2 - z_A^2)\,\frac{\sqrt{I}}{1 + 1.6\sqrt{I}}, \qquad A = 0.4918 + 6.6\times10^{-4}\,T + 3.7\times10^{-6}\,T^2

Henderson-Hasselbalch base and acid fractions ²

r = 10^{\,\mathrm{pH} - pK_a'}, \qquad f_B = \frac{r}{1+r}, \qquad f_A = 1 - f_B

Moles and mass of each component

n_i = c_i \cdot V, \qquad m_i = n_i \cdot M_i

Ionic strength of buffer species and salt needed for target ³

I = \tfrac{1}{2}\sum_i c_i z_i^2, \qquad \Delta I = I_{target} - I_{buffer}, \qquad m_{salt} = \frac{\Delta I \cdot V}{i}\,M_{salt}

van Slyke buffer capacity

\beta = 2.303\,C\,f_A f_B + 2.303\left([H^+] + [OH^-]\right) = 2.303\,C\,\frac{K_a[H^+]}{(K_a + [H^+])^2} + 2.303\left([H^+] + \frac{10^{-14}}{[H^+]}\right)

Expected pH at the use temperature

\mathrm{pH}_{use} = \mathrm{pH}_{prep} + \frac{dpK_a}{dT}\,(T_{use} - T_{prep})

Polyprotic species fractions and average charge (charge-balance pH solver)

\alpha_j = \frac{\prod_{k=1}^{j} 10^{-pK_{a,k}}/[H^+]}{\sum_{m=0}^{n}\prod_{k=1}^{m} 10^{-pK_{a,k}}/[H^+]}, \qquad [H^+] - \frac{10^{-14}}{[H^+]} + C_{cat} + \sum C_t \sum_j (z_0 - j)\alpha_j = 0

Frequently asked questions

Why is the effective pKa different from the pKa printed on my bottle?

The pKa used here is the standard value for 25 °C at zero ionic strength, which can differ from the number a supplier prints.¹ This tool shifts that value twice.

First for your prep temperature, using dpKa/dT.
Then for ionic strength, using the Debye-Hückel correction.

The result is the effective pKa, an estimate of the pKa your pH meter will see. A correction of this kind can be off by about 0.05 to 0.10 pK units, and by more at ionic strengths above 0.1 mol/L.¹

Can I make a 10x or 50x stock and dilute it later?

Yes. For TAE, TE, TBST, and TEN, type the multiple in the Concentration Multiple box. The tool scales every component.

Very strong stocks can crash out of solution when cold, so warm and mix before use. A 5x TBE stock, for example, keeps better than a 10x stock because its solutes do not settle out.

My bottle has a hydrate form that is not in the list. What do I do?

Pick the closest listed form only if the molar mass matches. If it does not, scale the mass yourself: divide the shown mass by the listed molar mass, then multiply by your bottle's molar mass.

Are any of these buffer chemicals hazardous?

Some are. Sodium cacodylate contains arsenic. Barbital is a Schedule IV substance under the Convention on Psychotropic Substances of 1971, and its salts, such as sodium barbitone, are under international control too.⁴ Glacial acetic acid and sodium hydroxide can cause severe burns to the eyes or skin, and hydrochloric acid is also a corrosive.⁵

Read the safety data sheet, wear splash goggles, gloves and a lab coat, and work in a fume hood when needed.⁵

Why does PBS have set amounts of NaCl and phosphate?

PBS is a standard recipe: 137 mM NaCl, 2.7 mM KCl, and 10 mM total phosphate. Those numbers match the salt level in the body, so cells stay happy. One standard recipe at pH 7.4 makes up the phosphate from 8.0 mM Na₂HPO₄ and 1.5 mM KH₂PO₄.⁶

You only choose the volume and the pH.

What is the difference between monobasic and dibasic phosphate?

Monobasic (NaH₂PO₄) is the acid partner. Dibasic (Na₂HPO₄) is the base partner. Mixing equal amounts of the two gives a pH near the phosphate pKa of 7.198 in very dilute solution.¹ Added salt lowers it, to about 6.8 at 150 mM.

More dibasic raises the pH, more monobasic lowers it.

Should I turn the ionic strength adjustment off?

Turn it off when the recipe does not call for extra salt, or when salt would hurt your test, such as some enzyme assays and gel runs.

Keep it on for cell, protein, and binding work where a set salt level matters.

Does adding salt change the pH?

Yes. Salt changes the effective pKa, which shifts the pH. At about 150 mM salt, the shift is about 0.1 pH unit for HEPES or Tris and about 0.4 for phosphate near pH 7. The tool already builds that into its numbers.

Add the salt before your final pH check so the reading is right.

My buffer capacity number looks small. Is that a problem?

Only if your test adds a lot of acid or base. A low β means the pH will drift more easily.

Raise β by using a higher buffer concentration, or by picking a buffer with a pKa closer to your target pH.²

Can I use tap water to make the buffer?

No. Tap water holds calcium, magnesium, chlorine, and other ions that shift the pH and ionic strength. Calcium and magnesium can also bind buffers such as phosphate. Use distilled or deionized water.

Sources

Goldberg RN, Kishore N, Lennen RM. Thermodynamic quantities for the ionization reactions of buffers. Journal of Physical and Chemical Reference Data. 2002;31(2):231-370, Section 8. Accessed September 29, 2026.

Flowers P, Theopold K, Langley R, Robinson WR. Chemistry 2e, 14.6 Buffers. OpenStax. 2019;section 14.6. Accessed September 29, 2026.

Harvey D. 6.9: Activity Effects. Analytical Chemistry 2.1. LibreTexts. section 6.9. Accessed November 12, 2026.

Green List: List of Psychotropic Substances under International Control. International Narcotics Control Board. 2025;36th edition, Substances in Schedule IV. Accessed November 12, 2026.

SOPs: Corrosives. St. Olaf College, Chemical Hygiene. Definitions; Standard Operating Procedures. Accessed November 12, 2026.

Phosphate-Buffered Saline (PBS; 1×, pH 7.4). Cold Spring Harbor Protocols. Cold Spring Harbor Laboratory Press. 2014;recipe pdb.rec081240. Accessed November 12, 2026.