the actual arithmetic for splitting a weekly mg total into two draws, worked example
Getting DMs asking to show the math instead of just saying "split it evenly" so here's a full worked example using round numbers.
Say a vial is labelled 10mg and reconstituted with 2mL of bacteriostatic water. Concentration = 10mg ÷ 2mL = 5mg/mL. On a standard U-100 insulin syringe, 1mL = 100 units, so 5mg/mL means 20 units = 1mg.
If someone's total weekly amount from that vial is 2.5mg, that's 2.5 × 20 = 50 units for the whole week drawn in one go. Splitting that into two even draws is 25 units each. Splitting unevenly (say a bigger draw early in the week) just means the two numbers add back to 50 — the arithmetic doesn't care how you split it, it only cares that both draws come from the same concentration and add up correctly.
The part people get wrong isn't the division, it's forgetting to redo this whole calculation when they open a new vial with a different mg amount or reconstitute with a different volume. The units-per-mg ratio changes every time either number changes.
best — the order this archive was captured in
this is the cleanest worked version of this I've seen posted, and the closing point is the actual root cause of nearly every dosing math error in this community — reusing an old units number after changing either the vial strength or the reconstitution volume.
yeah I see that exact error pattern constantly, someone reconstitutes differently "because they ran low on bac water" and just keeps using the old unit count
and dead space compounds it, that 2-5 unit loss per draw becomes a bigger relative error once you're doing two smaller draws instead of one bigger one
which is an argument for reconstituting with a slightly higher concentration if you're going to split, fewer total units drawn means dead space eats a smaller fraction
good addition, higher concentration + splitting does pair better than low concentration + splitting for exactly that reason
would add: weigh the vial before and after reconstitution if your scale is precise enough, it's a nice sanity check on your own math independent of the syringe.
Flairing Lab since this is a worked calculation rather than a general discussion. Genuinely useful reference post. edit: fixed a units typo in my own comment, said "50 units" when quoting the post, post itself is correct.
U-100 syringe assumption is worth stating explicitly like you did, people occasionally have U-40 on hand from somewhere else and the whole conversion breaks if you mix that up.
nice worked example, this kind of arithmetic-with-actual-numbers post is worth ten "just be careful with your math" comments.
slightly tangential but this is also why "trust the vendor's suggested reconstitution" is a bad habit, your own math should always match what's on the vial regardless of what a website suggests
worth adding residual water content into the "actual mg in the vial" side of this equation too if you have a COA with it listed, the label mg and the true peptide mg aren't always identical
good point, that's a refinement past what most people need but genuinely correct for anyone chasing precision this far
this post is a great example of showing work instead of just stating a rule, more of this please
- 1this is the cleanest worked version of this I've seen posted, and the…5 comments in this branch · started by u/units_not_mg