Five reconstitution examples, worked start to finish
The same four steps applied to five common vial and water combinations, with every intermediate number shown.
Reconstitution arithmetic is easier to trust once you have watched it run a few times. These are five combinations people commonly land on, each worked through completely, plus one that goes wrong and how to fix it.
Every example uses the same procedure:
2. Convert the portion to mg (mcg ÷ 1000)
3. Volume = portion (mg) ÷ concentration (mg/mL)
4. Units = volume (mL) × 100
Example 1 — 5 mg vial, 2 mL water, 250 mcg
The most common arrangement, and the one most reference tables assume.
- Concentration: 5 ÷ 2 = 2.5 mg/mL
- Portion in mg: 250 ÷ 1000 = 0.25 mg
- Volume: 0.25 ÷ 2.5 = 0.1 mL
- Units: 0.1 × 100 = 10 units
Doses per vial: 5 ÷ 0.25 = 20. Ten units sits a third of the way along a 30-unit barrel, which is comfortable to read.
Example 2 — 10 mg vial, 2 mL water, 500 mcg
Double the vial and double the portion, and the reading is unchanged.
- Concentration: 10 ÷ 2 = 5 mg/mL
- Portion in mg: 500 ÷ 1000 = 0.5 mg
- Volume: 0.5 ÷ 5 = 0.1 mL
- Units: 10 units
Doses per vial: 10 ÷ 0.5 = 20. Identical barrel reading to example 1 despite twice the concentration, because the portion doubled too. This is worth noticing: the number on the syringe tells you nothing on its own.
Example 3 — 2 mg vial, 1 mL water, 100 mcg
A small vial, reconstituted small.
- Concentration: 2 ÷ 1 = 2 mg/mL
- Portion in mg: 100 ÷ 1000 = 0.1 mg
- Volume: 0.1 ÷ 2 = 0.05 mL
- Units: 5 units
Doses per vial: 2 ÷ 0.1 = 20. Five units is readable but tight — it sits in the first sixth of a 30-unit barrel. Reconstituting this one with 2 mL instead would put it at 10 units, if the vial takes it.
Example 4 — 15 mg vial, 3 mL water, 1 mg
Larger vial, larger portion, a draw well into the barrel.
- Concentration: 15 ÷ 3 = 5 mg/mL
- Portion in mg: already 1 mg
- Volume: 1 ÷ 5 = 0.2 mL
- Units: 20 units
Doses per vial: 15 ÷ 1 = 15. Twenty units is two thirds of the way along a 30-unit barrel — easy to read, but leaves little room if the portion ever increases. A 50-unit syringe gives more headroom here.
Example 5 — 30 mg vial, 3 mL water, 2.5 mg
A concentrated vial with a substantial portion.
- Concentration: 30 ÷ 3 = 10 mg/mL
- Portion in mg: already 2.5 mg
- Volume: 2.5 ÷ 10 = 0.25 mL
- Units: 25 units
Doses per vial: 30 ÷ 2.5 = 12. At 25 units this nearly fills a 30-unit barrel. Workable, but a 50-unit syringe is the more sensible instrument.
All five at a glance
| Vial | Water | Conc. | Portion | Volume | Units | Doses |
|---|---|---|---|---|---|---|
| 5 mg | 2 mL | 2.5 | 250 mcg | 0.10 mL | 10 u | 20 |
| 10 mg | 2 mL | 5 | 500 mcg | 0.10 mL | 10 u | 20 |
| 2 mg | 1 mL | 2 | 100 mcg | 0.05 mL | 5 u | 20 |
| 15 mg | 3 mL | 5 | 1 mg | 0.20 mL | 20 u | 15 |
| 30 mg | 3 mL | 10 | 2.5 mg | 0.25 mL | 25 u | 12 |
Concentration in mg/mL. These are arithmetic demonstrations using plausible round numbers, not recommendations about what to draw.
One that goes wrong — 5 mg vial, 1 mL water, 100 mcg
Same procedure, unusable answer.
- Concentration: 5 ÷ 1 = 5 mg/mL
- Portion in mg: 100 ÷ 1000 = 0.1 mg
- Volume: 0.1 ÷ 5 = 0.02 mL
- Units: 2 units
The arithmetic is fine. The measurement is not. Two units on any barrel is a distance comparable to the width of the printed lines, so a slip of half a unit — well within normal reading error — is a 25% miss.
The fix is dilution, not care. Reconstituting the same vial with 3 mL instead gives 1.67 mg/mL, putting the same 100 mcg at 6 units. Same peptide, same 50 doses in the vial, three times the resolution.
This is the general shape of the problem: when a draw is too small to read, the answer is almost always to change the concentration of the next vial rather than to aim more carefully at the current one.
Checking your own numbers
Three tests that catch most errors:
- Doses per vial should be a sensible number. Vial mg ÷ portion mg. If a 5 mg vial appears to hold 200 portions or half a portion, the mcg-to-mg step went wrong.
- Units should be between about 2 and 100. Under 2 is unreadable; over 100 does not fit any insulin syringe.
- Volume should be less than the water you added. If a single draw appears larger than the whole vial, something is inverted.