How much water should I add?

Whatever your prescriber or the product's instructions specify. There is no universally correct amount: it sets the concentration, and therefore how big and how readable the mark on the syringe is. This page takes that figure as an input rather than recommending one.

Why does it answer in units rather than millilitres?

Because that is what the syringe is marked in. Both are shown — the millilitre figure is the underlying volume, and the unit figure is the mark to draw to.

Can I use this for insulin?

No. Insulin is supplied at a stated concentration and is dosed in units directly; there is no reconstitution step and this arithmetic does not apply. Follow your prescriber's instructions.

Peptide Reconstitution Calculator (Dose to Syringe Units)

Vial strength and water to a volume on the syringe.

Inputs

This page does arithmetic only. It turns a vial size and the amount of liquid you added into a volume to draw. It does not tell you what dose to take, whether a product is safe, or whether it is legal where you live. Dosing belongs to the prescriber or pharmacist who supplied the medication — follow their instructions over anything shown here. A misplaced decimal point changes a dose tenfold: check the result against your prescription label before you draw. CalculateEveryday is not a medical provider and accepts no responsibility for how this number is used.

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Formula

Concentration = vial mg ÷ diluent mL. Volume to draw = dose mg ÷ concentration. Units = volume × the syringe's units per mL

V = \frac{D_{\text{mg}}}{C_{\text{mg/mL}}} \qquad C = \frac{\text{vial mg}}{\text{diluent mL}}

Adding liquid to a freeze-dried vial makes a solution of a known strength: 5 mg in 2 mL is 2.5 mg per mL. The volume holding your dose is the dose divided by that strength. Insulin syringes are marked in units rather than millilitres, so the last step multiplies by the marks per millilitre — 100 on a U-100 syringe.

Worked example

A 5 mg vial with 2 mL of bacteriostatic water gives 2.5 mg per mL. A 250 mcg dose is 0.25 mg, so 0.25 ÷ 2.5 = 0.1 mL — the 10-unit mark on a U-100 syringe. The vial holds about 20 of those doses.

Where this can give the wrong answer

Micrograms and milligrams are a thousand apart, and mixing them up is the commonest and most dangerous error here. 250 mcg is 0.25 mg; if a figure looks ten or a hundred times out, check the unit first.
How much water you add changes nothing about the dose, only the volume you draw. Less water means a stronger solution and a smaller, harder-to-measure mark; more water means an easier mark and fewer doses per vial.
Check the syringe scale before reading the answer. The same 0.1 mL is 10 units on a U-100 syringe, 5 on a U-50 and 4 on a U-40.
A draw below about 2 units cannot be measured accurately on a standard syringe. The page says so rather than printing a number you cannot act on.
This is arithmetic about a volume. It makes no claim about what any substance does, whether it is safe, or whether it is legal where you live.

FAQ

: Whatever your prescriber or the product's instructions specify. There is no universally correct amount: it sets the concentration, and therefore how big and how readable the mark on the syringe is. This page takes that figure as an input rather than recommending one.

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