Concentration units for peptides: mg/mL, mM, µM, percent and ppm

Peptide concentrations are written in at least four systems: mass per volume, molarity, percentage, and parts per million. They describe the same solution, and converting between them needs nothing but the molecular weight and attention to prefixes. This page is the conversion reference: the definitions, the factors, and a table of the figures a laboratory meets most.

The units and what they mean

Concentration units used for peptides

Unit	Means	Equal to	Used for
mg/mL	Milligrams of solute per milliliter of solution	1 g/L; 1,000 µg/mL	Stocks, labels, certificates
µg/mL	Micrograms per milliliter	1 mg/L; 1 ppm (in water)	Working solutions
ng/mL	Nanograms per milliliter	1 µg/L; 1 ppb	Assay concentrations, limits of detection
M (mol/L)	Moles per liter	1,000 mM	Chemistry; comparing peptides
mM	Millimoles per liter	1,000 µM; 10-3 M	Stocks
µM	Micromoles per liter	1,000 nM; 10-6 M	In vitro working concentrations
nM	Nanomoles per liter	10-9 M	Binding and cell-based assays
% w/v	Grams per 100 mL	10 mg/mL per 1%	Reagent labels (0.9% NaCl, 0.9% benzyl alcohol)
ppm	Parts per million by mass	1 µg/g; ≈ 1 µg/mL in dilute aqueous solution	Trace contaminants, residual solvents

Mass per volume to molarity, and back

The one conversion that needs the peptide's identity: divide a mass concentration in mg/mL by the molecular weight in g/mol to get molarity in mol/L, because the milli prefixes cancel. In the other direction, multiply molarity in mol/L by molecular weight to get g/L, which is mg/mL.

1 mg/mL expressed as molarity, for a range of peptide sizes

Molecular weight (g/mol)	Typical of	1 mg/mL is	1 mM is
340	A tripeptide	2.94 mM	0.34 mg/mL
500	A tetrapeptide	2.00 mM	0.50 mg/mL
850	A nonapeptide	1.18 mM	0.85 mg/mL
1,400	A pentadecapeptide	0.71 mM	1.40 mg/mL
3,400	A 29-residue peptide	0.29 mM	3.40 mg/mL
5,100	A 44-residue peptide	0.20 mM	5.10 mg/mL

The molecular weight to use is the peptide's own, and the mass is the mass of peptide rather than of powder; molarity for peptides works through both corrections.

Prefixes, and where the factors of a thousand hide

Each step from M to mM to µM to nM to pM is a factor of 1,000, and the same ladder runs from g to mg to µg to ng to pg. A dilution from a 1 mM stock to a 10 µM working solution is a factor of 100, not 1,000, which is easy to misread as "one step down" on the ladder. Writing both concentrations in the same unit before dividing (1,000 µM ÷ 10 µM = 100) removes the mistake. The serial dilutions article uses that discipline throughout.

Percent solutions

A percentage on a reagent label is grams per 100 mL unless it says otherwise, so 0.9% sodium chloride is 9 g/L, or 0.154 M. Percentages are also written weight per weight for solids and volume per volume for liquid mixtures; 0.9% benzyl alcohol in bacteriostatic water is by volume. Which convention applies matters when a reagent is used as a solvent: 0.9% v/v benzyl alcohol is 9.4 mg/mL, since its density is 1.044 g/mL. The reagent solvents article gives both compositions in all three units.

Parts per million

Parts per million is by mass: 1 ppm is 1 µg per gram of solution. In a dilute aqueous solution a milliliter weighs very nearly a gram, so 1 ppm and 1 µg/mL are interchangeable to within the density of water. The unit appears on certificates for residual solvents and trace metals, and in ICH guidance limits, rather than for the peptide itself.

From a vial's label to a concentration

A label states a quantity, such as 10 mg, and a certificate states a verified quantity and, if measured, a net peptide content. The peptide in the vial is quantity × content. Divided by the volume of solvent added, that is the mass concentration; divided further by the molecular weight, it is the molarity. The arithmetic is three lines and is laid out in preparing peptide solutions.

Frequently asked questions

Why do assay papers quote nanomolar and stock labels milligrams per milliliter?

Because a balance produces mass and a binding curve produces moles. Stocks are made by mass; experiments are described by molarity. The conversion is the molecular weight.

Is 1 ppm the same as 1 µg/mL?

In dilute water, yes, to within about 0.3%. In a dense solvent, or at high concentration, the density correction matters.

What does 0.9% mean on a saline label?

0.9 g of sodium chloride per 100 mL, that is 9 mg/mL or 0.154 M; it is the ionic strength of many physiological buffers.

Does the calculator convert units?

It gives the molecular weight for a sequence, which is the only number the conversions need; the tables here do the rest.

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. International Union of Pure and Applied Chemistry. Mass concentration. IUPAC Compendium of Chemical Terminology (the "Gold Book"). doi:10.1351/goldbook.M03713
3. Bureau International des Poids et Mesures. The International System of Units (SI), 9th edition, 2019; the SI prefixes and the mole.
4. International Council for Harmonisation. ICH Q3C (R8): Impurities: Guideline for Residual Solvents, 2021; limits expressed in ppm.

Cosmic Peptides. (2026, October 30). Concentration units for peptides: mg/mL, mM, µM, percent and ppm. Cosmic Peptides Research Library. Retrieved October 30, 2026, from https://cosmicpeptides.com/research/methods/concentration-units-for-peptides

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  title        = {Concentration units for peptides: mg/mL, mM, µM, percent and ppm},
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  urldate      = {2026-10-30},
  note         = {Last reviewed 2026-10-30}
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