practicalhow-to
Reconstitution: doing the maths together
The arithmetic behind mixing a powder vial, worked slowly, plus the decimal-point errors that have put people in hospital and the limits nobody can calculate away.
Arithmetic, checked twice
Reconstitution calculator
Work out how many units on an insulin syringe correspond to a given dose, from the amount in the vial and the volume of solvent you added.
Updated 18 June 2026Runs in your browser only
This is the calculation members most often get wrong, usually at night, usually while tired, and it is the one where getting it wrong matters most. So do it here, then do it again on paper, then have somebody else do it — that is the actual advice from our quality-and-testing circle and it is not a joke.
The tool assumes a standard U-100 insulin syringe, where 100 units is one millilitre. It does not know what you should take, it cannot know whether your vial contains what the label says, and it is not a substitute for the instructions that came with your product or for the person who prescribed it. If those disagree with this page, they are right and this page is wrong.
Read this before you trust the number
The result is only as good as the two figures you typed. The commonest error in this community is not the arithmetic; it is entering the total powder weight where the peptide content was wanted, which produces a confident and completely wrong answer. If a certificate gives a peptide content as a percentage of the powder, that percentage is the figure this calculator wants applied first.
A second, quieter error: drawing a volume smaller than your syringe can actually resolve. If the answer comes out at a very small number of units, the honest reading is that your solution is too concentrated to measure accurately, and the fix is more solvent rather than a steadier hand.
There is a longer walkthrough at reconstitution, doing the maths together, and the storage side is at storage and the cold chain.
Guides that go with it
startinghow-to
Your first injection, step by step
A slow walkthrough for the injection you have been putting off, written by people who put it off too — plus what to do if your hands are shaking.
practical
Pens, vials and syringes: a plain comparison
What each format actually is, what each gets right and wrong, and the one confusion — units versus milligrams — that has sent people to hospital.
practical
Storage and the cold chain
What the fridge is protecting, what happens when the chain breaks, and how to think about deliveries, heatwaves, power cuts and a pen left in a car.
sourcing
What purity actually means
A purity figure is a statement about one sample, measured one way, against one detector — here is what it covers, what it silently omits, and why content is a different question.