Accepted answer
At 2 mg/mL a 0.25 mg dose is 12.5 units on a U-100 barrel and a 1 mg dose is 50 units. Volume is dose divided by concentration and one unit is 0.01 mL, so the unit count is dose ÷ 2 × 100. Both land in a readable part of the barrel, which is what choosing the volume deliberately buys you.
Answering this needs the syringe you actually own, because the barrel graduations decide what "readable" means.
Round to a diluent volume you can measure accurately. Measuring 1.00 mL on a 1 mL syringe is reliable; measuring 1.37 mL on anything is not, and the error propagates into every dose.
Concentration and unit conversion at a glance
| Vial | Diluent | Concentration | 0.25 mg | 0.5 mg | 1 mg | 2.5 mg |
|---|
| 5 mg | 1 mL | 5 mg/mL | 5 u | 10 u | 20 u | 50 u |
| 5 mg | 2 mL | 2.5 mg/mL | 10 u | 20 u | 40 u | 100 u |
| 10 mg | 1 mL | 10 mg/mL | 2.5 u | 5 u | 10 u | 25 u |
| 10 mg | 2 mL | 5 mg/mL | 5 u | 10 u | 20 u | 50 u |
| 10 mg | 3 mL | 3.33 mg/mL | 7.5 u | 15 u | 30 u | 75 u |
Units are U-100 insulin units, where 1 unit = 0.01 mL. Divide dose by concentration for millilitres, then multiply by 100.
Vial headspace is the hard constraint. A nominal 2 mL vial typically holds a little over 2 mL to the shoulder; adding 3 mL is not an option and attempting it wastes the lot.
Insulin syringe barrel graduations are typically 1 unit on a 0.3 mL barrel, 1 unit on a 0.5 mL barrel and 2 units on a 1 mL barrel, which is why the barrel size changes what is readable.
Nothing here is medical advice, and research-use material is not approved for human use.
Check the vial can physically hold the volume before you draw it up.