Peptide Stock Solutions: Molarity and Concentration Math
Concentration in mg/mL is mass divided by volume. Molarity is that concentration divided by molecular weight. For accurate work, correct the gross powder mass for net peptide content, because water and counter-ions add weight without adding peptide.
Mass concentration
Mass concentration equals peptide mass divided by solvent volume. Dissolving 5 mg in 2.5 mL gives 2 mg/mL. This is a laboratory stock concentration, not an administration amount.
Converting to molarity
Divide mg/mL by molecular weight in g/mol to get mol/L. A 2 mg/mL solution of a 1,000 g/mol peptide is 0.002 M, or 2 mM.
Use the molecular weight for the exact construct, including terminal modifications and salt form when stated. Each product page lists the catalog molecular weight where available.
Correcting for net peptide content
Lyophilized powder contains water and counter-ions such as acetate or trifluoroacetate. Net peptide content is often below the gross weight. If a lot report gives 80% content, 5 mg of powder holds about 4 mg of peptide.
Use a measured content value only when it is reported for the same lot. Without one, state that the concentration is based on gross weight.
Peptide Purity vs Identity vs Content
Making working dilutions
Use C1 × V1 = C2 × V2 to dilute a stock. To make 1 mL of 0.2 mg/mL from a 2 mg/mL stock, take 0.1 mL of stock and add 0.9 mL of diluent.
Record the lot, solvent, preparation date and calculation basis on each tube. The calculator page can help check the arithmetic.
Evidence guides
- How Peptide Lab Verification Works — How HPLC, LC-MS, content testing and lot records fit together.
- How to Verify Peptide Authenticity — A checklist for identity, lot matching and report provenance.
- How to Choose a Research Peptide Supplier — Evaluate suppliers on specifications, documentation and handling.
- Peptide Purity vs Identity vs Content — Why purity, identity and net peptide content are separate measurements.
Frequently asked questions
How do I convert peptide mg/mL to molarity?
Divide the concentration in mg/mL by the molecular weight in g/mol. The result is in mol/L. For example, 2 mg/mL of a 1,000 g/mol peptide is 2 mM.
Why correct for net peptide content?
Lyophilized powder includes water and counter-ions. Using gross weight alone overstates the amount of peptide in solution.
Is a stock concentration a dose?
No. A stock concentration describes a laboratory solution. These materials are for research use only and no administration guidance is provided.
References
Related reading
More peptide science
- Research Peptides: Types, Vial Strengths and Lot Reports
- What Is a Peptide? Amino Acids, Bonds and Sequence
- How Research Peptides Are Made: Solid-Phase Peptide Synthesis
- Why Peptides Degrade: Oxidation, Deamidation and Hydrolysis
- Reading a Mass Spectrum: How LC-MS Confirms Peptide Identity
- The Semax Family and Adamax’s Adamantane Modification
- IGF-1 LR3 vs IGF-1 DES: How Two Growth-Factor Analogs Differ
- Fmoc vs Boc Peptide Synthesis: Protecting Groups Compared
- Peptide Solubility: Choosing a Solvent for Lab Stock Solutions
- Storing Lyophilized Peptides: Aliquoting and Freeze-Thaw Protocol
Research Use Only. Educational laboratory information; not medical advice.
