Reconstitution math: worked examples (vial mg + water mL to units)
5 mg in 2 mL is 2.5 mg/mL, so 250 mcg is 0.1 mL = 10 units on a U-100 syringe. The math has two steps. First, concentration = vial mg ÷ water mL (5 mg in 2 mL = 2.5 mg/mL = 2,500 mcg/mL). Second, volume per draw = amount ÷ concentration, and U-100 units = mL × 100.
Want your own numbers? Open the reconstitution calculator: enter vial mg, water mL and amount to get units on U-100, U-50 or U-40.
The math
Step 1: concentration (mg/mL) = vial mg ÷ water mL
concentration (mcg/mL) = mg/mL × 1000
Step 2: volume (mL) = amount ÷ concentration (same mass unit)
Step 3: units = mL × 100 (U-100), × 50 (U-50), × 40 (U-40)
Shortcut (U-100): units = amount mcg × water mL ÷ (vial mg × 10)| Vial + water | mcg/mL | Example amount | mL | U-100 units |
|---|---|---|---|---|
| 5 mg + 1 mL | 5000 | 250 mcg | 0.05 | 5 |
| 5 mg + 2 mL | 2500 | 250 mcg | 0.1 | 10 |
| 10 mg + 2 mL | 5000 | 500 mcg | 0.1 | 10 |
| 10 mg + 3 mL | 3333.333 | 1000 mcg | 0.3 | 30 |
| 2 mg + 1 mL | 2000 | 100 mcg | 0.05 | 5 |
Worked example, step by step
A 10 mg vial mixed with 3 mL of water: 10 ÷ 3 = 3.333 mg/mL, or 3,333 mcg/mL. To draw 1,000 mcg: 1,000 ÷ 3,333 = 0.3 mL, which is 30 units on a U-100 syringe, 15 on U-50, or 12 on U-40.
Check it with the shortcut: 1,000 × 3 ÷ (10 × 10) = 30 units. Same answer.
What changing the water does
Water changes concentration, never the total in the vial. Doubling the water halves the concentration, which doubles the units for the same amount. 250 mcg is 5 units from 5 mg + 1 mL, but 10 units from 5 mg + 2 mL. More water gives a larger, easier-to-read draw; less water keeps each draw small.
Draws per vial do not depend on water at all: vial amount ÷ amount per draw. A 5 mg vial holds 20 draws of 250 mcg whether you add 1 mL or 3 mL.
Working backwards: pick the water for a target mark
If you want an amount to land on a specific unit mark, solve for water: water mL = target units × vial mg × 10 ÷ amount mcg. Example: to make 250 mcg read as 20 units from a 5 mg vial, water = 20 × 5 × 10 ÷ 250 = 4 mL. The calculator below has a reverse mode for this.
Common mistakes
Mixing mg and mcg (1 mg = 1,000 mcg) is the most common error, and it is off by exactly 1,000×. The second is using a U-40 or U-50 syringe with U-100 numbers. Write the concentration on the vial so you never have to redo the math.
Math only, not medical advice. This page converts numbers; it does not recommend any compound or amount.
Concentration = 5 mg ÷ 2 mL = 2.5 mg/mL Volume = 250 mcg ÷ 2500 mcg/mL = 0.1 mL Units = 0.1 mL × 100 (U-100) = 10 units
U-100 syringe · 100-unit barrel. Illustration only; read the marks on your actual syringe.
Math only, not medical advice. Verify every number yourself.