What this tool does
- Stock concentration: peptide amount ÷ diluent volume, in mg/mL, µg/mL, mM and µM.
- Diluent volume: how much liquid to add to reach a target concentration.
- Dilution: how much stock and diluent to combine, using C₁V₁ = C₂V₂.
In this guide
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Blends contain more than one compound, so they don't have a single molecular weight. For a blend, calculate each component separately using its own amount.
1 Stock concentration
2 Diluent volume for a target concentration
3 Dilution (C₁V₁ = C₂V₂)
The target concentration must be lower than the stock concentration.
How the calculations work
Mass concentration
Concentration in mg/mL is the amount of peptide divided by the volume of diluent: C = m ÷ V. Dissolving 10 mg in 2 mL gives 5 mg/mL, which is the same as 5,000 µg/mL.
Molarity
Molarity expresses concentration as the number of molecules rather than their mass, which matters when comparing compounds of different sizes. It's calculated from the mass concentration and the molecular weight: mM = (mg/mL ÷ g/mol) × 1,000. For example, 1 mg/mL of BPC-157 (1,419.56 g/mol) is about 0.704 mM, or 704 µM.
Dilution
Diluting a stock keeps the amount of peptide constant while increasing the volume, so C₁ × V₁ = C₂ × V₂. To make 1 mL at 1 mg/mL from a 5 mg/mL stock, you need 0.2 mL of stock plus 0.8 mL of diluent.
Getting accurate numbers
- Use the measured amount when you have it. The amount in a vial can differ slightly from the label. A third-party certificate of analysis reports the measured content for the tested batch.
- Purity isn't the same as content. A lyophilized powder can also contain water and counter-ions, so peptide content by weight is usually below 100%. See how peptide purity is tested.
- Molecular weights here are for the base compound. Salt forms (such as acetate or iodide) have a different formula weight, so check which form a protocol refers to.
- Measure volumes with calibrated equipment. Small errors in volume make large differences at low volumes.
For the hands-on side, see how to reconstitute lyophilized peptides and bacteriostatic vs sterile water.
Frequently asked questions
How do I calculate peptide concentration in mg/mL?
Divide the amount of peptide in milligrams by the volume of diluent in millilitres. For example, 10 mg dissolved in 2 mL gives a concentration of 5 mg/mL.
How do I convert mg/mL to molarity?
Divide the concentration in mg/mL by the molecular weight in g/mol, then multiply by 1,000 to get millimolar (mM). For example, 1 mg/mL of a 1,000 g/mol peptide is 1 mM.
What does C1V1 = C2V2 mean?
It's the dilution equation: stock concentration times stock volume equals final concentration times final volume. It tells you how much stock solution to dilute to reach a lower target concentration.
Why is the measured amount in my vial different from the label?
Vials are often filled slightly over the stated amount, and lyophilized powder can contain water and counter-ions. The measured content on a certificate of analysis gives the most accurate starting figure.
