The units, and the one that causes real harm
| Unit | Measures | What to know |
|---|---|---|
| mg milligram | Mass of the dry powder | How vials are sold, typically 2–30 mg |
| mcg microgram | Mass, smaller | 1 mg = 1000 mcg. Most peptide doses are quoted here |
| mL millilitre | Liquid volume | How much bacteriostatic water you add, and what you inject |
| units | Volume, on an insulin syringe | 100 units = 1 mL. A volume marking, not an amount of drug |
| IU international unit | Biological activity | Defined separately for each substance. Never convertible between compounds |
Reading mcg as mg, or the reverse, moves the dose by a factor of a thousand. It is the most common serious arithmetic mistake in this whole subject, and it is entirely silent — the syringe looks fine either way. Write the unit next to every number you put on paper, and check it again before you draw.
The units marking deserves a second look, because it trips people up. An insulin syringe measures volume, not drug. "10 units" means 0.1 mL of whatever is in the barrel. How much peptide that contains depends entirely on how much water you reconstituted with.
Concentration is the whole idea
Everything else follows from one relationship. The powder in the vial is a fixed amount. You choose how much water to dissolve it in. That choice sets how concentrated the solution is, and therefore how much liquid one dose comes to.
concentration = amount in the vial ÷ water added
A 10 mg vial in 2 mL of water gives 5 mg per mL. The same 10 mg in 5 mL gives 2 mg per mL. Identical powder, different strength per millilitre — and therefore a different number on the syringe for the same dose.
Because doses are usually quoted in micrograms, it is worth converting the concentration to mcg/mL immediately and staying there. Multiply by 1000: 5 mg/mL becomes 5000 mcg/mL. Working in one unit throughout is most of how you avoid the error above.
The method, in three steps
Two of the three numbers are facts about your vial. Only the third is a choice.
- Concentration. Vial amount ÷ water added. Convert to mcg/mL.
- Injection volume. Your dose ÷ concentration. This gives millilitres.
- Syringe reading. Millilitres × 100 = insulin units. In practice, move the decimal point two places right.
Worked through: a 5 mg vial reconstituted with 2 mL is 2.5 mg/mL, which is 2500 mcg/mL. A 250 mcg dose is 250 ÷ 2500 = 0.1 mL, which is 10 units on an insulin syringe.
The same arithmetic, three ways
Vial size and water added are facts you read off the vial. The dose column is whatever your protocol specifies — the figures below are there to show the working, not to suggest a number.
| Vial | Water | Concentration | Dose | Volume | Units |
|---|---|---|---|---|---|
| 5 mg | 2 mL | 2500 mcg/mL | 250 mcg | 0.1 mL | 10 |
| 2 mg | 2 mL | 1000 mcg/mL | 100 mcg | 0.1 mL | 10 |
| 5 mg | 1 mL | 5000 mcg/mL | 250 mcg | 0.05 mL | 5 |
| 10 IU | 1 mL | 10 IU/mL | 2 IU | 0.2 mL | 20 |
Compare rows one and three. Same vial, same dose, half the water — and the syringe reading halves. Nothing about the drug changed. This is why a number someone quotes you in "units" is meaningless unless you also know how they reconstituted, and why copying someone else's syringe marking is a genuine way to take the wrong amount.
Choosing how much water to add
Within reason this is your call, and it is a trade-off between precision and practicality.
| Approach | Gains | Costs |
|---|---|---|
| Less water higher concentration |
Smaller injection volume; vial lasts longer in mL terms | Each syringe marking covers more drug, so measurement error costs more |
| More water lower concentration |
Dose spread over more markings, so small adjustments are readable | Larger volume to inject; more of the vial is water |
A common landing point is 1–2 mL, which for typical vial sizes puts a dose somewhere in the readable middle of an insulin syringe rather than crowded against the first mark. If your calculation lands under about 5 units, the solution is probably too concentrated to measure reliably — add more water next vial and recalculate.
Checking the answer before it matters
- Reverse it. Multiply your injection volume by the concentration. It should give back your intended dose. If it does not, the error is upstream.
- Sanity-check the size. Peptide doses usually land between roughly 0.05 and 0.5 mL. A result far outside that range is a unit conversion gone wrong far more often than it is a real answer.
- Watch for factor-of-1000 answers. If the volume comes out absurdly large or vanishingly small, you have almost certainly mixed mg and mcg.
- Recalculate from scratch whenever anything changes — different vial size, different water, different dose. Do not adjust the old answer; redo it.
- Have it checked. A second person, or the same calculation done a different way, catches what re-reading your own working will not.
Erring low is recoverable; erring high may not be. When a calculation does not feel right, the answer is to stop and resolve it, not to estimate and proceed.
What this page does not cover
The source material this was drawn from also gives testosterone dosing targets calculated from bodyweight. That is not reproduced here, for two reasons. Testosterone is not a peptide and sits outside what this site covers; and clinical replacement is titrated against trough blood levels, haematocrit and symptoms rather than derived from a milligram-per-kilogram figure, so publishing target ranges would be prescribing rather than explaining arithmetic. If you are on hormone therapy, that calculation belongs with whoever monitors your bloods.
Sterile technique, storage and injection-site practice are covered in the risk primer. What to reconstitute with, and how long a vial stays good once mixed, are properties of the specific compound and should come from its documentation.
Before you act on any of this
This page explains a calculation. It is not a recommendation to use any compound, and it does not tell you what dose is appropriate for you — that requires someone who knows your history, your bloods and your medications. Getting the arithmetic right does not make a dose safe; it only makes it the dose you intended.