GLP-1 Reconstitution Calculator: Units to Draw

Compounded semaglutide and tirzepatide are usually shipped as a lyophilised powder, a freeze-dried pellet at the bottom of a small glass vial, rather than as a ready-to-use pen. Before the first dose you add bacteriostatic water to dissolve the powder, a step called reconstitution. From that point every dose is measured not in milligrams but in the number of units you pull back on an insulin syringe, and translating one into the other is where most of the confusion, and most of the risk, lives. This calculator handles that translation.

How to use this calculator

Enter three numbers from your own vial and prescription. The vial strength is the total milligrams of medication in the vial, printed on the label. The bacteriostatic water is the volume in millilitres you added, or plan to add, to reconstitute it. The prescribed dose is the amount in milligrams your clinician told you to take. The tool returns the concentration, the volume to draw, and the mark to fill to on a U-100 insulin syringe, along with how many doses the vial holds. It does not decide your dose; it only converts the dose you were given into a syringe reading.

The math behind it

Reconstitution is two short steps of arithmetic. First, the concentration is the vial strength divided by the water volume. A 5 mg vial reconstituted with 2 mL of bacteriostatic water gives 2.5 mg per mL. Second, the volume for your dose is the dose divided by that concentration. A 0.25 mg dose at 2.5 mg/mL works out to 0.1 mL. Because a U-100 insulin syringe reads 100 units per millilitre, that 0.1 mL is 10 units on the barrel. Every result the calculator shows is those two steps and the unit conversion, nothing more.

Why the water volume does not change your dose

A common worry is that adding more or less water changes how much drug you get. It does not. The water only sets how concentrated the solution is, and the calculator adjusts the units to match. Add more water and the same dose is a larger, easier-to-read mark; add less and it is a smaller, more concentrated one. The total drug you inject stays the same as long as the dose in milligrams stays the same. What matters is that the water volume you enter here is the volume you actually used, so the units line up. When in doubt, use the volume your pharmacy specified.

Reading the syringe correctly

The single most important check is the syringe type. This tool assumes a U-100 insulin syringe, the standard for GLP-1 injections, where 100 units equal 1 mL. The "units" on that barrel are a volume scale, not a measure of the drug, so they only give the right dose at the concentration you calculated. A different syringe scale will not match these marks, and mismatched syringes are one of the best-documented causes of accidental overdose. Confirm your syringe reads to 100 units per mL before you rely on any figure here. For where and how to inject once the syringe is filled, see our guide to semaglutide injection sites and technique.

Where reconstitution comes up

Vials and reconstitution are almost entirely a feature of the compounded market, which is where the cheapest GLP-1 pricing now sits. Branded pens like Wegovy and Zepbound arrive premixed and dose themselves, so their users never touch this math. People using compounded semaglutide or tirzepatide through a licensed telehealth clinician do, and so do those following a lower-dose approach; both are covered in our explainers on getting compounded GLP-1 online and the GLP-1 micro-dosing trend. If you are weighing that route against branded pens on price, the GLP-1 cost calculator lays out what each option actually costs.

What this tool will not do

It will not tell you what dose to take, when to increase it, or how much water to add, because those are clinical decisions specific to you. It assumes the numbers you enter are correct and simply does the conversion. Two safeguards are worth repeating: verify the vial strength and your prescribed dose against your own paperwork, and verify the syringe is U-100. A credentialed compounding pharmacy and a prescriber who has evaluated you are not optional parts of this process, and choosing that provider well matters as much as the arithmetic, which is why we wrote a guide to choosing a telehealth GLP-1 prescription. Used alongside that guidance, this calculator takes the guesswork out of the one step that is pure math.