Dose calculator

A lyophilised vial contains a dry powder measured in milligrams. It has no volume you can draw, so before any of it can be measured out it has to be dissolved in bacteriostatic water. The moment you add that water you have created a concentration, and every number that follows comes out of it.

The concentration is the vial strength divided by the volume of water added. Five milligrams of powder in 2.5 mL of water is 2 mg per mL — which is also 2,000 mcg per mL, and that second form is usually the more useful one, because doses are commonly written in micrograms while vials are labelled in milligrams.

To find the volume of a single dose, divide the dose by that concentration. A 250 mcg dose out of a 2,000 mcg/mL vial is 0.125 mL. That is a real volume but a useless instruction, because an insulin syringe has no millilitre markings on it.

The U number on an insulin syringe is how many units it divides one millilitre into. A U-100 syringe has 100 marks to the millilitre, so one unit is 0.01 mL. A U-50 has 50, so one unit is 0.02 mL. A U-40 has 40, so one unit is 0.025 mL.

Multiply the dose volume by that number and you have the mark to draw to. The 0.125 mL above is 12.5 units on a U-100 syringe, 6.25 units on a U-50, and 5 units on a U-40 — the same quantity of drug in all three cases, described by three different rulers. This is the single most common place the arithmetic goes wrong: a number worked out for one syringe, drawn on another.

The more useful calculation is often the reverse one. You know the dose you want to measure and you have not mixed the vial yet, so the question is how much water to add to make that dose land somewhere you can read.

Pick the units you want to draw, then work up: units divided by the syringe's units per millilitre gives the dose volume, and vial strength divided by that volume, multiplied by the number of doses, gives the water to add. In practice the reverse tab does it in one step — enter the dose and the mark you would rather draw to, and it tells you the volume, or says so plainly when no sensible volume gets you there.

It also refuses to hand you an answer that will not physically fit. A 3 mL vial cannot take 5 mL of water no matter what the arithmetic says, and a calculator that returns that number anyway has told you something false in a confident font.

The concentration is fixed from that point. You cannot change it by adding more water later without changing every dose that comes out of the vial afterwards, so it is worth deciding the volume before the first one goes in rather than after.

Add the water slowly, down the side of the vial, and let it dissolve on its own rather than shaking it. Then check the mark on your own syringe before you draw, every time. This page divides numbers; it cannot see your syringe, and the markings are the thing that decides what actually gets measured.

Every formula above, along with how the app works out doses remaining and reorder dates, is written out in full on the methodology page.