Every peptide reconstitution calculator runs on three small formulas, and you can check any of its outputs by hand in under a minute. Formula 1: concentration = peptide mass ÷ diluent volume. Formula 2: volume to draw = amount wanted ÷ concentration. Formula 3: units on a U-100 insulin syringe = mL × 100. This guide works through each one with real lab numbers, shows where the common tenfold and thousandfold errors come from, and explains what our reconstitution calculator is doing behind the interface. All of this is laboratory solution arithmetic for research material — it tells you what is in a syringe barrel, not what to do with it.
The units you are converting between
- mg (milligram) — how peptide vials are labelled: a BPC-157 vial holds 5, 10, 15 or 20 mg of powder.
- mcg (microgram, µg) — how working amounts are usually expressed. 1 mg = 1,000 mcg. This conversion is the single most common source of error; see micrograms vs milligrams.
- mL (milliliter) — how diluent and draws are measured.
- Units — the graduations on an insulin syringe. On the U-100 scale, 100 units = 1 mL, so 1 unit = 0.01 mL. Units measure volume only; they say nothing about peptide amount until you know the concentration.
Formula 1: concentration
Concentration (mg/mL) = vial mass (mg) ÷ diluent volume (mL).
Reconstitute a 5 mg vial with 2 mL of bacteriostatic water: 5 ÷ 2 = 2.5 mg/mL. Multiply by 1,000 to get 2,500 mcg/mL. The physical procedure — swabbing, adding diluent down the glass, swirling — is covered in the peptide reconstitution guide; this page is only about the numbers.
Formula 2: volume for a given amount
Volume (mL) = amount wanted (mcg) ÷ concentration (mcg/mL).
Suppose an experiment calls for 250 mcg of peptide from the 2,500 mcg/mL stock above: 250 ÷ 2,500 = 0.1 mL. From a denser 5,000 mcg/mL stock the same 250 mcg is only 0.05 mL. Same amount, half the volume — which is why a syringe reading means nothing without the concentration written on the vial label.
Formula 3: milliliters to syringe units
Units = mL × 100 (on a U-100 syringe). So 0.1 mL = 10 units and 0.05 mL = 5 units. Combining all three formulas into one line:
Units to draw = amount wanted (mcg) ÷ [vial mass (mg) × 1,000 ÷ diluent volume (mL)] × 100.
That single expression is the entire calculator. A worked pass: 10 mg vial, 3 mL diluent, 300 mcg wanted. Concentration = 10 × 1,000 ÷ 3 = 3,333 mcg/mL. Volume = 300 ÷ 3,333 = 0.09 mL. Units = 9.
Reference table: mcg per unit at common concentrations
| Reconstitution | mg/mL | mcg per 1 unit | Units for 100 mcg | Units for 250 mcg | Units for 500 mcg |
|---|---|---|---|---|---|
| 5 mg + 5 mL | 1 | 10 | 10 | 25 | 50 |
| 5 mg + 2.5 mL | 2 | 20 | 5 | 12.5 | 25 |
| 5 mg + 2 mL | 2.5 | 25 | 4 | 10 | 20 |
| 10 mg + 2 mL | 5 | 50 | 2 | 5 | 10 |
| 10 mg + 1 mL | 10 | 100 | 1 | 2.5 | 5 |
Read the table row-wise before you reconstitute: if the amounts your protocol uses fall at 1–2 units, the stock is too dense to measure accurately; if they exceed the barrel, it is too dilute. Draws between about 5 and 40 units keep the percentage error on a 1 mL 31G insulin syringe small; the 0.5 mL 29G half-barrel syringe spreads the same volumes over larger graduations for finer reading. Choosing hardware is its own topic — see choosing syringes and needles for peptide research.
The three classic errors
- The ×1,000 slip. Treating 0.25 mg as 25 mcg (or 2,500 mcg) — always write concentrations in both mg/mL and mcg/mL on the vial label so the conversion is done once, at the bench, and never again mentally.
- The ×10 slip. Confusing units with mL, or misreading a U-100 barrel as if 10 units were 1 mL. 10 units is 0.1 mL.
- The wrong-denominator slip. Dividing by the vial mass instead of the concentration, which makes the answer insensitive to how much diluent was added. If a result does not change when you imagine doubling the diluent, the formula was applied wrong.
When mass units are not enough: moles and IU
Cross-peptide comparisons are usually made at equal molarity, not equal mass, because molecular weights differ enormously — semaglutide is 4113.58 g/mol while TB-500 is 889.02 g/mol, so 1 mg of TB-500 contains about 4.6 times as many molecules as 1 mg of semaglutide. Converting mg/mL to molar concentration is covered in molecular weight, moles and molarity. A few biologicals are labelled in international units (IU), a bioassay-defined measure that does not convert to mass by any universal factor; the product's COA states the relationship for that material where one exists.
Checking the calculator, and yourself
A good habit: run the numbers once on the calculator, once by hand with the one-line formula, and accept the result only when they agree. Then record vial mass, diluent, concentration and date on the label and in a log sheet. The most common real-world failures are not arithmetic at all — they are unlabelled vials and mid-study changes of diluent volume, both covered in 10 common reconstitution and storage mistakes. For unit conversions outside this workflow, the unit converter and molarity calculator cover mass, volume and molar forms.