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Tool

Reconstitution calculator

Work out the concentration, the draw volume, and where that lands on the barrel of an insulin syringe. It does the arithmetic on numbers you give it. It does not decide what your dose should be.

What this tool is and is not

This converts milligrams and millilitres into a mark on a syringe. Choosing a dose is a medical decision that belongs with a doctor who knows your history. If a number went in wrong, a confident-looking answer will come out.

Where to draw to Insulin syringe showing the draw volume A U-100 syringe barrel marked in units, with the calculated draw shaded from the plunger end. 020406080100 enter your numbers

Drawn for a U-100 barrel. Always read the syringe you are actually holding — U-50 and U-30 barrels carry different markings for the same volume.

How the maths works

Reconstitution is a dilution problem. The vial holds a fixed mass of powder. The water you add decides how concentrated the solution is, and that decides how much liquid carries your dose.

Concentration is the vial contents divided by the water added. A 10 mg vial in 2 mL gives 5 mg per mL, or 5000 mcg per mL.

Draw volume is your dose divided by that concentration. A 250 mcg dose at 5000 mcg per mL needs 0.05 mL.

Units are the marks on an insulin barrel. A U-100 syringe divides 1 mL into 100 marks, so 0.05 mL is 5 units. This is the step people most often get wrong, because "units" sounds like a dose when it is really a volume.

More water makes small doses easier to measure accurately. If your draw lands under about two units, add more water and recalculate rather than trying to eyeball a sliver.