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A research-focused comparison of Melanotan II and PT-141, covering melanocortin receptor selectivity, evidence, safety signals and regulatory status.
The peptide calculator helps you avoid mistakes when working with research peptides. The most common error is not a reconstitution error or a storage error. The most common error is an arithmetic error – confusion between milligrams and micrograms, between milliliters and “units” on an insulin syringe, between mass concentration and molar concentration. It rarely costs the health of the sample and often the precision of the experiment – which, in protocols where results are interpreted with statistical power, means that a single careless calculation brings the entire measurement session into disrepute.
The logic of peptide calculations is only seemingly simple. The lyophilisate in the vial has a declared net weight (e.g. 5 mg or 10 mg). You reconstitute it in a volume of solvent (e.g. 2 mL of bacteriostatic water). You get the concentration in mg/mL – a chemical unit that is not immediately translatable to the graduations of an insulin syringe. The syringe is calibrated in U100 or U40 “units”, where 1 unit of U100 is 0.01 mL – a value derived from insulin protocols from the 1970s-1980s. (US moved to U100 as a standard introduced by the FDA in 1973), not from peptide chemistry. To summarize: for each experimental dose, you need to make two unit transitions (mg → µg → mL → “u”), each of which is an opportunity for an order of magnitude error.
This article compiles the complete logic for calculating research peptide concentration and volume – basic formulas, reference tables for typical lyophilized weights, a guide to insulin syringes, common calculation errors and how to avoid them.
📖 The following article is educational and describes the practice of calculating peptide concentrations when working with research reagents (Research Use Only). All dose examples refer to experimental protocols in vitro and in animal models – they do not constitute medical advice or a protocol for the use of peptides in humans.
In brief. Working-solution concentration follows one equation: concentration [mg/mL] = peptide mass [mg] / solvent volume [mL]. For example, 5 mg of lyophilisate in 2 mL of bacteriostatic water gives 2.5 mg/mL, or 2,500 micrograms/mL. Volume can be read with a U-100 insulin syringe, in which 1 unit = 0.01 mL; at 2.5 mg/mL, one unit corresponds to 25 micrograms. These calculations concern the preparation of RUO research reagents.
Concentration in mg/mL is calculated by dividing peptide mass in milligrams by solvent volume in millilitres.
When working with peptides in a research context, you use three basic equations. Understanding them is a condition for any further calculation.
Concentration [mg/mL] = peptide mass [mg] / solvent volume [mL]Example: 5 mg of lyophilisate in 2 mL of bacteriostatic water gives a concentration of 2.5 mg/mL.
Move to micrograms (µg):
Concentration [µg/mL] = concentration [mg/mL] × 1000In the same example: 2.5 mg/mL × 1000 = 2500 µg/mL.
Volume [mL] = dose [µg] / concentration [µg/mL]For a concentration of 2500 µg/mL and an experimental dose of 250 µg:
Volume = 250 / 2500 = 0.1 mL = 100 µLU100 insulin syringes (commonly available) are calibrated in units where 100 units = 1 mL. That is:
1 unit U100 = 0.01 mL = 10 µLHence:
Number of units [u U100] = volume [mL] × 100In the example: 0.1 mL × 100 = 10 U100 units. You pull the plunger of the syringe up to the “10 u” mark on the scale.
The table below shows the peptide concentrations after reconstitution of typical masses of lyophilisate in various volumes of bacteriostatic water. Values in mg/mL and µg/mL below.
| Weight of lyophilisate | 1 mL | 2 mL | 3 mL | 5 mL | 10 mL |
|---|---|---|---|---|---|
| 2 mg | 2.0 mg/mL | 1.0 mg/mL | 0.67 mg/mL | 0.4 mg/mL | 0.2 mg/mL |
| 5 mg | 5.0 mg/mL | 2.5 mg/mL | 1.67 mg/mL | 1.0 mg/mL | 0.5 mg/mL |
| 10 mg | 10.0 mg/mL | 5.0 mg/mL | 3.33 mg/mL | 2.0 mg/mL | 1.0 mg/mL |
| 20 mg | 20.0 mg/mL | 10.0 mg/mL | 6.67 mg/mL | 4.0 mg/mL | 2.0 mg/mL |
| 50 mg | 50.0 mg/mL | 25.0 mg/mL | 16.67 mg/mL | 10.0 mg/mL | 5.0 mg/mL |
In research practice, the most common reconstitutions are 5 mg in 2 mL (concentration 2.5 mg/mL) or 10 mg in 2 mL (5 mg/mL) – they allow convenient collection of volumes of 50-200 µL with an insulin syringe. The procedure for dissolving the lyophilisate is described in the guide how to dissolve peptides step by step.
The number of units depends on solution concentration; on a U-100 scale, one unit corresponds to 0.01 mL.
The table below shows how many units on the U100 syringe correspond to each µg dose for the most common concentrations. Values rounded to the nearest tenth of a unit – in practice, an insulin syringe shows a scale of 1-2 units.
| Dose | 1000 µg/mL | 2000 µg/mL | 2500 µg/mL | 5000 µg/mL |
|---|---|---|---|---|
| 100 µg | 10 u (0.10 mL) | 5 u (0.05 mL) | 4 u (0.04 mL) | 2 u (0.02 mL) |
| 200 µg | 20 u (0.20 mL) | 10 u (0.10 mL) | 8 u (0.08 mL) | 4 u (0.04 mL) |
| 250 µg | 25 u (0.25 mL) | 12.5 u (0.125 mL) | 10 u (0.10 mL) | 5 u (0.05 mL) |
| 500 µg | 50 u (0.50 mL) | 25 u (0.25 mL) | 20 u (0.20 mL) | 10 u (0.10 mL) |
| 750 µg | 75 u (0.75 mL) | 37.5 u (0.375 mL) | 30 u (0.30 mL) | 15 u (0.15 mL) |
| 1000 µg (1 mg) | 100 u (1.00 mL) | 50 u (0.50 mL) | 40 u (0.40 mL) | 20 u (0.20 mL) |
| 1500 µg (1.5 mg) | 150 u (1.50 mL) | 75 u (0.75 mL) | 60 u (0.60 mL) | 30 u (0.30 mL) |
For U40 syringes (less often used in human medicine, still present in veterinary medicine – calibration 40 units = 1 mL) we divide the number of units by 2.5 in relation to U100. So 10 u U100 = 4 u U40 for the same physical volume. Always check the syringe type before calculating – replacing U100 with U40 leads to an error of 2.5x.
The calculations in this section use the U-100 scale; the U-40 scale applies a different volume conversion.
In laboratory work with peptides in animal models, disposable insulin syringes with an integral needle are standard. The choice of type and capacity affects the measurement precision:
| Capacity | Calibrated as | Typical use | The smallest readings |
|---|---|---|---|
| 0.3 mL (30 u U100) | U100 | Doses <200 µg, precise sampling | 0.5 u (5 µL) |
| 0.5 mL (50 u U100) | U100 | Doses 100–500 µg, most commonly used | 1 u (10 µL) |
| 1.0 mL (100 u U100) | U100 | Doses >500 µg | 2 u (20 µL) |
The smaller the capacity of the syringe, the higher the precision of the scale in the small volume area. For doses <100 µg, it is worth using a 0.3 mL (30 u) syringe instead of a 1 mL (100 u) syringe, even if the second one is enough.
Standard needles for subcutaneous injections in animal models: 29G–31G, length 8–13 mm. Thicker needles (27G and lower) increase the trauma of the injection point, needles longer than 13 mm are unnecessary for subcutaneous tissue.
Three scenarios showing the full calculation logic from purchasing a vial to taking a dose with a syringe.
You receive a 5 mg vial of BPC-157 lyophilisate. You are planning a research protocol in a rat model with an experimental dose of 250 µg per animal.
Step 1 – Reconstitution: water volume selection 2 mL (allows ~20 doses of 100 µL); concentration 5 mg / 2 mL = 2.5 mg/mL = 2500 µg/mL.
Step 2 – Single dose volume: dose 250 µg / concentration 2500 µg/mL = 0.1 mL.
Step 3 – U100 syringe: 0.1 mL × 100 = 10 units of U100. You withdraw 0.3 mL with a syringe to the “10 u” mark.
You receive a 50 mg vial of Semax lyophilisate. You are planning a study in a mouse model with an experimental dose of 50 µg per animal.
Step 1 – Reconstitution: selecting a water volume of 5 mL (typical for a peptide of this mass); concentration 50 mg / 5 mL = 10 mg/mL = 10,000 µg/mL.
Step 2 – Single dose volume: dose 50 µg / concentration 10,000 µg/mL = 0.005 mL = 5 µL.
Step 3 – Syringe: 5 µL = 0.5 unit U100. Such a small volume requires a Hamilton precision syringe (10 µL micro syringe) or a higher working dilution.
Scenario 2 conclusion: the starting concentration was too high for the planned dose. Practical solution – prepare a working stock by diluting 1:10 in bacteriostatic water: working concentration 1 mg/mL = 1000 µg/mL; 50 µg dose volume: 50 µL = 5 U100 units – readable on a 0.3 mL syringe.
You receive a 10 mg vial of Selank lyophilisate. You are planning a protocol on rats with an average weight of 250 g, an experimental dose of 100 µg/kg body weight.
Step 1 – Reconstitution: 2 mL bacteriostatic water; concentration 10 mg / 2 mL = 5 mg/mL = 5000 µg/mL.
Step 2 – dose per animal (250 g = 0.25 kg): dose = 100 µg/kg × 0.25 kg = 25 µg.
Step 3 – Volume: 25 µg / 5000 µg/mL = 0.005 mL = 5 µL.
Step 4 – Syringe: again, such a small volume requires working dilution to a concentration of 500 µg/mL. After dilution: dose 25 µg = 0.05 mL = 5 units of U100.
The same pattern in all three scenarios: if the volume of a single dose is less than 0.02 mL (2 U100 units), it is worth preparing a working dilution. Smaller volumes read inaccurately on a typical insulin syringe.
In the research peptide literature, doses are usually given in µg/kg body weight. This allows for scaling across species (mice, rats, larger models) and comparison of results between research groups. Typical indicative ranges from the literature (reference values for preclinical models, NO recommendations for people):
| Peptide | Approximate dose range in animal models |
|---|---|
| BPC-157 | 10–500 µg/kg/day |
| TB-500 | 100–500 µg/kg/day |
| Semax | 50–500 µg/kg/day |
| Selank | 100–300 µg/kg/day |
| GHK-Cu | 100–500 µg/kg/day |
| DSIP | 25–100 µg/kg/day |
The above values are generalizations from various preclinical studies (rats, mice, cell cultures) – specific doses in the research protocol require confirmation from the current source literature for the selected peptide and animal model. Extrapolation to other species requires appropriate allometric calculations and is not equivalent to recommendations for humans (peptides are RUO reagents, not medicinal products).
To convert the dose between species, body surface area (BSA) scaling is used, described in detail by Reagan-Shaw et al. (2008). Basic formula:
HED [mg/kg] = animal_dose [mg/kg] × (Km_animal / Km_human)where the Km coefficients (mg/kg → mg/m²) are, among others: mouse 3, rat 6, rabbit 12, human 37. For example: a dose of 100 µg/kg in a rat corresponds to approx. 16 µg/kg in a human (HED = 100 × 6/37). This simplification — full scaling takes into account metabolic differences (allometric coefficient ~0.75 in Kleiber’s law).
The most common errors are confusing milligrams with micrograms, volume with mass, and U-100 units with U-40 units.
In contacts with researchers starting to work with peptides, the same individual misunderstandings are repeated.
Mistake 1: Confusing U100 and U40. The U40 syringe has a calibration of 40 units = 1 mL. Taking “10 units” from a 2.5 mg/mL vial on a U40 syringe gives 0.25 mL = 625 µg, not the planned 250 µg. This is an error of 2.5×, which leads to excess doses with serious consequences in toxicological studies. Always check the calibration on the syringe packaging.
Mistake 2: Confusing mg with µg. 1 mg = 1000 µg. A dose of “5 mg” instead of “5 µg” is an error of 1000×. In the peptide literature, doses are almost always in µg or µg/kg – the mg value for a single dose is a signal for immediate verification.
Mistake 3: Confusing concentration with dose. The peptide concentration in the reconstituted vial is constant (e.g. 2500 µg/mL). The dose per administration is variable (depends on the protocol). The formula “dose = concentration × volume” should be written on a piece of paper above the workstation.
Mistake 4: Omitting counterions. The value “5 mg” on the vial refers to the net weight of the peptide without the counterion (acetate, trifluoroacetate). For highly basic peptides (with lysine, arginine – e.g. PT-141, Selank), the mass of the counterion may be 5-15% of the total salt mass. A professional supplier lists the net weight of peptide in COA – but it’s worth verifying whether “5 mg” means net peptide or total salt weight. The details of the COA interpretation are described Peptide Quality Standards Guide.
Mistake 5: Assuming linearity of dosage regardless of species. A dose of 100 µg/kg for a rat is not the same as a dose of 100 µg/kg for larger models – there are allometric principles (metabolic scaling is not linear, but power-law with a factor of ~0.75 in Kleiber’s law). For studies in species other than classic rodent models, consult specific literature and publications by Reagan-Shaw 2008 or Nair and Jacob 2016.
Error 6: Incorrect syringe scale reading. U100 insulin syringes are graduated every 1 or 2 units depending on the capacity. 0.5 mL (50 u) syringes usually have 1 u graduations – a minimum reading of 1 u = 10 µL. 1 mL (100 u) syringes usually have 2 u graduations – a minimum reading of 2 u = 20 µL. For single doses <0.02 mL, insulin syringes do not provide adequate precision – a Hamilton microsyringe or preparation of a working dilution is required.
Peptide calculator on the One Peptides website automates the above calculations. After entering three parameters (peptide mass in the vial, planned solvent volume, experimental dose in µg), the calculator returns:
The tool allows you to minimize the risk of individual errors – when planning a protocol in which you use several peptides with different masses and doses, the calculator is faster and less prone to errors than an Excel spreadsheet or “on foot” calculations.
Concentration [mg/mL] = mass of peptide in vial [mg] / volume of solvent [mL]. Example: 5 mg of lyophilisate in 2 mL of bacteriostatic water gives a concentration of 2.5 mg/mL = 2500 µg/mL. The peptide calculator performs this conversion automatically.
The U100 insulin syringe is calibrated in units where 100 units = 1 mL. That is, 1 unit of U100 = 0.01 mL = 10 µL. The U100 scale comes from insulin protocols – when working with research peptides we use it as a precise tool for collecting small volumes, but the “u” unit is a measure of volume, not peptide activity.
U40 is an older calibration of insulin syringes: 40 units = 1 mL, i.e. 1 unit of U40 = 0.025 mL. U40 syringes are rare in human medicine today (they were superseded by the U100 in the US in the 1970s and in most countries thereafter), but still available – especially in veterinary medicine. Confusing U100 with U40 leads to an error of 2.5x in the volume withdrawn – always check the calibration on the packaging.
Formula: U100 units = (dose in µg / peptide concentration in µg/mL) × 100. Example: for a concentration of 2500 µg/mL and a dose of 250 µg → (250 / 2500) × 100 = 10 units of U100. Reference tables for typical concentrations are provided in the “Conversion Table” section above.
For a 0.3 mL (30 u U100) syringe with 0.5 u graduations, the minimum precise volume is ~5 µL (0.5 units). For a 1 mL (100 u U100) syringe with 2 u graduations, the minimum precise volume is ~20 µL (2 units). For doses requiring smaller volumes, it is worth preparing a working dilution or using a Hamilton microsyringe.
An intermediate peptide concentration, lower than the starting concentration after reconstitution, used when the planned doses are too small for direct collection from stock. Example: a peptide with a concentration of 10 mg/mL is diluted 1:10 in bacteriostatic water to 1 mg/mL. The working dilution should be prepared immediately before use and stored for a short time (24-48 hours at 2-8°C).
Linear scaling (mg/kg) is a simplification and only works within certain ranges. Allometric scaling is more precise – taking into account differences in metabolic rate between species according to Kleiber’s law (factor ~0.75). The classic Body Surface Area approach is described in the work by Reagan-Shaw et al. 2008: HED [mg/kg] = animal_dose × (Km_animal / Km_human), where Km is, among others, 3 for mouse, 6 for rat, 37 for human.
The peptide calculator on the One Peptides website automates concentration, volume and syringe unit conversion calculations. Each vial of peptide from the catalog has a net peptide weight according to the certificate of analysis available at quality and certificates website.
ℹ️ Global disclaimer
All One-Peptides products are reagents intended exclusively for laboratory and scientific research (Research Use Only). They are not medicines, dietary supplements or products intended for human consumption. The information in this article is educational in nature and is a review of published scientific literature; does not constitute medical, pharmaceutical or dietary advice.
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