What does a peptide calculator actually calculate?
A peptide calculator is a reconstitution and dose-conversion tool, not a dosing recommender. It answers one narrow question: given a vial of freeze-dried powder and a measured volume of bacteriostatic water, how much liquid do you draw into the syringe for the dose your protocol or your prescribing clinician specified? Four formulas do all of the work, and every one of them is arithmetic you can check by hand in under a minute.
Here they are in full. Concentration in mg/mL equals vial strength in mg divided by BAC water volume in mL. Injection volume in mL equals dose in mg divided by concentration in mg/mL. Units on a U-100 syringe equal injection volume in mL multiplied by 100. Doses per vial equal vial strength in mg divided by dose in mg. Underneath all four sits the conversion that causes most of the trouble on this page: 1 mg equals 1000 mcg.
If you would rather not run those by hand every time, PeptideDeck publishes a peptide calculator built on exactly these formulas. You pick a syringe type (U-100 insulin in a 1 mL, half mL or third mL barrel), enter a vial size from 1 mg to 100 mg, enter a BAC water volume from 0.5 mL to 10 mL, and enter your desired dose in either mcg or mg. It returns concentration, volume per dose, total doses per vial, and the units to draw on a U-100 scale. It carries presets for BPC-157, TB-500, CJC-1295 (No DAC), Ipamorelin, Retatrutide, GHK-Cu, MOTS-c and Semax, plus a half-life dose calculator and a microdosing schedule generator.
A calculator of any kind converts a dose into a volume. It does not choose the dose, it does not know what is actually inside your vial, and it cannot detect that you typed mcg when you meant mg. Every failure described further down this page is a failure of input, not a failure of arithmetic, which is why the five checks at the end are worth more than the four formulas at the top.
How do you calculate peptide dosage step by step?
Calculating a peptide dose is five steps in a fixed order, and taking them out of order is where people slip. Work a concrete case: a 5 mg vial, 2 mL of bacteriostatic water, and a dose of 250 mcg specified by your protocol.
Step one, convert the dose into milligrams so it matches the units on the vial. 250 mcg divided by 1000 equals 0.25 mg. Step two, calculate concentration: 5 mg divided by 2 mL equals 2.5 mg/mL. Step three, calculate injection volume: 0.25 mg divided by 2.5 mg/mL equals 0.1 mL. Step four, convert volume into syringe units: 0.1 mL multiplied by 100 equals 10 units on a U-100 syringe. Step five, sanity check the vial: 5 mg divided by 0.25 mg equals 20 doses.
Step one is not decoration. Concentration is expressed in mg/mL, so a dose written in mcg cannot be divided into it directly. Skip the conversion and divide 250 by 2.5 and you get 100, which looks like a plausible answer and is in fact 100 mL, fifty times the 2 mL of liquid in the entire vial. A result that cannot physically exist is the most useful error message a calculator ever gives you.
The table below runs those same five steps across eight different vial and water combinations. Every concentration, volume, unit count and dose count in it was recomputed from the four formulas before publication, and each row carries enough of its own inputs that you can redo the arithmetic yourself.
| Vial strength | BAC water | Concentration | Dose | Injection volume | U-100 units | Doses per vial |
|---|---|---|---|---|---|---|
| 5 mg | 2 mL | 2.5 mg/mL | 250 mcg (0.25 mg) | 0.1 mL | 10 units | 20 |
| 5 mg | 1 mL | 5 mg/mL | 250 mcg (0.25 mg) | 0.05 mL | 5 units | 20 |
| 10 mg | 2 mL | 5 mg/mL | 500 mcg (0.5 mg) | 0.1 mL | 10 units | 20 |
| 10 mg | 5 mL | 2 mg/mL | 500 mcg (0.5 mg) | 0.25 mL | 25 units | 20 |
| 15 mg | 3 mL | 5 mg/mL | 1 mg (1000 mcg) | 0.2 mL | 20 units | 15 |
| 20 mg | 2 mL | 10 mg/mL | 2 mg (2000 mcg) | 0.2 mL | 20 units | 10 |
| 30 mg | 3 mL | 10 mg/mL | 1.5 mg (1500 mcg) | 0.15 mL | 15 units | 20 |
| 50 mg | 5 mL | 10 mg/mL | 5 mg (5000 mcg) | 0.5 mL | 50 units | 10 |
Every row: concentration = vial mg / water mL, injection volume = dose mg / concentration, units = injection volume mL x 100, doses = vial mg / dose mg. All figures recomputed before publication. Doses per vial are a theoretical maximum and ignore the residual volume left behind in the vial.
What are units on a U-100 syringe, and why are they not milligrams?
Units on a U-100 insulin syringe are a volume marking. One unit is one hundredth of a millilitre, or 0.01 mL. That is the entire definition. Units are not milligrams, not micrograms, and not international units of anything. This is the single most common misunderstanding in peptide dosing, and it is the one that produces the largest errors.
Because a unit is a fixed volume, the same unit count delivers a different amount of peptide from every differently reconstituted vial. Ten units is always 0.1 mL. Ten units drawn from a 5 mg vial in 2 mL of water, which is 2.5 mg/mL, delivers 0.25 mg. Ten units drawn from that same 5 mg vial in 1 mL of water, which is 5 mg/mL, delivers 0.5 mg. Same syringe, same marking, double the peptide.
Here is the check that kills the units-equal-milligrams belief permanently. A full 1 mL U-100 barrel is 100 units. Drawn from a vial reconstituted to 2.5 mg/mL, that entire full syringe contains 2.5 mg. If 100 units meant 100 mg, a 5 mg vial would not hold enough peptide to fill a single syringe even once. Any time your mental model produces a number larger than the whole vial, the model is wrong, not the vial.
U-100 describes the scale, not the barrel size, and the three common barrels hold different totals. A 0.3 mL barrel holds 30 units, a 0.5 mL barrel holds 50 units, and a 1 mL barrel holds 100 units. All three carry the same U-100 scale on which one unit is 0.01 mL. If your calculation returns 50 units and you own a 0.3 mL syringe, that dose does not fit in a single draw, and it is a reconstitution problem to solve before injection day rather than at the bedside.
How many units is 250 mcg?
There is no single answer, and any source that gives you one without asking about your vial is guessing. 250 mcg is a mass. Units on a U-100 syringe are a volume. Converting between them requires the concentration of your specific vial, and that depends on how much bacteriostatic water you personally added.
Three correct answers to the same question. In a 5 mg vial reconstituted with 2 mL, concentration is 2.5 mg/mL and 250 mcg is 0.1 mL, which is 10 units. In a 5 mg vial reconstituted with 1 mL, concentration is 5 mg/mL and 250 mcg is 0.05 mL, which is 5 units. In a 20 mg vial reconstituted with 1 mL, concentration is 20 mg/mL and 250 mcg is 0.0125 mL, which is 1.25 units.
That third answer is a warning rather than a data point. 1.25 units sits a quarter of the way past a tick mark on a U-100 syringe, and nobody draws that repeatably. A vial concentrated that heavily for a dose that small has created a measurement problem at reconstitution, and the fix is more bacteriostatic water next time, not a steadier hand tonight.
So the honest form of the question is never "how many units is 250 mcg". It is "how many units is 250 mcg from a vial of X mg reconstituted with Y mL". Any dosing chart that answers the short version has quietly assumed values for X and Y that may not be yours.
How much bacteriostatic water should I use?
Bacteriostatic water volume is the one input you fully control, and it does not change how much peptide you own. It changes only how that fixed amount of peptide is spread through the liquid, which changes the volume you draw and therefore the units on the syringe. The number of doses in the vial does not move at all.
The table below holds the vial and the dose constant, a 10 mg vial and a 500 mcg dose, and varies only the water. Concentration falls, injection volume rises, syringe units rise, and doses per vial sit unchanged at 20 in every single row. That last column is the proof that adding water does not dilute your dose. It only changes the arithmetic of drawing it.
The practical choice is about measurability. A volume that lands your dose in the readable middle of the syringe means a small slip in drawing produces a small error in dose. Very low water volumes push doses down toward one or two units, where half a tick mark is a large percentage error. Very high water volumes can push a dose past the capacity of a small barrel, or past the volume the vial itself will hold.
One constraint is easy to miss: the reconstituted vial has to be one you can store and finish before the peptide degrades. We publish no stability figures here, because they vary by compound and we have not tested them. Ask whoever supplied your protocol, and treat any vendor claim you cannot trace with the same suspicion we apply in our scam checker.
| BAC water added | Concentration | Injection volume for 500 mcg | U-100 units | Doses per vial |
|---|---|---|---|---|
| 1 mL | 10 mg/mL | 0.05 mL | 5 units | 20 |
| 2 mL | 5 mg/mL | 0.1 mL | 10 units | 20 |
| 3 mL | 3.33 mg/mL | 0.15 mL | 15 units | 20 |
| 4 mL | 2.5 mg/mL | 0.2 mL | 20 units | 20 |
| 5 mL | 2 mg/mL | 0.25 mL | 25 units | 20 |
One 10 mg vial, one 500 mcg (0.5 mg) dose, five water volumes. The 3 mL concentration is 10 divided by 3, a repeating decimal shown rounded to 3.33 mg/mL, while the 0.15 mL injection volume is exact. Doses per vial never move, because water does not change how much peptide is in the vial.
Why does copying someone else's dosing chart go wrong?
A dosing chart is only valid for the exact reconstitution it was built on. Vendor charts and forum screenshots frequently omit the water volume they assume, and that one missing number is what turns a chart into a hazard.
Work the failure through. A chart built on a 5 mg vial in 2 mL of water, which is 2.5 mg/mL, tells you to draw 10 units for a 250 mcg dose. You add 1 mL instead, because that is what came in your kit. Your actual concentration is 5 mg divided by 1 mL, which is 5 mg/mL. You follow the chart and draw 10 units, which is 0.1 mL. At 5 mg/mL, 0.1 mL delivers 0.5 mg, which is 500 mcg. You have taken exactly double the intended dose while following printed instructions correctly.
The error runs in both directions. A chart built on 1 mL that you follow after adding 2 mL tells you to draw 5 units, which is 0.05 mL. At 2.5 mg/mL that delivers 0.125 mg, which is 125 mcg, half of what you intended. Underdosing is less immediately dangerous and more insidious, because nothing happens and the natural conclusion is that the compound does not work.
The structural point is the same one we make about sources generally in our ranked comparison of peptide and GLP-1 references. A chart published by the company selling the vial has an incentive to look simple, and simplicity here means dropping the assumption that made the numbers valid in the first place. Recompute from your own vial and your own measured water volume every time. It costs you one division.
What happens when mcg and mg get swapped in a calculator?
Every peptide calculator that accepts a dose in both mcg and mg has a unit selector, and that selector is the highest-consequence control on the page. Getting it wrong moves the answer by a factor of 1000 in one direction or the other.
Type 250 into an mg field when you meant 250 mcg, working from a 5 mg vial in 2 mL of water. The calculator divides 250 by 2.5 and returns 100 mL, which is 10,000 units. The correct answer was 0.1 mL, which is 10 units. The error is a thousandfold, and the output is so absurd that it protects you: 100 mL will not fit in a 1 mL barrel, and 250 mg is fifty times the entire 5 mg vial.
The reverse is the dangerous one, because it looks like nothing at all. Type 0.25 into an mcg field when you meant 0.25 mg. 0.25 mcg is 0.00025 mg. Divided by 2.5 mg/mL that is 0.0001 mL, which is 0.01 units. Nobody can draw a hundredth of a unit, so the syringe comes out visually empty and you inject essentially none of the compound while believing you dosed correctly.
The defence is a habit rather than a rule. Before trusting any calculator output, restate your dose in both units. 250 mcg is 0.25 mg. 1000 mcg is 1 mg. 2 mg is 2000 mcg. If the number you typed does not match the unit label printed next to the field, you have found the error before the syringe did.
The table below sets out the mix-ups that account for most of the arithmetic damage, with the size of each error computed rather than described.
| The mix-up | What you meant to give | What the arithmetic actually returns | Size of the error |
|---|---|---|---|
| 250 typed into an mg field, not mcg | 250 mcg = 0.1 mL = 10 units | 250 / 2.5 = 100 mL = 10,000 units | 1000 times too much, and it will not fit a 1 mL barrel |
| 0.25 typed into an mcg field, not mg | 0.25 mg = 0.1 mL = 10 units | 0.00025 / 2.5 = 0.0001 mL = 0.01 units | One thousandth of the dose, and the syringe looks empty |
| Chart assumed 2 mL of water, you added 1 mL | 250 mcg | 10 units = 0.1 mL at 5 mg/mL = 500 mcg | 2 times too much |
| Chart assumed 1 mL of water, you added 2 mL | 250 mcg | 5 units = 0.05 mL at 2.5 mg/mL = 125 mcg | Half the intended dose |
| Units read as milligrams | 10 units = 0.25 mg | 10 units is 0.1 mL, always, whatever the vial | Impossible: a full 100-unit barrel at 2.5 mg/mL holds 2.5 mg |
Baseline for every row: a 5 mg vial and an intended dose of 250 mcg (0.25 mg) drawn on a U-100 syringe. Reconstituted with 2 mL of bacteriostatic water that vial is 2.5 mg/mL, and the correct draw is 0.1 mL, which is 10 units.
The five-check self-test to run before every injection
Run these five checks in this order, every time, before the needle goes anywhere. They take under a minute and they catch every error described above. None of them requires you to trust the calculator, the vendor, or this article.
Check one, the unit check. State your dose in both mcg and mg, then confirm the unit selector matches the unit you actually typed. Check two, the water check. Confirm the BAC water volume you entered is the volume you measured into this specific vial, not the volume a chart assumed.
Check three, the concentration check. Divide vial mg by water mL in your head and confirm the calculator's concentration matches. That is one division, and it validates every output downstream of it. Check four, the ceiling check. Confirm the injection volume is smaller than the liquid in the vial, and that the unit count fits your barrel: 30 units on a 0.3 mL, 50 on a 0.5 mL, 100 on a 1 mL.
Check five, the reverse check. Multiply the units you are about to draw by 0.01 to get millilitres, multiply that by your concentration to get milligrams, and confirm you land back on the dose you started with. On the worked example: 10 units times 0.01 is 0.1 mL, and 0.1 mL times 2.5 mg/mL is 0.25 mg, which is 250 mcg. If the reverse calculation does not return your intended dose, stop and find out why before you draw anything.
Check five is independent of the calculator's own output, so it catches unit-selector slips and transcription errors. It cannot catch a wrong water volume, because that error lives in the vial rather than in the arithmetic, and the reverse calculation will happily confirm a concentration that was never true. Only check two catches that one, which is why measuring the water rather than eyeballing it matters more than any other step here.
Where the arithmetic stops and a clinician starts
Everything on this page is arithmetic. None of it is a protocol. A peptide calculator converts a dose that someone qualified has already decided into a volume you can draw, and it holds no opinion about whether that dose suits you, whether the compound is appropriate, or whether the vial contains what the label claims.
Research peptides such as BPC-157, TB-500, CJC-1295 and Ipamorelin are sold for research use and are not approved treatments. There is no approved label to convert, which means the dose you are calculating came from a protocol, a forum or a vendor rather than from a regulator. That does not change the arithmetic by a single decimal place, but it should change how much confidence you attach to the input.
For prescription GLP-1 medication supplied as a vial rather than a pre-filled pen, the same conversion applies and the same rule applies harder. Confirm your exact injection volume with the prescribing clinician before the first dose, and reconfirm it any time the concentration changes. Our methodology explains how we evaluate every provider and figure we publish, and our affiliate disclosure lists the commercial relationships behind this site, including the one named on this page.
The arithmetic is checkable, and that is the whole reason to prefer it to a chart. Every number in this article was recomputed from the four formulas at the top before it was published. If you find one that does not reconcile, the formulas are right there for you to prove it with.