Laboratory Practice
Six steps from a lyophilized vial to labeled aliquots, each with its formula and a worked line you can check
Most mistakes in preparing peptide solutions are not chemistry mistakes. They are arithmetic: a label mass used where the measured one belonged, two peptides compared in mg/mL as if that were molar, a dilution worked the wrong way round. Each step below states its formula and then runs it, and the examples carry through from one step to the next.
The example values are illustrative rather than any specific lot's figures, and they describe preparing laboratory solutions only. Every compound mentioned is supplied for laboratory research use only. Nothing below is dosing, medical or legal guidance.
The right solvent depends on the peptide's charge and hydrophobicity, and on what your assay tolerates. Test solubility on a small portion first if you are unsure, add solvent gently down the vial wall, and swirl rather than vortex, which can promote aggregation. Technique is covered in more depth in the reconstitution lab guide.
Common first choices, by peptide character:
Concentration (mg/mL) = peptide mass (mg) ÷ solvent volume (mL)
Example A. A BPC-157 vial's certificate reports a measured content of 5.0 mg, and you add 2.0 mL of solvent: 5.0 mg ÷ 2.0 mL = 2.5 mg/mL. That is the same as 2.5 µg/µL, a convenient form when pipetting.
Most assays are designed in molar terms. Use the molecular weight (MW) from the certificate or from PubChem,[1][2] and remember that 1 mg/mL = 1 g/L.
Molarity (M) = concentration (g/L) ÷ MW (g/mol)
Example A, continued. BPC-157 has a MW of 1419.5 g/mol.[1] 2.5 g/L ÷ 1419.5 g/mol = 0.001761 M = 1.76 mM.
Example B. Tirzepatide has a MW of about 4813.5 g/mol.[2] With a measured content of 10.0 mg in 1.0 mL: 10.0 g/L ÷ 4813.5 g/mol = 0.002077 M = 2.08 mM.
A larger peptide gives a lower molarity at the same mass concentration. At 2.5 mg/mL, tirzepatide would be 2.5 ÷ 4813.5 × 1000 = 0.52 mM, against 1.76 mM for BPC-157. Comparing peptides at equal mg/mL is not comparing them at equal numbers of molecules.
C1 × V1 = C2 × V2, where C1 is the stock concentration, V1 the volume of stock needed, C2 the target concentration and V2 the final volume.
Example C. You need 1.0 mL of 10 µM BPC-157 from the 1.76 mM (1,761 µM) stock: V1 = (10 µM × 1.0 mL) ÷ 1,761 µM = 0.00568 mL = 5.68 µL. Pipette 5.68 µL of stock into 994.3 µL of assay buffer.
For very small transfers (under about 2 µL), make an intermediate dilution first, because small-volume pipetting errors otherwise dominate. Applied to Example C, diluting the stock 1:10 (to 176.1 µM) turns the 5.68 µL transfer into 56.8 µL, made up to 1.0 mL with 943.2 µL of buffer.
Example D. The 2.0 mL BPC-157 stock from Example A will be used across 20 experiments, each needing up to 80 µL of stock.
If you already prepared a stock from the label claim and later find that the measured content differs, correct it:
True concentration = nominal concentration × (measured content ÷ label claim)
A stock made up as "5.0 mg/mL" from a "10 mg" vial (that is, in 2.0 mL) that actually measured 9.6 mg is 5.0 × (9.6 ÷ 10) = 5.0 × 0.96 = 4.8 mg/mL. The post on HPLC purity vs net peptide content explains why the label and the measured content differ.
Select strength
Should I use the free-base or salt MW?
Use the MW that matches how content was measured. Certificates typically report peptide content as free peptide, so use the free-peptide MW for molarity. The peptide calculator uses free-peptide molecular weights for its built-in compounds.
Why do my results drift over weeks?
Repeated freeze-thaw and solution-phase degradation are common causes. Aliquot, and record dates. See shipping and storage.
Does the Peptide Calculator handle molarity?
Yes. It has five connected tools: stock concentration, mg to mM molarity, working dilution and serial series (C1V1 = C2V2), an aliquot split and cold-storage planner, and a sequence tool that estimates molecular weight, pI and hydropathy.
This article summarizes published preclinical research literature. Compounds referenced are supplied by Eppix Labs strictly as research materials for laboratory investigation within the United States. They are not approved by the FDA for human or veterinary use, and nothing on this page should be interpreted as medical advice or guidance on human or animal administration.
