How to work out peptide reconstitution math (with worked examples)

A plain-English walkthrough of peptide reconstitution math: how mg, ml and units relate, how to read a U-100 syringe, and worked examples you can follow step by step.

Reconstitution math trips almost everyone up at first, because three different units are in play at once — milligrams (mg) of powder, millilitres (ml) of liquid, and the "units" marked on an insulin syringe. Get the relationship between them straight and the arithmetic becomes simple. This is a conceptual explainer with worked examples; it is not dosing advice, and every real decision belongs with a qualified clinician.

The three numbers that matter

Vial strength (mg). How much peptide is in the vial as dry powder — printed on the label, e.g. 5 mg or 10 mg.
Diluent volume (ml). How much bacteriostatic water you add. You choose this — it changes the concentration but not the total amount of peptide.
Concentration (mg/ml). Simply strength divided by volume. This is the number everything else is built on.

Step 1: work out your concentration

Concentration = vial strength ÷ diluent volume. If you put 2 ml of water into a 10 mg vial, that's 10 ÷ 2 = 5 mg/ml. Add only 1 ml to the same vial and it becomes 10 mg/ml — same peptide, twice as concentrated, so half the liquid carries the same amount.

Step 2: convert to the units on the syringe

Insulin syringes are almost always U-100, which means 100 units = 1 ml. Units are just a finer way of reading the same millilitres: 50 units is 0.5 ml, 10 units is 0.1 ml. So once you know your mg/ml, you can translate any amount in mcg into a mark on the barrel.

The one conversion people forget: 1 mg = 1000 mcg. Most amounts people record are in mcg, while the vial is labelled in mg, so this factor of 1000 is where mistakes creep in.

A worked example, start to finish

Vial: 10 mg. Water added: 2 ml. Concentration = 10 ÷ 2 = 5 mg/ml = 5000 mcg/ml.
Say you want to draw 250 mcg. As a fraction of 1 ml that's 250 ÷ 5000 = 0.05 ml.
On a U-100 syringe, 0.05 ml = 0.05 × 100 = 5 units. So 250 mcg sits at the 5-unit mark.

Change the water and the mark moves. Reconstitute the same 10 mg vial with 1 ml instead (10 mg/ml), and 250 mcg becomes 2.5 units — half as far up the barrel. Neither is "right"; they're just different concentrations, which is exactly why writing down how much water you used matters.

Do it without the arithmetic

If you'd rather not do this by hand every time, the free reconstitution calculator takes your vial strength, water volume and the amount you want, and shows the concentration and the syringe mark. It's a calculator for numbers you enter — it does not recommend a dose.

Was this helpful?

FAQ

How do you calculate peptide reconstitution?

Divide the vial strength (mg) by the water you add (ml) to get concentration in mg/ml. Convert your target amount to the same units, then divide by the concentration to get millilitres, and multiply by 100 for units on a U-100 syringe.

How many units is 250 mcg?

It depends entirely on concentration. In a vial reconstituted to 5 mg/ml (5000 mcg/ml), 250 mcg is 0.05 ml, which is 5 units on a U-100 syringe. At a different concentration the same 250 mcg lands on a different mark.

What does U-100 mean on an insulin syringe?

U-100 means the scale is calibrated so 100 units equal 1 millilitre. So 1 unit is 0.01 ml. It's just a finer-grained way of reading the same volume.

Why does the amount of water change the dose mark?

Because water changes concentration, not the amount of peptide. Less water means a higher mg/ml, so the same amount of peptide sits in fewer millilitres — a smaller mark on the syringe. That's why recording your diluent volume is essential.

Keep reading

How to keep a peptide log (and why spreadsheets fail)A practical guide to keeping an organised peptide research log — what to record,…How to store peptides: fridge, freezer and travelA plain-English guide to peptide storage — why cold and dark matter, the differe…What is reconstitution? A conceptual explainerWhat reconstitution means, why peptides are supplied freeze-dried, what bacterio…

Educational content only. Peptide Wizard does not calculate or recommend doses and is not medical advice — always consult a qualified healthcare professional.