cost
Cost per mg: Using Net Peptide Content, Not Vial Size
How to calculate cost per mg using net peptide content instead of vial size, with a worked comparison showing why the two figures can diverge sharply.
Medically reviewed by Natalia Sorokin, PhD, research scientist and biochemist — Last reviewed
Natalia Sorokin, PhD holds a doctorate in biochemistry from Moscow State University and postdoctoral training at the Scripps Research Institute, with over 18 years in synthetic peptide chemistry and pharmaceutical-grade peptide production.
Calculating cost per mg using net peptide content instead of vial size means dividing a listing’s price by the actual peptide mass in the vial, not by the round number printed on the label. A vial marketed as “10 mg” is a gross figure — the total lyophilized weight before accounting for counter-ions, buffering agents, and moisture. Net peptide content is the fraction of that weight that is actually peptide. Using the labeled vial size as the denominator instead of net content produces a cost-per-mg figure that looks better than the vial actually delivers.
Why Vial Size Alone Is the Wrong Denominator
A vial’s stated size is a manufacturing specification, not a purity-adjusted measurement. It reflects how much freeze-dried material went into the vial, which includes the peptide chain plus whatever counter-ion or bulking agent traveled with it through synthesis and lyophilization. Two vials both labeled “10 mg” from two different sources can contain meaningfully different amounts of actual peptide if their net content percentages differ, even though the sticker price and the label look identical.
Net peptide content is usually reported as a percentage of gross weight, sometimes alongside a purity figure from HPLC testing. Purity describes how much of the peptide fraction is the correct sequence; net content describes how much of the total vial weight is peptide fraction at all. Both numbers are needed to know what a vial actually contains, and neither one is interchangeable with the round vial size printed at the top of a listing.
The Two Different Cost-per-mg Calculations
There are two ways to compute cost per mg, and they answer different questions:
- Cost per mg (gross) = vial price ÷ labeled vial size
- Cost per mg (net) = vial price ÷ (labeled vial size × net peptide content percentage)
The gross calculation is the one most listings implicitly invite a buyer to do, since it only requires the price and the number on the label. The net calculation requires a certificate of analysis or a stated net content figure, and it is the more accurate one, because it reflects the mass of peptide a buyer is actually paying for rather than the mass of freeze-dried material in the vial.
A Worked Example
Consider a vial labeled 10 mg, priced at $90, with a certificate of analysis stating net peptide content of 85%.
Gross calculation: $90 ÷ 10 mg = $9.00 per mg.
Net calculation: actual peptide mass = 10 mg × 0.85 = 8.5 mg. Cost per mg = $90 ÷ 8.5 mg = $10.59 per mg.
Recomputing to check: 10 × 0.85 is 8.5, and 90 divided by 8.5 is approximately 10.588, which rounds to $10.59. The gross figure understates the real cost per mg by more than a dollar and a half on this vial. Across a full order of several vials, that gap compounds into a noticeably different total spend than the gross figure would suggest.
A second vial, also labeled 10 mg and also priced at $90, but with a certificate of analysis stating 95% net content, tells a different story: actual peptide mass = 10 mg × 0.95 = 9.5 mg, and cost per mg = $90 ÷ 9.5 mg ≈ $9.47. Same label, same price, same gross cost-per-mg figure of $9.00 — but the two vials differ by roughly a dollar in real cost per mg once net content is factored in.
| Figure | Vial A (85% net content) | Vial B (95% net content) |
|---|---|---|
| Labeled vial size | 10 mg | 10 mg |
| Vial price | $90 | $90 |
| Actual peptide mass | 8.5 mg | 9.5 mg |
| Cost per mg (gross, using label) | $9.00 | $9.00 |
| Cost per mg (net, using actual mass) | $10.59 | $9.47 |
The two vials look identical on price and label alone. The net calculation is what separates them.
Where the Net Content Figure Comes From
Net peptide content is not printed on every listing. It typically appears on a batch-specific certificate of analysis, sometimes as a standalone percentage and sometimes derived from an amino acid analysis or mass balance the testing lab performed. A listing that publishes only a purity percentage from an HPLC trace, without a separate net content figure, has not given a buyer enough information to run the net calculation — purity and net content measure different things, and a high purity percentage does not imply a high net content percentage.
When a certificate of analysis is available, checking whether it states net content directly, rather than assuming the label’s gross weight is close enough, is the step that determines whether a cost-per-mg comparison across listings is meaningful. Some suppliers, including the semaglutide listing published alongside a batch-specific certificate of analysis, state net content directly rather than leaving a buyer to assume the label’s gross weight is close enough.
Comparing Listings Fairly
Cost per mg only functions as a fair comparison tool when every listing being compared uses the same denominator. Comparing one listing’s gross cost per mg against another listing’s net cost per mg produces a mismatch that looks like a price difference but is actually a measurement difference. Before comparing prices across two or more sources, it helps to confirm whether each one publishes a net content figure at all, and if only one does, converting the other using its purity trace and any available mass data brings both numbers onto the same basis.
This matters most when vial sizes differ across listings, since price per vial alone is not comparable across different sizes. Once cost per mg is calculated, using net content rather than the labeled size as the denominator, the comparison reflects the peptide mass actually being purchased rather than the round number a manufacturer chose to print on the vial.
Summary
Calculating cost per mg using net peptide content instead of vial size means multiplying the labeled vial size by its net content percentage to get the actual peptide mass, then dividing the vial price by that mass rather than by the label alone. Two vials with identical prices and identical labeled sizes can carry a meaningfully different real cost per mg once net content is factored in, which is why a certificate of analysis stating this figure is worth checking before treating any cost-per-mg comparison as settled.