How to work out what to draw

By Tessaro Pty Ltd, the maker of Pepti · Reviewed October 2026

The same vial can give you two completely different draws. It depends entirely on how much water you added. Here is the maths, in three steps.

The three formulas

Most insulin syringes sold for at-home use are U-100, marked 0 to 100 units. If your syringe is marked in units and reaches 100, it is U-100 and the formula above applies. If it is marked in millilitres only, read the volume directly and skip the last step.

A worked example

Concentration table

The mistake almost everyone makes

If you change how much water you add, every unit number you had memorised is wrong. The vial has not changed — the concentration has. Work it out again from the top whenever your water volume changes. If you are choosing that volume for the first time, how much bacteriostatic water for a 5 mg vial explains what the choice changes and what it does not.

How many doses a vial gives you

Doses per vial = water added (mL) ÷ volume per dose (mL)

2 mL of water added, drawing 0.1 mL per dose, gives 20 doses. Work this out once when you reconstitute and count down from it.

Which number on the vial is which

The formulas need two facts from the vial and one from you. A vial of freeze-dried powder states a total amount; it has no concentration until liquid goes in, and the concentration it ends up with is entirely a product of how much you add. A vial or pen that arrives as a solution already states its strength per millilitre, so the first formula does not apply: the label has done it for you, and you start at the second line. The two kinds of number are easy to confuse and the arithmetic is different for each, so read the label before the calculator. Our guide to reading a vial label goes through every field.

Units, millilitres and why they get mixed up

An insulin syringe is marked in units, not millilitres, and the scale is a convention: on a U-100 syringe, one millilitre is divided into one hundred marks. Syringes come in several capacities on the same scale, so a 1 mL syringe reads to 100 units, a 0.5 mL syringe to 50 and a 0.3 mL syringe to 30; a mark means the same volume on each, the barrel simply stops sooner. The third formula is nothing more than that convention written down. The trap is the word “units” itself, which some product labels use for an entirely different measure, international units (IU) of activity. A syringe unit is a volume; an IU is not. This page converts volumes only and gives no IU conversion.

Checking a result before you trust it

Arithmetic errors with vials are usually input errors, and they show up in the size of the answer. A draw volume larger than the syringe holds, or so small it sits between the first two marks, means a number went in wrong: the amount and the strength were swapped, the water volume was entered in the wrong unit, or the figure you hold was in micrograms and was typed as milligrams. Run the three lines again from the label rather than adjusting the answer until it looks plausible. The calculator keeps all three inputs visible for exactly this reason.

What to write down when you mix

The water volume is printed nowhere once the vial is mixed, which is why every later draw depends on a note made at the time: the date and time, the amount as printed, the diluent and the exact volume added, the resulting concentration, the discard-by date you were given, and where the vial is kept. The reconstitution log guide sets out each field and why it earns its place, and storage and expiry logging covers the dates. A vial record without the water volume is a vial you will later be guessing about.

Common questions

What does mg/mL mean?

Milligrams of substance in each millilitre of solution: the concentration. It is the amount in the vial divided by the volume of liquid you added. Two vials with the same amount of powder but different water volumes have different concentrations, which is why the same vial strength can give two different draws.

Why does the same vial give a different draw after I change the water volume?

Because the water volume sets the concentration, and the draw is the figure you hold divided by that concentration. Double the water and the concentration halves, so the same figure needs twice the volume. The amount in the vial has not changed; the arithmetic has. Work it out again from the top whenever the water volume changes.

How do I convert millilitres to insulin syringe units?

On a U-100 syringe, multiply the volume in millilitres by 100: 0.1 mL is 10 units and 0.25 mL is 25 units. The conversion is the same on 1 mL, 0.5 mL and 0.3 mL syringes, which all use the U-100 scale and differ only in how far the barrel goes. It applies to syringe units only, not to international units (IU) on a product label.

Does this page tell me what dose to take?

No. Every example on it is an illustration of division with made-up inputs. The figure you work to comes from whoever directs your use; this page and the calculator convert that figure into a volume and syringe units, and nothing more.

This converts a target you already have. It does not tell you what that target should be.

Before you start, the guide to reading a vial label explains which number on the label is which, and the reconstitution log guide sets out what to write down when you mix.

Once the arithmetic is done, the Pepti peptide tracker records your protocol, injection sites and logs, with each vial’s lot and reconstitution date; the app itself does not do this calculation. See the peptide tracking app guide for what a tracker logs.

Not medical advice. Pepti is a record-keeping tool, not a medical device.

Pepti – Peptide Tracker · Guides · Reviewed October 2026 · Editorial policy · About