Molarity for peptides: mass to moles and back

Molarity is moles of a substance per liter of solution. For a peptide it is the concentration that chemistry answers to, because a reaction, a binding event or a column sees molecules, not milligrams. Converting a mass of peptide to a molarity takes one equation and one number, the molecular weight, and most mistakes come from using the wrong number. This article works the arithmetic through, with the places it goes wrong marked.

The equation

Amount of substance in moles equals mass divided by molecular weight. Concentration in moles per liter equals that amount divided by the volume in liters:

The three forms of the same relation

To find	Use	Units that must agree
Molarity (M)	M = mass (g) ÷ MW (g/mol) ÷ volume (L)	grams, g/mol, liters
Mass to weigh (g)	mass = M × volume (L) × MW	mol/L, liters, g/mol
Volume to dissolve in (L)	volume = mass (g) ÷ MW ÷ M	grams, g/mol, mol/L

Keeping the units in grams, liters and moles until the last step avoids the commonest slip. Milligrams and milliliters cancel conveniently: mg/mL ÷ (g/mol) gives mol/L directly, because the two factors of a thousand cancel. So 1 mg/mL of a peptide of molecular weight 1,000 g/mol is 1 mM.

Which molecular weight

A product page or a reference entry gives the molecular weight of the peptide as a free base: the molecule with no counter-ions. A certificate of analysis may give the same figure or, less often, the mass of the salt as supplied. A freeze-dried powder is the salt, and each charged basic residue carries one acid counter-ion, trifluoroacetate (114 g/mol) or acetate (60 g/mol). For a short peptide the difference is large: a tripeptide of free-base mass 340 g/mol as a bis-trifluoroacetate salt weighs 568 g/mol, a 67% difference.

The rule: use the free-base molecular weight with the mass of peptide, which is the weighed mass multiplied by the net peptide content. Or use the salt's molecular weight with the weighed mass, if the certificate states the salt's mass and the water content separately. Mixing the two conventions is the error that produces solutions off by a factor that happens to look plausible. Net peptide content and purity explains the correction; peptide salt forms explains where the counter-ions come from.

A worked example

Take a 15-residue peptide with a free-base molecular weight of 1,419.5 g/mol, supplied as 5 mg of powder with a net peptide content of 82% on its certificate. The aim is a 1 mM stock.

1. Peptide in the vial: 5 mg × 0.82 = 4.1 mg = 0.0041 g.
2. Amount: 0.0041 g ÷ 1,419.5 g/mol = 2.89 × 10^-6 mol = 2.89 µmol.
3. Volume for 1 mM (0.001 mol/L): 2.89 × 10^-6 mol ÷ 0.001 mol/L = 2.89 × 10^-3 L = 2.89 mL.

Dissolving the whole vial in 2.89 mL gives a 1 mM stock, which is 1.42 mg/mL of peptide, or 1.73 mg/mL of the powder as weighed. Skipping the net peptide content correction and dissolving in 3.52 mL (5 mg ÷ 1,419.5 ÷ 0.001) gives a solution of 0.82 mM labeled 1 mM: an 18% error carried into every experiment that follows.

Why the whole vial beats the balance

For a few milligrams of a hygroscopic powder, the error in weighing a portion out on a laboratory balance is often larger than the error in the vial's stated quantity. A lot's verified quantity on a certificate was measured on an analytical balance under controlled conditions, so dissolving the entire contents in a calculated volume, as above, is usually the more accurate route. Weighing milligrams of peptide gives the numbers behind that claim.

Using the calculator's numbers

The peptide calculator gives the average molecular weight for any sequence, including termini and modifications, and that is the figure to use for molarity. The monoisotopic mass it also reports is for mass spectrometry and is not the right number here; for a 15-residue peptide the two differ by about 0.9 g/mol, which is immaterial, but for larger peptides the gap grows, and the average mass is the correct one by definition for a bulk quantity.

Checking by absorbance

A stock made this way can be checked directly if the peptide contains tryptophan, tyrosine or a disulfide. The calculator gives a molar absorbance at 280 nm; dividing the measured absorbance of the solution (in a 1 cm cell) by that figure gives the molarity. A stock that measures within 10% of the calculated value confirms the whole chain. One that measures 20% low usually means the net peptide content correction was skipped, or part of the material did not dissolve.

Frequently asked questions

Is 1 mg/mL the same concentration for every peptide?

As a mass concentration, yes; as a molarity, no. A 500 g/mol tripeptide at 1 mg/mL is 2 mM; a 5,000 g/mol peptide at 1 mg/mL is 0.2 mM. Comparing peptides needs molarity.

Which molecular weight does the product page show?

The peptide's own, as a free base, matching its reference entry. The powder weighs more because of counter-ions and water, which is what net peptide content accounts for.

What if the certificate gives no net peptide content?

Then the mass of peptide is not known better than roughly 70–90% of the weighed mass for a trifluoroacetate salt. A measured concentration by absorbance or amino acid analysis is the way to pin it down.

Does a disulfide bond change the molecular weight?

By 2.016 g/mol per bond, the two hydrogens lost. The calculator applies it when the sequence is written with the bond.

Sources & Further Reading

1. International Union of Pure and Applied Chemistry. Amount concentration. IUPAC Compendium of Chemical Terminology (the "Gold Book"). doi:10.1351/goldbook.A00295
2. Pace CN, Vajdos F, Fee L, Grimsley G, Gray T. How to measure and predict the molar absorption coefficient of a protein. Protein Science 1995;4(11):2411–2423. doi:10.1002/pro.5560041120
3. Westwood S, Choteau T, Daireaux A, Josephs RD, Wielgosz RI. Mass balance method for the SI value assignment of the purity of organic compounds. Analytical Chemistry 2013;85(6):3118–3126. doi:10.1021/ac303329k
4. Burkitt WI, Pritchard C, Arsene C, Henrion A, Bunk D, O'Connor G. Toward Système International d'Unité-traceable protein quantification: from amino acids to proteins. Analytical Biochemistry 2008;376(2):242–251. doi:10.1016/j.ab.2008.02.010

Cosmic Peptides. (2026, October 30). Molarity for peptides: mass to moles and back. Cosmic Peptides Research Library. Retrieved October 30, 2026, from https://cosmicpeptides.com/research/methods/peptide-molarity-calculations

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  author       = {{Cosmic Peptides}},
  title        = {Molarity for peptides: mass to moles and back},
  howpublished = {Cosmic Peptides Research Library},
  year         = {2026},
  month        = oct,
  url          = {https://cosmicpeptides.com/research/methods/peptide-molarity-calculations},
  urldate      = {2026-10-30},
  note         = {Last reviewed 2026-10-30}
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