In plain English
Lab work with peptides involves a lot of unit conversions. Mistakes are easy to make and can be large, because the prefixes jump by factors of 1,000.
There are three ways to say “how much”. Mass says how heavy the material is, in milligrams (mg) or micrograms (µg). Amount of substance counts molecules, using the mole (mol), a fixed very large number of molecules [1]. Activity, in international units (IU), measures what a substance does in a test compared with an official reference sample, rather than what it weighs [2, 3].
Molar concentration (molarity) matters because experiments compare how many molecules are present. Two peptides at the same mg/mL can contain very different numbers of molecules if one is heavier.
The powder in a vial is also not all peptide. It holds water and counter-ions (small charged molecules) too, so careful calculations correct for that [4].
The short version
- Each prefix step (m, µ, n) is a factor of 1,000.
- mg/mL ÷ molar mass (g/mol) = mol/L.
- IU cannot be converted to mg without a defined reference value.
- C₁V₁ = C₂V₂ for every dilution.
SI prefixes and the units that matter
| Prefix | Symbol | Factor | Mass example | Amount example | Volume example |
|---|---|---|---|---|---|
| milli | m | 10⁻³ | 1 mg = 0.001 g | 1 mmol | 1 mL |
| micro | µ | 10⁻⁶ | 1 µg = 0.001 mg | 1 µmol | 1 µL |
| nano | n | 10⁻⁹ | 1 ng = 0.001 µg | 1 nmol | 1 nL |
| pico | p | 10⁻¹² | 1 pg | 1 pmol | — |
The mole is the SI unit of amount of substance, defined by fixing the Avogadro constant at exactly 6.02214076 × 10²³ per mole. The litre is a non-SI unit accepted for use with the SI [1]. In Australia, the National Measurement Institute (NMI) is responsible for measurement standards, trade measurement and chemical and biological measurement services [5].
Concentration units used with peptides:
| Unit | Meaning | Useful identity |
|---|---|---|
| mg/mL | milligrams per millilitre | 1 mg/mL = 1 g/L = 1 µg/µL |
| µg/mL | micrograms per millilitre | 1 µg/mL = 1 mg/L |
| M (molar) | moles per litre (mol/L) | 1 M = 1,000 mM = 1,000,000 µM |
| mM | millimoles per litre | 1 mM = 1 µmol/mL |
| µM | micromoles per litre | 1 µM = 1 nmol/mL |
| % w/v | grams per 100 mL | 0.1% w/v = 1 mg/mL |
International units (IU): activity, not mass
An international unit is defined by a WHO international standard: a reference preparation to which a value in IU has been assigned. Results reported in IU from assays calibrated against that standard can be compared between laboratories [2]. The WHO Expert Committee on Biological Standardization establishes these WHO biological reference materials [3].
Two consequences follow:
- There is no universal IU-to-mg factor. The conversion depends on the specific standard and on the activity of the specific preparation. International units are a different kind of quantity from SI units, and a joint IUPAC–IFCC document recommends using SI units (the mole) wherever the molecule being measured can be identified [6].
- IU are used mostly for biologicals whose activity is measured by assay. Most short synthetic peptides of known sequence are described by mass and molarity.
Molar mass: which number to use
The molar mass (g/mol) is numerically equal to the molecular weight in daltons. Databases such as PubChem list the molar mass of the neutral peptide [7]. Synthetic peptides are often supplied as salts (for example trifluoroacetate or acetate) and the powder contains water, so the gross mass of powder is more than the mass of peptide. Pharmacopoeial work on peptide reference standards uses a mass-balance approach that accounts for these components when assigning a content value [4]. See Purity vs Peptide Content.
The worked examples below use bradykinin (RPPGFSPFR), molar mass 1060.2 g/mol [7], as an illustrative peptide.
Worked examples
Example 1: mass unit conversions
- 2.5 mg = 2.5 × 1,000 = 2,500 µg
- 750 µg = 750 ÷ 1,000 = 0.75 mg
- Moles in 1.0 mg of bradykinin: 0.0010 g ÷ 1060.2 g/mol = 9.43 × 10⁻⁷ mol = 943 nmol
Example 2: mass concentration
2.0 mg of peptide dissolved to a final volume of 1.0 mL gives:
2.0 mg ÷ 1.0 mL = 2.0 mg/mL
Example 3: mg/mL to mM and µM
2.0 mg/mL = 2.0 g/L.
2.0 g/L ÷ 1060.2 g/mol = 0.001886 mol/L = 1.886 mM = 1,886 µM
A quick check: for a peptide near 1,000 g/mol, 1 mg/mL is roughly 1 mM.
Example 4: correcting for net peptide content
The COA reports a net peptide content of 75% (an illustrative figure). The actual peptide in 2.0 mg of powder is:
2.0 mg × 0.75 = 1.5 mg
In 1.0 mL: 1.5 mg/mL ÷ 1060.2 g/mol = 1.415 mM. Ignoring net peptide content would overstate the concentration by a third (1.886 versus 1.415 mM).
Example 5: volume for a target molarity
To make a 1.0 mM stock from 5.0 mg of peptide (net content taken as 100% for simplicity):
volume (L) = mass (g) ÷ [molar mass (g/mol) × concentration (mol/L)]
volume = 0.0050 g ÷ (1060.2 g/mol × 0.0010 mol/L) = 0.004716 L = 4.72 mL
Example 6: a single dilution with C₁V₁ = C₂V₂
To make 0.50 mL of a 100 µM working solution from the 1,886 µM stock in example 3:
V₁ = (C₂ × V₂) ÷ C₁ = (100 µM × 0.50 mL) ÷ 1,886 µM = 0.0265 mL = 26.5 µL of stock
Make up to 0.50 mL with 473.5 µL of diluent. C and V can be in any units, provided C₁ and C₂ share one unit and V₁ and V₂ share another.
Example 7: a serial dilution
To make 10 µM and 1 µM solutions from the 100 µM working solution, use two 1-in-10 steps:
- 100 µL of 100 µM + 900 µL diluent = 1.0 mL of 10 µM
- 100 µL of 10 µM + 900 µL diluent = 1.0 mL of 1 µM
Serial steps keep each pipetted volume large enough to measure accurately.
Example 8: µg/mL to nM
1.0 µg/mL = 1.0 mg/L. 1.0 mg/L ÷ 1060.2 g/mol = 9.43 × 10⁻⁷ mol/L = 943 nM (0.943 µM).
Common errors and how to avoid them
| Error | Effect | Check |
|---|---|---|
| Mixing mg and µg | 1,000-fold error | Write units on every line of the calculation |
| Using gross powder mass | Concentration overstated | Apply net peptide content from the COA [4] |
| Using the salt or a different molar mass | Small to moderate error | State which molar mass was used |
| Volume added versus final volume | Small error at high concentration | Specify “made up to” or “added” |
| Pipetting very small volumes | Poor accuracy | Use an intermediate dilution |
| Loss to container walls | True concentration lower than calculated | Use low-binding tubes for dilute solutions [8] |
| Treating IU as mass | Meaningless result | Use IU only with its defined standard [2, 6] |
For solvent choice and handling, see Predicting Peptide Solubility.
What this means in Australia
- Legal units of measurement. The National Measurement Institute is responsible for Australia’s measurement standards [5]. Safety data sheets supplied in Australia must express units in Australian legal units of measurement under the National Measurement Act 1960 [9].
- Concentration maths has no bearing on legal status. Unapproved peptide products are not entered on the Australian Register of Therapeutic Goods (ARTG) and have not been assessed by the TGA [10].
- Scheduling depends on the substance. Several peptides are named in the current Poisons Standard (October 2026), some as Schedule 4 (prescription-only) and Appendix D, clause 5 (possession without authority is illegal) [11].
- Labels don’t change status. The TGA has stated that a “research use only” disclaimer does not make supply lawful, change regulatory status, permit importation, or remove advertising or supply obligations [10].
This is general information, not legal advice. Laws change; check the current Poisons Standard and TGA guidance.
Frequently asked questions
How do I convert mg/mL to µM for a peptide?
Divide mg/mL (equal to g/L) by the molar mass in g/mol to get mol/L, then multiply by 1,000,000. For example, 2.0 mg/mL of a 1060.2 g/mol peptide is 1,886 µM [7].
What does C₁V₁ = C₂V₂ mean?
It states that the amount of solute is the same before and after dilution. Any three values give the fourth, provided the concentration units match and the volume units match.
Can international units be converted to milligrams?
Only with a defined reference value for that specific substance and standard. IU measure activity against a WHO international standard, not mass [2, 6].
Why is my calculated concentration higher than the measured one?
Common causes are using gross powder mass instead of net peptide content, incomplete dissolution, or adsorption to container walls [4, 8].
What molar mass should be used for a peptide salt?
Use the neutral-peptide molar mass together with net peptide content from the COA. Using the gross powder mass with the neutral molar mass overstates concentration [4, 7].
References
- Bureau International des Poids et Mesures (BIPM). The International System of Units (SI). 9th edition. 2019. bipm.org. Accessed 11 October 2026.
- Baylis SA, Wallace P, McCulloch E, Niesters HGM, Nübling CM. Standardization of nucleic acid tests: the approach of the World Health Organization. J Clin Microbiol. 2019;57(1):e01056-18. doi:10.1128/JCM.01056-18. PMID: 30257900
- World Health Organization. Expert Committee on Biological Standardization. who.int. Accessed 11 October 2026.
- McCarthy D, Han Y, Carrick K, et al. Reference standards to support quality of synthetic peptide therapeutics. Pharm Res. 2023;40(6):1317–1328. doi:10.1007/s11095-023-03493-1. PMID: 36949371
- Department of Industry, Science and Resources. National Measurement Institute. industry.gov.au. Accessed 11 October 2026.
- Dybkaer R, Storring PL. Application of IUPAC-IFCC recommendations on quantities and units to WHO biological reference materials for diagnostic use. Eur J Clin Chem Clin Biochem. 1995;33(9):623–625. PMID: 8611672
- National Center for Biotechnology Information. PubChem Compound Summary for CID 439201, Bradykinin. pubchem.ncbi.nlm.nih.gov/compound/439201
- Kristensen K, Henriksen JR, Andresen TL. Adsorption of cationic peptides to solid surfaces of glass and plastic. PLoS One. 2015;10(5):e0122419. doi:10.1371/journal.pone.0122419. PMID: 25932639
- Safe Work Australia. Model Code of Practice: Preparation of safety data sheets for hazardous chemicals. Last updated 16 June 2023. safeworkaustralia.gov.au. Accessed 11 October 2026.
- Therapeutic Goods Administration. Understanding your responsibilities when importing, compounding and supplying unapproved peptide products. Safety advisory, 13 April 2026. tga.gov.au. Accessed 11 October 2026.
- Therapeutic Goods (Poisons Standard—October 2026) Instrument 2026 (F2026L01327). Federal Register of Legislation. legislation.gov.au. Accessed 11 October 2026.
Important information and disclaimer
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Regulatory status. Many peptides are scheduled substances under the Poisons Standard. Some are prescription-only (Schedule 4), and some may not be possessed without authority (Appendix D). The TGA has stated that a “research use only” label does not make supply lawful, change a product’s regulatory status, or remove advertising or supply obligations. State and territory laws may also apply. You are responsible for knowing and complying with all laws that apply to you. Regulatory information in this article may change after the review date.
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