Pump rate: mL/hr
mL/hr = total volume (mL) ÷ time (hr)
- 1,000 mL ÷ 8 hr
= 125 mL/hr
When the time is in minutes, convert to hours or multiply by 60:
- 50 mL ÷ 30 min × 60 min/hr
= 100 mL/hr
Gravity rate: gtt/min
gtt/min = volume (mL) × drip factor (gtt/mL) ÷ time (min)
The drip factor is printed on the tubing package. Macrodrip sets are usually 10, 15 or 20 gtt/mL; microdrip sets are 60 gtt/mL. You cannot give part of a drop, so round to the whole drop.
| Drip factor | Setup | Rate |
|---|---|---|
| 10 gtt/mL | 1,000 × 10 ÷ 480 = 20.83 | 21 gtt/min |
| 15 gtt/mL | 1,000 × 15 ÷ 480 = 31.25 | 31 gtt/min |
| 20 gtt/mL | 1,000 × 20 ÷ 480 = 41.67 | 42 gtt/min |
| 60 gtt/mL | 1,000 × 60 ÷ 480 = 125 | 125 gtt/min |
The microdrip shortcut
With a 60 gtt/mL set, drops per minute equal mL per hour, because the 60 drops per mL and the 60 minutes per hour cancel. 125 mL/hr on a microdrip set is 125 gtt/min.
Infusion time
time (hr) = total volume (mL) ÷ rate (mL/hr)
- 1,000 mL ÷ 120 mL/hr × 60 min/hr = 500 min
- 500 min = 480 min (8 hr) + 20 min
= 8 hr 20 min
Heparin and other unit-based drips
For drips ordered in units per hour, find the concentration first, then divide.
- 25,000 units ÷ 250 mL = 100 units/mL
- 1,200 units/hr ÷ 100 units/mL
= 12 mL/hr
Common mistakes
- Answering 125 when the question asks for gtt/min on a 15 gtt/mL set (that is the pump rate).
- Forgetting to change hours to minutes: 1,000 × 15 ÷ 8 gives 1,875, drops per hour, not per minute.
- Rounding 31.25 up to 32. Round to the nearest whole drop: 31.
