Gravity drips and drop factors
When an IV bag runs without a pump, the nurse sets the flow with a roller clamp and counts drops in the drip chamber. Each brand of tubing makes drops of a fixed size, given by the drop factor printed on the package: the number of drops (gtt, from the Latin guttae) that make 1 mL. Macrodrip sets give 10, 15 or 20 gtt/mL and suit adults and large volumes; microdrip sets give 60 gtt/mL and suit children and slow rates.
The formulas
- Flow rate (mL/h) = volume (mL) ÷ time (h)
- Drip rate (gtt/min) = volume (mL) × drop factor (gtt/mL) ÷ time (min)
- Equivalently, gtt/min = mL/h × drop factor ÷ 60
With a 60 gtt/mL set, the drip rate in gtt/min equals the flow rate in mL/h, a handy shortcut.
Worked example
Infuse 1,000 mL of saline over 8 hours with a 20 gtt/mL set.
- Flow rate = 1,000 ÷ 8 = 125 mL/h.
- Drip rate = 125 × 20 ÷ 60 = 41.7, rounded to 42 gtt/min.
- Time between drops = 60 ÷ 41.7 = 1.4 s.
- Every 15 minutes, about 31.3 mL should leave the bag, which gives a quick check on the volume marks.
Same infusion with other sets
| Drop factor | Drops per minute at 125 mL/h |
|---|---|
| 10 gtt/mL | 21 |
| 15 gtt/mL | 31 |
| 20 gtt/mL | 42 |
| 60 gtt/mL | 125 |
At 125 drops per minute, a microdrip is too fast to count reliably, which is why microdrip sets are kept for low rates, usually under 60 to 100 mL/h.
Counting in practice
Count the drops for a full minute, or for 15 seconds and multiply by 4. Recheck after the patient moves, after the bag height changes, and every hour: gravity drips slow down as the bag empties and as the patient bends the arm. Label the bag with time marks so drift is easy to see.
When a pump is required
Use an electronic infusion pump for potent medicines such as heparin, insulin, potassium and vasoactive drugs, for neonates and small children, for chemotherapy and whenever the prescription requires an exact dose. A calculated drip rate is a guide for gravity infusions, not a substitute for the protocols of your facility.
This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.