How to calculate peptide concentration after reconstitution

Last reviewed 8 August 2026

Reconstitution has exactly one equation in it. Everything else on this page is a consequence of that equation, a unit conversion, or an honest account of where the equation stops being exact. This article works the arithmetic for every fill mass in the NovoVita catalogue and converts each result to U-100 graduations. The companion tool is the reconstitution calculator.

The one equation

Concentration is mass divided by volume:

C = m ÷ V

Where m is the mass of lyophilised solid stated on the vial label, V is the volume of diluent added in millilitres, and C is the resulting concentration in milligrams per millilitre.

Nothing else enters. Not the compound’s molecular weight, not its sequence length, not its molar mass, not the supplier. Two vials of the same fill mass reconstituted with the same volume of diluent hold the same concentration whether the solid is a seven-residue peptide or a forty-three-residue protein. This is the reason a single calculator serves an entire catalogue: the arithmetic is indifferent to the chemistry.

The three forms worth committing to memory:

• Concentration: C = m ÷ V — milligrams per millilitre.
• Mass per graduation: C ÷ 100 — because one U-100 graduation is 0.01 ml.
• Volume for an arbitrary mass: V = m ÷ C, or 100 × m ÷ C in graduations.

The choice of diluent does not enter the arithmetic

Bacteriostatic water and sterile water differ in composition — bacteriostatic water carries a preservative, conventionally benzyl alcohol at 0.9%, and sterile water carries none. That difference governs whether a vial is suited to repeated withdrawals, and it is a real consideration. It is not an arithmetic one. Only the volume added appears in C = m ÷ V, so 10 mg made up with 2 ml of either water gives the same 5 mg/ml.

What the label mass actually is

Two distinctions sit behind m, and neither is pedantry.

Chromatographic purity is not a mass fraction. Purity determined by reversed-phase HPLC is an area percentage across the peptide-related species the detector resolves. It answers the question what proportion of the peptide material is the intended sequence. It does not answer what proportion of the weighed solid is peptide. A supplier purity figure of the form “greater than 99%” is a statement about identity and composition. It is not a statement about mass.

Net peptide content is a separate determination. Synthetic peptides are commonly supplied as an acetate salt and retain residual water from lyophilisation. Both the counter-ion and the residual water carry mass and both are inside the vial. Net peptide content — the fraction of the weighed solid that is peptide — is therefore reported separately from purity on a certificate of analysis, precisely because the two numbers answer different questions.

The consequence for the arithmetic is small but worth stating: C computed from the label mass is an upper bound on the concentration of the intended peptide, not a measurement of it. Every concentration in the tables below inherits that caveat.

Concentration for every fill mass in the catalogue

The columns below are the same five diluent volumes offered as shortcuts in the calculator. The rows are every distinct fill mass in the catalogue. All figures are milligrams per millilitre, to three significant figures where the division recurs.

The table is a convenience, not the method. The method is the division, and it takes any fill mass and any diluent volume, including combinations no table lists.

Which presentations sit at which fill mass

Among the compounds documented in this library:

1 mg

IGF-1 LR3. At this fill mass micrograms are the natural working unit: 1 mg in 1 ml is 1 mg/ml, which is 1,000 mcg/ml.

5 mg

TB-500, cagrilintide.

10 mg

The most common fill mass in the catalogue: BPC-157, ipamorelin, CJC-1295 + ipamorelin, tesamorelin, SS-31, semax, selank, kisspeptin, 5-Amino-1MQ and melanotan II.

20 mg

MOTS-c, and the combined BPC-157 + TB-500 presentation — see the note on blends below.

80 mg

KLOW, also a blend.

100 mg

GHK-Cu.

500 mg

NAD+. This is the one fill mass in the catalogue at which solid displacement stops being negligible; see below.

Converting concentration to U-100 graduations

A U-100 barrel is graduated in units, not in millilitres. The nomenclature comes from a concentration standard of 100 units per millilitre, but once the barrel is in hand it is simply a volume scale with a fixed relationship to millilitres:

1 graduation = 0.01 ml

That relationship holds on every U-100 barrel regardless of capacity. A 0.3 ml barrel carries 30 graduations, a 0.5 ml barrel 50, and a 1 ml barrel 100. Capacity changes how many graduations there are; it never changes how much volume one of them represents.

It follows that the mass contained in one graduation is the concentration divided by one hundred. Expressed in micrograms, since 1 mg is 1,000 mcg:

Divide by 1,000 for milligrams. A 10 mg vial in 2 ml is 5 mg/ml, so one graduation holds 50 mcg, or 0.05 mg. A 500 mg vial in 5 ml is 100 mg/ml, so one graduation holds 1 mg exactly — twenty times the mass in the same graduation of the previous example. Across the whole table the range runs from 2 mcg to 10,000 mcg per graduation, a factor of five thousand. That is the practical reason a habit of reading the barrel formed on one vial does not transfer to another.

The rearrangement, and the line the calculator does not cross

The equation rearranges as any equation does. For an arbitrary mass m:

V = m ÷ C, and in graduations, u = 100 × m ÷ C

The algebra is not in dispute. What is worth stating plainly is where each input comes from.

C is derived from two facts NovoVita can state: the mass printed on the vial and the volume of diluent the operator added. m is not one of those facts. It is not on the label, it is not in the catalogue, it is not in the product data, and it is not pre-filled anywhere in the calculator. The tool’s 23 vial presets fill in the vial strength field and nothing else — every other field opens on a value the operator sets and changes only when they change it.

This is the whole of NovoVita’s position on the subject, and it is deliberate rather than incidental: the volume is ours to calculate, the target amount is not.

Calculators elsewhere in this category ship presets that fill in a target amount alongside the vial strength, sometimes unlabelled. That pattern was considered here and rejected. A pre-filled target amount is not arithmetic; it is a figure asserted by the publisher, and asserting one would change what this site is saying about the material it sells. Nothing on this page and nothing in the tool states, implies or recommends an amount of any compound, for any purpose, in any species.

Where the arithmetic stops being exact

Four things move the real number away from the tabulated one. The first is negligible across most of the catalogue and substantial at the top of it; the last can be arbitrarily large, because it is not an error in the division at all but a limit on what the division was ever asked to tell you.

The denominator is diluent added, not final volume

C = m ÷ V divides by the volume of diluent introduced, not by the volume of the solution that results. The dissolved solid occupies space of its own, so the final volume is always slightly greater than V and the true concentration always slightly lower than the table says.

At the milligram scale this is negligible. Taking the conventional partial specific volume for proteins of roughly 0.73 ml per gram (Perkins, 1986), 10 mg of solid occupies about 0.007 ml — some 0.4% of a 2 ml addition, and well below the precision with which that 2 ml was measured in the first place.

At the top of the catalogue it stops being negligible. Five hundred milligrams of solid occupies of the order of 0.3–0.4 ml. Made up with 2 ml of diluent, the final volume is nearer 2.35 ml than 2.00, and the true concentration nearer 210–215 mg/ml than the 250 mg/ml the division returns — a discrepancy around 15%. Two caveats attach: 0.73 ml/g is a protein average and a small molecule such as a dinucleotide will differ from it, so the correction is an estimate rather than a measurement; and every calculator in this category, NovoVita’s included, divides by diluent added. Read the tabulated figure as mass per millilitre of diluent, because that is what it is.

The denominator is also measured by hand

An error of 0.1 ml on a 1 ml addition is a 10% error in C. The same absolute error on a 5 ml addition is 2%. Larger diluent volumes are arithmetically more forgiving of the same slip, which is a genuine consideration when the fill mass allows a choice.

Recurring decimals and false precision

Ten milligrams in 3 ml is 3.333… mg/ml. The calculator prints volumes to three decimal places and graduations to one, and it is worth being clear that this is a property of the division rather than of the glassware. The finest thing a U-100 barrel resolves is one graduation, 0.01 ml. Digits beyond the second decimal place in millilitres exist in the arithmetic and nowhere else.

Blends report a total, not a split

Where a vial’s label mass is a total across more than one peptide, C = m ÷ V returns total peptide per millilitre and nothing more. Per-component concentration requires the ratio, and where the ratio is not stated on the label it cannot be recovered from the total. An even split is an assumption, not a derivation, and it should not be made silently. Two presentations in this catalogue are blends: the combined BPC-157 and TB-500 vial, and KLOW.

What the barrel will and will not resolve

Two limits bracket every result, and both are properties of the instrument rather than of the compound.

The ceiling is capacity. Thirty graduations on a 0.3 ml barrel, 50 on a 0.5 ml, 100 on a 1 ml. A volume larger than the barrel cannot be measured on it. Arithmetically there are two ways out: lower the concentration by adding more diluent, or use a barrel with more capacity. The calculator states which is happening when it happens.

The floor is resolution. Below roughly two graduations, the reading is dominated by how accurately anyone can judge a meniscus against a scale ruled in whole units. The calculator flags anything under two graduations for that reason, and the flag is a statement about measurement error, not about quantity.

For a given fill mass, the diluent volume is the only variable the operator controls, and it moves both limits at once. More diluent lowers C, which lifts a small volume off the resolution floor and pushes a large one toward the capacity ceiling. That trade-off is the actual reason to think about diluent volume at all, and it is a measurement question from beginning to end.

Vials filled in international units

Two presentations in this catalogue are filled by international units rather than by mass, and the arithmetic above does not apply to them.

An international unit is a unit of biological activity, defined against a WHO International Standard preparation held for that specific substance. The mass equivalent of one IU is a property of that standard; it is not a general conversion, and there is no factor that turns IU into milligrams across substances. A vial labelled in IU therefore has no milligram fill mass to put into m.

This is why those two vials are absent from the calculator’s preset list. A preset fills a milligram field, and a milligram field is the wrong field for an IU-filled vial. The division itself is unit-agnostic — enter an IU figure and the ratio returned is IU per millilitre, correctly computed — but the tool will label that ratio in milligrams, because it has no way to know what was typed. Anyone working with an IU-filled vial should read the output as a ratio in the units they entered and disregard the label.

The figure changes after the solution is made

Concentration describes the solution at the moment it is prepared. Two well-documented effects move it afterwards, and both belong to the general pharmaceutical stability literature rather than to any particular compound in this catalogue.

The solid state is the stable one. Lyophilised peptide held sealed, desiccated, cold and out of light degrades slowly. Once taken into aqueous solution, degradation rates rise by orders of magnitude relative to the solid state. The chemical routes described for peptides in both states are asparagine and glutamine deamidation, aspartate-mediated backbone hydrolysis, methionine and cysteine oxidation, and physical aggregation. Solid-state deamidation kinetics and the influence of excipients and residual water are set out in Li and colleagues (2005) and in DeHart and Anderson (2012).

Aggregation removes material from the number without removing it from the vial. Aggregated peptide is still mass in solution but is no longer the species the concentration figure was calculated to describe. Repeated freeze-thaw cycling of solutions is avoided in laboratory practice for this reason, and concentration is itself one of the variables in aggregation kinetics.

Neither observation changes C = m ÷ V. Both change how long the answer remains a good description of what is in the vial.

Using the calculator

The reconstitution calculator is a single static page and the arithmetic runs in the browser. It carries:

• 23 vial presets, grouped by catalogue category, which fill in the vial strength field and nothing else.
• Diluent shortcuts at 0.5, 1, 2, 3 and 5 ml — the same five columns as the tables above.
• Three barrel capacities, 0.3, 0.5 and 1.0 ml, all U-100.
• A scale diagram placing the computed volume against the graduations of the selected barrel, so the figure and the instrument can be checked against each other.
• Both limit warnings described above — capacity exceeded, and below two graduations.
• Three reference sections covering units and conversions, the mechanics of reconstituting a vial, and storage.

It takes figures, divides them, and shows its working. It supplies the vial strength because that is printed on the label. It supplies nothing else.

References

1. Perkins SJ. Eur J Biochem. 1986;157(1):169–180. Partial specific volumes of proteins calculated from amino acid composition; source of the 0.73 ml/g figure used above.
2. Li B, et al. J Pharm Sci. 2005;94(8):1723–35. PMID 15986465. Solid-state deamidation kinetics and the influence of excipients and residual water.
3. DeHart MP, Anderson BD. J Pharm Sci. 2012;101(9):3142–56. PMID 22437444. Degradation pathways of peptides in the solid and solution state.