unpopular opinion: most of what gets said here about mg/mL is guesswork
unpopular opinion: most of what gets said here about mg/mL is guesswork, and I am aware this is a minority view on this board. I keep a laminated card in the drawer with the arithmetic on it. Looks ridiculous. Has never once let me draw the wrong volume. Shook the first vial because nobody told me not to. It was…
The whole calculation, once, with real numbers.
You have a 5mg vial. You add 3mL of bacteriostatic water. Concentration is mass over volume: 1.67mg/mL. A U-100 insulin syringe is graduated at 100 units per millilitre, so one unit is 0.01mL.
To draw 1.5mg you need 1.5 divided by 1.67 millilitres, which is 90 units on the barrel. Write "1.67mg/mL" on the vial in marker before you put it away, and you never have to do this again for that vial.
the number you write down is the number you will trust at 6am
That is sterile water, not bacteriostatic. Same appearance, different product, and it matters for a multi-draw vial.
The arithmetic in full: concentration is mass over volume, so 20mg into 3mL gives 6.67mg/mL. A U-100 syringe is graduated at 100 units per millilitre, so 2mg is 2 divided by 6.67 millilitres, which is 30 units.
Adding more water does not change how much peptide is in the vial. It changes the concentration, which changes the volume you draw for the same amount. That is the entire relationship.