Reconstitution arithmetic: concentration from mass and volume

Written 24 September 2026 by the trade desk. About 3 minutes to read.

One division, and the reasons it is that one and not another. This is bench arithmetic for the record on the vial; what is done with the solution afterwards is the laboratory's own protocol.

The sum

Concentration is the mass in the vial divided by the volume of diluent added. Milligrams over millilitres gives milligrams per millilitre; that is the whole sum. A 10 mg vial made up with 2 mL holds 5 mg/mL. Multiply by 1,000 if the record is kept in micrograms per millilitre.

The volume in the sum is the volume added, not the volume in the vial afterwards. A dissolved cake of 5 to 30 mg occupies at most a few hundredths of a millilitre, below the precision of the transfer, so it is ignored.

Worked examples

Three vial strengths against three volumes: label mass divided by volume, rounded to two places.

A 30 mg vial in 3 mL and a 10 mg vial in 1 mL give the same figure.

Why purity is not in the sum

The label mass is the fill weight: the weight of material put into the vial, within 5 percent. Purity is a share of chromatogram peak area, not a weight. Multiplying 10 mg by 99 percent describes nothing: the other one percent is related substances of similar mass, also in the vial, while the water and acetate in the vial are not in the purity figure at all.

If a record needs peptide by weight rather than material, the figure to use is net peptide content, stated on the certificate where the batch was assayed and otherwise available from the desk on request. It replaces the label mass in the numerator. Purity still does not enter.

Why the fill weight might

The fill weight is released at the labelled amount within plus or minus 5 percent, so a 10 mg vial holds between 9.5 and 10.5 mg of material, and in 2 mL that is 4.75 to 5.25 mg/mL. Most records use the nominal figure and note the tolerance. Where the record must be exact, the measured fill weight on the batch certificate is the number to divide by, since it was weighed rather than assumed; and where the record needs peptide rather than material, net peptide content is the numerator, as above.

Choosing the diluent

The diluent does not change the arithmetic; 2 mL is 2 mL whichever water is used. What changes is how the vial is handled afterwards. Bacteriostatic water, with 0.9 percent benzyl alcohol, is the usual choice for a vial that will be entered more than once. Sterile water without preservative is used when the method that follows cannot tolerate benzyl alcohol, and the vial is then a single-entry article. The choice is a laboratory preparation matter set by the protocol, and it is written on the vial with the volume. Run the water down the inside of the glass and swirl; shaking foams the solution.

Diluent for a box of ten

A box holds ten vials of one strength, so diluent for a box is ten times the volume per vial. We list bacteriostatic water as BAC3 (3 mL vials) and BAC10 (10 mL vials), ten to a box.

• 1 mL per vial: 10 mL, one BAC10 vial if its overfill covers the last draw; otherwise two.
• 2 mL per vial: 20 mL, two BAC10 vials.
• 3 mL per vial: 30 mL, three BAC10 vials or one box of BAC3, one water vial to one peptide vial.

Pairing a BAC3 with each peptide vial keeps every water vial to a single entry. Entering a BAC10 several times is what the preservative is for, and the 28-day rule after first puncture then applies.

After reconstitution

A reconstituted vial is kept at 2 to 8 degrees Celsius, not frozen. It is no longer the sealed vial the certificate describes, so its label should carry what the certificate cannot: the batch number, the date of reconstitution, the diluent and the concentration worked out above. What is done with the solution after that is the laboratory's own protocol, outside this guide.

Applies to

This guide was written with Bacteriostatic Water, Retatrutide and Semaglutide in mind, and it holds for the other lines of their kind; the full list is on the compounds page.

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