mg ÷ mL, that is the whole formula, why did i find this hard
mg ÷ mL, that is the whole formula, why did i find this hard, which sounds obvious until you try to state the evidence for it.
Started reconstituting everything to a round concentration and the six-in-the-morning error rate went to zero.
The whole calculation, once, with real numbers.
You have a 30mg vial. You add 3mL of bacteriostatic water. Concentration is mass over volume: 10mg/mL. A U-100 insulin syringe is graduated at 100 units per millilitre, so one unit is 0.01mL.
To draw 5mg you need 5 divided by 10 millilitres, which is 50 units on the barrel. Write "10mg/mL" on the vial in marker before you put it away, and you never have to do this again for that vial.
The failure modes I have actually seen posted here, in order of frequency.
One: reconstituting at a different volume than intended and not writing it down, so the next draw is based on a remembered concentration. Two: treating units as a dose rather than a volume, which breaks the moment the concentration changes. Three: assuming a syringe is U-100 when it is not.
All three are solved by the same two habits — do the arithmetic before the water goes in, and write the result on the glass. Neither costs anything and between them they cover almost every question this board gets.
Sceptical readings welcome. The confident ones are the ones I distrust.
best — the order this archive was captured in
The arithmetic in full: concentration is mass over volume, so 20mg into 2mL gives 10mg/mL. A U-100 syringe is graduated at 100 units per millilitre, so 1.5mg is 1.5 divided by 10 millilitres, which is 15 units.
The arithmetic in full: concentration is mass over volume, so 20mg into 2mL gives 10mg/mL.
Right, and the arithmetic behind it: 5mg in 2mL is 2.5mg/mL, so 0.25mg is 10 units.
Reminder: this board does maths, not dosing. Nobody here tells another member what amount to take.
What concentration did you actually reconstitute at, rather than what did you intend to?
I would not round that figure. At 5mg/mL a rounding error of two units is a real fraction of the intended amount.
Wrote the date on the vial as well as the concentration after losing track of which one was opened first.
reconstituting at a round concentration makes every later dose easy
reconstituting at a round concentration makes every later dose easy
Disagreeing with this line only: adding more water is not a safety margin, it is a different divisor.
Did the maths give you a whole number of units, or a fraction?
Asked WWB what volume they reconstitute to for their own stability testing and they answered in one line. Useful thing to know.
That only works if the fill weight is exactly the label weight, and it usually is not exactly anything.
What is written on the vial right now?
label the vial with the date as well as the concentration
Is that a U-100 syringe? The barrel markings change the answer completely.
Spent 10 weeks confused about this until somebody drew the table for me. 20mg, 2mL, 10mg/mL, 2.5mg is 25 units. That is the whole thing.
Disagree with the "more water is safer" framing. It is not safer, it is just a different number to divide by.
Bacteriostatic or plain sterile water?
Careful — that arithmetic is wrong. 10mg in 3mL is 3.33mg/mL, so 0.5mg draws to 15 units, not what you wrote.
If your calculation produces a fraction of a unit, the concentration is wrong for your dose. Change the water volume until the numbers land on whole units.
The dead space question sent me down a rabbit hole for a week. It is a fraction of a unit and it is the same every draw.
How many mg in the vial and how many mL are you adding?
Correction: that gives 50 units, not what you have written. 15mg into 3mL is 5mg/mL and the division follows from there.
Shook the first vial because nobody told me not to. It was fine, but I have swirled every one since.
Pushback on the syringe advice: a U-100 barrel reads units of volume. Calling them "doses" is how the confusion starts.
- 1The arithmetic in full: concentration is mass over volume, so 20mg into 2mL…7 comments in this branch · started by u/yusuf_nakamura
- 2Did the maths give you a whole number of units, or a fraction?6 comments in this branch · started by u/georgi_adebayo