Peptide Math at the Bench: Units, Molarity, Concentration and Dilution Series
Bench mistakes in peptide work are usually arithmetic, not chemistry, and the same four calculations account for most of them. This page starts with the unit habits that prevent the worst slips, then covers molar conversion, concentration and draw volume, dilution series, and a lookup table.
Unit habits that stop tenfold errors
- Spell out “mcg” for micrograms. A handwritten “µg” is easy to misread.
- Keep the conversion in front of you: 1mg = 1000mcg.
- Find out whether a graduated syringe is marked in mL or in hundredths of a mL before its markings go into any calculation.
Mass to molarity
Papers on cell culture report peptide concentrations in molar units, so you will convert often. µM equals µg/mL divided by molecular weight in kDa, and a figure in mg/L works identically.
Example: at 1 µg/mL, BPC-157 (molecular weight about 1.42 kDa) works out as 1 ÷ 1419 × 1,000,000, close to 0.70 µM. Semaglutide, around 4.11 kDa, gives only about 0.24 µM at that same mass concentration. Equal masses of two peptides can be far apart in molar terms.
Concentration and the amount per draw
Concentration after reconstitution is peptide mass in mg divided by diluent volume in mL. A 10mg vial plus 2mL of bacteriostatic water gives 5 mg/mL; the same vial plus 1mL gives 10 mg/mL. Pick the volume before opening anything, so that the amount you are after corresponds to a draw you can measure accurately. Thinner solutions make small amounts easier to measure.
The amount in a draw is concentration multiplied by the volume drawn. With a 5 mg/mL solution, 0.1mL contains 0.5mg (500mcg), and 0.05mL contains 0.25mg (250mcg).
Dilution series for dose-response work
In-vitro protocols typically call for a ladder of concentrations such as 10, 1, 0.1 and 0.01 µg/mL. Each rung is a 1:10 step: 0.1mL of the previous solution goes into 0.9mL of diluent, is mixed gently, and becomes the source for the next tube. Draw from the freshly mixed tube each time, switch to a new tip between steps, and make the whole series fresh, because dilute peptide solutions lose material to the tube walls faster than concentrated stock.
Lookup table
| Peptide in vial | Diluent volume | Concentration | Amount in 0.1mL |
|---|---|---|---|
| 10mg | 3mL | 3.33 mg/mL | 333mcg |
| 10mg | 2mL | 5 mg/mL | 500mcg |
| 10mg | 1mL | 10 mg/mL | 1000mcg (1mg) |
| 5mg | 2mL | 2.5 mg/mL | 250mcg |
| 5mg | 1mL | 5 mg/mL | 500mcg |
Applying it to Welon Peptide vials
The 5mg and 10mg rows match the strengths of our BPC-157 listing, and Bacteriostatic Water comes in 3mL, 5mL and 10mL. Keep enough diluent on hand that you never have to improvise volumes partway through a protocol, and label each vial with its concentration and the date as soon as it is mixed (see reconstituting peptides step by step).
Don’t look for molecular weights in the summary rows on our COAs page: for each Finnrick test of a Welon Peptide Solutions product they show the product, test date, a pass and a purity figure, but not molecular weight. The figures used above come from the compound guides on this site. For how impurities shift the effective molar concentration, see the purity percentage guide. For research use only. Not for human or veterinary use.
