How to calculate peptide concentration: mg/mL and molar

Concentration errors are the quiet cause of a great many irreproducible peptide experiments, and most of them happen at the reconstitution step rather than in the assay. This note sets out the arithmetic for mg/mL, µg/µL and molar concentrations, the two corrections that are easy to forget (net peptide content and purity), and a worked example for each compound in the Alphex catalogue. All Alphex products are for laboratory and in-vitro research only.

The basic calculation

Concentration equals mass divided by volume. If you add V mL of diluent to a vial containing M mg, the nominal concentration is M/V mg/mL. Because 1 mg/mL is the same as 1 µg/µL, the number does not change when you switch units, which makes bench arithmetic easier.

• 5 mg + 1 mL = 5 mg/mL = 5 µg/µL
• 5 mg + 2 mL = 2.5 mg/mL
• 10 mg + 1 mL = 10 mg/mL
• 50 mg + 5 mL = 10 mg/mL

Choose V so that the volume you will pipette per sample is comfortably measurable (tens of µL rather than single µL), and so that the resulting stock is within the peptide's solubility.

Correction one: net peptide content

The label mass is gross lyophilised solid, of which peptide is typically 70-90%. If the assay is concentration-critical, correct for it: a 10 mg vial at 80% net content reconstituted in 1 mL is 8 mg/mL peptide, not 10. See net peptide content versus gross vial weight for how to obtain or estimate the figure.

Correction two: purity

Purity is the fraction of peptide that is the target sequence. At 99+% it changes the answer by under 1% and can usually be ignored; at 95% it cannot. Multiply the net peptide mass by purity to get target-sequence mass. Alphex batches are 99+% by HPLC, so this correction is small, which is one practical benefit of the threshold. See what HPLC purity actually tells you.

Converting to molar

Molarity = (mass in g / molecular weight in g/mol) / volume in L. Using the molecular weight from the specification table:

• BPC-157 (1419.5 g/mol): 5 mg in 1 mL = 3.52 mM nominal; at 80% net content, 2.82 mM.
• TB-500 (889.0 g/mol): 5 mg in 1 mL = 5.62 mM nominal.
• GHK-Cu (403.9 g/mol complex): 50 mg in 5 mL = 24.8 mM nominal.
• Ipamorelin (711.9 g/mol): 5 mg in 1 mL = 7.02 mM nominal.
• GLP-3 (RT) (4731.3 g/mol): 5 mg in 1 mL = 1.06 mM nominal.

Use the molecular weight of the free peptide for molar calculations, not the salt; the counter-ion is already accounted for in the net content correction. Our note on molecular weight and molar calculations covers the salt question in detail.

Dilution to a working concentration

C1V1 = C2V2. To make 500 µL of a 10 µM working solution from a 3.52 mM stock: V1 = (10 µM × 500 µL) / 3520 µM = 1.42 µL. That is too small to pipette accurately, so make an intermediate: dilute the stock 1:100 (10 µL into 990 µL) to 35.2 µM, then take 142 µL of that into 358 µL of buffer. Two accurate steps beat one inaccurate one. Use low-binding tubes for the intermediate; dilute peptide solutions lose material to plastic.

Common mistakes

• Adding diluent by eye rather than by calibrated syringe or pipette. A 10% volume error is a 10% concentration error.
• Forgetting the volume already in the vial when topping up.
• Assuming 100% net content in a molar calculation.
• Using the salt molecular weight in one calculation and the free-peptide weight in another.
• Reporting a "dose" in mg without stating the stock preparation. State mass, diluent, volume, and any correction applied in the methods.

Alphex research peptides are supplied for laboratory and in-vitro research by qualified researchers only. They are not authorised for human or veterinary use.

Research use only. Alphex products are supplied exclusively for laboratory and in-vitro research by qualified researchers. They are not authorised for human or veterinary use, and nothing in this note is advice on any such use.

Published 22 July 2026. Alphex reviews research notes when the testing method, the catalogue or UK guidance changes.

Alphex Research Team

Alphex Limited, UK research peptide supplier

The Alphex research team writes and maintains these notes for UK laboratories buying research peptides. Every batch Alphex lists is verified by reverse-phase HPLC with a batch-matched Certificate of Analysis. Content covers laboratory and in-vitro research only.