Handling & storage
Reconstitution Maths: Three Formulas Behind Every Peptide Calculator
Concentration, volume to draw, syringe units. Verify any calculator by hand in sixty seconds and see precisely where thousandfold slips arise.
Every peptide reconstitution calculator runs on three short equations, and you can confirm any result it gives you by hand in under a minute. One: divide the peptide mass by the diluent volume to get concentration. Two: divide the amount you want by that concentration to get the volume to draw. Three: multiply millilitres by 100 to get units on a U-100 insulin syringe. This page works each equation through with realistic laboratory figures, points out where the familiar factor-of-ten and factor-of-thousand errors come from, and shows what our reconstitution calculator is doing behind the scenes. This is arithmetic for research solutions and describes only what ends up inside the barrel — never what should be done with it.
The units involved
- mg — milligrams, the unit printed on the vial: BPC-157, for instance, is supplied as 20, 15, 10 or 5 mg of powder.
- mcg — micrograms (µg), the unit in which working amounts are usually expressed; a milligram contains 1,000 of them. No conversion produces more mistakes than this one, which is why micrograms vs milligrams exists as a separate note.
- mL (millilitre), the unit for diluent and for draws.
- Units — the marks on an insulin syringe. Under the U-100 standard, one millilitre is 100 units, so a unit is 0.01 mL. They express volume only and say nothing about how much peptide is present until you know the concentration.
Formula one: concentration
Divide vial mass in mg by diluent volume in mL to obtain concentration in mg/mL.
Add 2 mL of bacteriostatic water to a 5 mg vial and you have 2.5 mg/mL; multiplied by 1,000 that is 2,500 mcg/mL. This page is about numbers only — the physical procedure of wiping the stopper, letting diluent run down the glass and swirling gently is described in the peptide reconstitution guide.
Formula two: volume for an amount
Divide the amount you want in mcg by the concentration in mcg/mL to obtain the volume in mL.
Needing 250 mcg from the 2,500 mcg/mL solution above, the arithmetic is 250 ÷ 2,500 = 0.1 mL. Prepare a more concentrated 5,000 mcg/mL stock and the same 250 mcg fits into 0.05 mL — the same mass in half the volume, which is precisely why a number of units on a syringe means nothing unless the concentration is recorded on the vial.
Formula three: millilitres to units
On a U-100 barrel, units are millilitres multiplied by 100, so 0.05 mL reads 5 units and 0.1 mL reads 10 units. Combining all three equations produces a single line:
Units to draw = mcg wanted ÷ [mg in vial × 1,000 ÷ mL of diluent] × 100.
That single line is the entire calculator. An example: 300 mcg needed, from a 10 mg vial made up with 3 mL. Concentration works out at 3,333 mcg/mL (10 × 1,000 ÷ 3); the volume is 0.09 mL (300 ÷ 3,333); on the barrel, 9 units.
Micrograms per unit at typical concentrations
Read across the rows of the table before you decide on a diluent volume. If the amounts in your protocol come out at only 1-2 units, the solution is too strong to measure dependably; if they run off the end of the scale, it is too weak. Draws falling roughly between 5 and 40 units keep the percentage error small on a 1 mL 31G insulin syringe, and the 0.5 mL 29G barrel spreads those same volumes over more widely spaced marks, which is easier to read accurately. Choosing hardware is its own topic — see choosing syringes and needles for peptide research.
Three recurring mistakes
- Getting a factor of 1,000 wrong: treating 0.25 mg as 25 mcg, or as 2,500 mcg. Record the concentration on the label in mcg/mL as well as mg/mL, so that the conversion is performed once, at the bench, and never repeated mentally afterwards.
- Getting a factor of 10 wrong: confusing units with millilitres, or reading a U-100 barrel as if 10 units meant 1 mL. Ten units is 0.1 mL.
- Dividing by the wrong thing: using the vial's mass in place of the concentration, which produces an answer that does not depend on the diluent volume at all. A quick test — if doubling the imagined diluent volume leaves your result unchanged, you applied the formula incorrectly.
Where mass units fall short: molarity and IU
Peptides are usually compared at equal molarity rather than equal mass, because their molecular weights are so different: TB-500 is 889.02 g/mol while semaglutide is 4113.58 g/mol, so one mg of TB-500 contains about 4.6 times as many molecules as one mg of semaglutide. How to convert mg/mL into a molar concentration is covered in molecular weight, moles and molarity. Note too that some biological materials are specified in international units — a bioassay-based measure with no general conversion to mass; where such a relationship exists for a particular product, the certificate will state it.
Cross-checking yourself
It is worth working every calculation out twice — once in the calculator, once by hand with the single-line formula — and trusting the result only when the two match. Then record the mass, the diluent used, the resulting concentration and the date, both on the label and in a log. Experience suggests that what actually goes wrong is rarely the arithmetic: it is vials with no label and diluent volumes that changed partway through a study, both covered in 10 common reconstitution and storage mistakes. For conversions beyond this workflow, the unit converter and the molarity calculator deal with mass, volume and molar forms.
Questions
How many micrograms does one syringe unit hold?
Entirely on the concentration of your solution, since a unit on the U-100 scale is nothing more than 0.01 mL. At 10 mg/mL each unit carries 100 mcg; at 5 mg/mL, 50 mcg; at 2.5 mg/mL — that is 2,500 mcg/mL — 25 mcg. Because the graduation measures volume, the same mark on the barrel represents different masses from differently prepared stocks.
Which formula does a reconstitution calculator use?
Three consecutive steps. Concentration (mcg/mL) = mass in mg × 1,000 ÷ diluent in mL. Draw volume (mL) = mcg wanted ÷ that concentration. Units = mL × 100. Condensed into a single expression: units = amount ÷ (mg × 1,000 ÷ mL) × 100. If a calculator's answer differs from that hand calculation, the inputs you gave it are wrong.
Does 1 mg equal 1,000 mcg?
Precisely so — the same relationship as between grams and milligrams. That mg-to-mcg conversion is where thousandfold errors creep into reconstitution arithmetic most often, so write both forms on the vial label — "2.5 mg/mL = 2,500 mcg/mL" — at the time you reconstitute.
Do units convert to millilitres identically on all syringes?
Within a single scale, yes. Under the U-100 convention, 1 mL corresponds to 100 units regardless of whether the barrel is 1 mL, 0.5 mL or 0.3 mL. But U-40 syringes also exist, with only 40 units to the millilitre, and their markings cannot be read using U-100 arithmetic. Everything we stock is U-100, so if you source barrels elsewhere, check the scale printed on them.
Why convert mg/mL into molarity?
Because equal masses of two peptides contain very unequal numbers of molecules. Molarity comes from dividing the concentration in g/L by the molar mass in g/mol: TB-500 (MW 889.02) at 1 mg/mL works out at roughly 1,125 µM, whereas semaglutide (MW 4113.58) at the same 1 mg/mL is only about 243 µM. Cell-based and receptor comparisons are interpretable only when molar concentrations match.
Which concentration should I choose?
The one that puts your habitual draws in the readable middle of the barrel — somewhere around 5 to 40 units on a U-100 scale. Start from the amounts the protocol calls for, divide each by a few candidate concentrations, and pick the diluent volume that brings those draws into that window without overfilling the vial; in a standard 3 mL research vial that usually means about 2 to 2.5 mL.