IV Drip Rate
Drops per minute for a gravity infusion, from the volume, the time, and the drop factor on the tubing.
Exact rate 31.25 gtt/min — drops cannot be split, so count 31 per minute.
Use this if the infusion is switched to a volumetric pump.
Multiply the volume in millilitres by the drop factor of the tubing, then divide by the infusion time in minutes. For 1000 mL over 8 hours with 15 gtt/mL tubing: (1000 × 15) ÷ 480 = 31.25, so you count 31 drops per minute.
Why gravity infusions are counted in drops
A volumetric pump is told a rate in millilitres per hour. A gravity line has no pump — flow is set by a roller clamp and checked by counting drops in the drip chamber. To convert a volume into a countable rate you need the drop factor: how many drops that particular tubing produces per millilitre. It is printed on the giving-set packaging, and it is a property of the tubing, not of the fluid or the patient.
Standard macrodrip sets deliver 10, 15, or 20 drops per millilitre. Microdrip (sometimes called minidrip) tubing delivers 60 drops per millilitre, which makes the arithmetic convenient: with a 60 gtt/mL set the drops per minute equal the millilitres per hour exactly.
Time must be in minutes
The formula divides by minutes, not hours, because the answer is a per-minute count. Converting the ordered time first — 8 hours becomes 480 minutes — removes the most frequent mistake in this calculation.
Drop factor comes from the tubing packaging: 10, 15, or 20 gtt/mL for macrodrip sets, 60 gtt/mL for microdrip. Round the answer to a whole number — you cannot count a fraction of a drop.
Worked example: 1000 mL over 8 hours, 15 gtt/mL tubing
Convert the time, multiply by the drop factor, then divide:
- 1 Read the drop factor off the tubing packet. This set is 15 gtt/mL. Never assume it — different sets differ.
- 2 Convert the ordered time to minutes. 8 hours × 60 = 480 minutes.
- 3 Multiply the volume by the drop factor. 1000 mL × 15 gtt/mL = 15,000 drops in total.
- 4 Divide by the time in minutes. 15,000 gtt ÷ 480 min = 31.25 gtt/min.
- 5 Round to whole drops. 31.25 rounds to 31 gtt/min — count 31 drops in the chamber over a full minute.
- 6 Set and re-check the roller clamp. Time the drops with a watch after adjusting, then re-check the rate on your next round.
Drip rate for 1000 mL by drop factor and time
Rounded to whole drops per minute. Calculated as (volume × drop factor) ÷ minutes.
| Infusion time | 10 gtt/mL | 15 gtt/mL | 20 gtt/mL | 60 gtt/mL |
|---|---|---|---|---|
| 4 hours (240 min) | 42 | 63 | 83 | 250 |
| 6 hours (360 min) | 28 | 42 | 56 | 167 |
| 8 hours (480 min) | 21 | 31 | 42 | 125 |
| 10 hours (600 min) | 17 | 25 | 33 | 100 |
| 12 hours (720 min) | 14 | 21 | 28 | 83 |
| 24 hours (1440 min) | 7 | 10 | 14 | 42 |
Checking your answer at the bedside
Because a gravity line drifts with patient position, bag height, and how full the bag is, the calculated rate is a starting point rather than a guarantee. Count the drops for a full 60 seconds after setting the clamp — counting for 15 seconds and multiplying by four magnifies any miscount fourfold.
A quick cross-check: divide the volume by the hours to get millilitres per hour, then confirm the drops per minute look plausible against it. For 1000 mL over 8 hours that is 125 mL/hr, and with 15 gtt/mL tubing 31 gtt/min is about a drop every two seconds, which is a rate you can realistically count.
This tool is a study aid for practising calculations. Infusion rates must be set and verified by a qualified professional against the prescription and local policy.