IV Flow Rates and Infusions
mL/h, drops per minute, infusion times and dose-based drips
Formulas
| Pump rate | mL/h = total mL ÷ hours |
| Gravity drip | gtt/min = (mL × drop factor) ÷ minutes |
| Infusion time | hours = total mL ÷ mL/h |
| Dose-based | mL/h = dose per hour ÷ concentration |
Drop factors: macrodrip 10, 15 or 20 gtt/mL; microdrip 60 gtt/mL (with microdrip, gtt/min = mL/h).
Concentration
Concentration = amount of drug ÷ volume of the bag.
- Heparin 25,000 units in 250 mL = 100 units/mL.
- Heparin 25,000 units in 500 mL = 50 units/mL.
- Dopamine 400 mg in 250 mL = 1.6 mg/mL = 1,600 mcg/mL.
Worked examples
| Problem | Setup | Answer |
|---|---|---|
| 1,000 mL NS over 8 h (pump) | 1,000 ÷ 8 | 125 mL/h |
| 1,000 mL over 8 h, drop factor 15 | (1,000 × 15) ÷ 480 = 31.25 | 31 gtt/min |
| 500 mL over 4 h, drop factor 20 | (500 × 20) ÷ 240 = 41.67 | 42 gtt/min |
| Antibiotic 100 mL over 30 min (pump) | 100 ÷ 0.5 h | 200 mL/h |
| 1,000 mL at 125 mL/h, started 0800 | 1,000 ÷ 125 = 8 h | Finishes 1600 |
| Heparin 1,000 units/h; bag 25,000 units/250 mL | 1,000 ÷ 100 units/mL | 10 mL/h |
| Pump at 20 mL/h; heparin 25,000 units/500 mL. Dose? | 20 mL/h × 50 units/mL | 1,000 units/h |
| Dopamine 5 mcg/kg/min, 80 kg; bag 400 mg/250 mL | 5 × 80 = 400 mcg/min × 60 = 24,000 mcg/h ÷ 1,600 mcg/mL | 15 mL/h |
| KCl 20 mEq in 100 mL over 2 h | 100 ÷ 2; 20 ÷ 2 | 50 mL/h (10 mEq/h) |
Weight-based heparin (example protocol)
Patient 80 kg. Bolus 80 units/kg = 6,400 units. Infusion 18 units/kg/h = 1,440 units/h. Bag 100 units/mL → 1,440 ÷ 100 = 14.4 mL/h. Then adjust by aPTT or anti-Xa per your facility's protocol.
Check your answer
- Does it make sense? A maintenance IV is usually 75–150 mL/h in adults.
- Minutes vs. hours is the most common mistake in drip problems (× 60 to go from per minute to per hour).
- mcg vs. mg is the second most common: convert first.