IV Drip Rate Calculator (gtt/min)
Calculate drops per minute for a gravity IV infusion from the volume, the time and the drop factor of the tubing, with each step explained.
Formula
gtt/min = (volume in mL × drop factor in gtt/mL) ÷ time in minutes
How to calculate it step by step
- Find the volume in mL and the infusion time on the order.
- Find the drop factor on the IV tubing package: 10, 15 or 20 gtt/mL for macrodrip tubing, 60 gtt/mL for microdrip tubing.
- Convert the time to minutes (hours × 60).
- Multiply the volume by the drop factor, then divide by the minutes.
- Round to the nearest whole drop, then count the drops in the drip chamber for one minute (or for 15 seconds and multiply by 4).
Worked examples
Example 1. Infuse 1,000 mL of D5W over 12 hours by gravity. The tubing drop factor is 15 gtt/mL. How many drops per minute will you count?
- Time in minutes: 720 min.
- gtt/min = (1,000 mL × 15 gtt/mL) ÷ 720 min = 20.83.
- Round to the whole drop: 21 gtt/min.
Answer: 21 gtt/min
Example 2. Infuse 1,000 mL of D5 0.45% sodium chloride over 4 hours by gravity. The tubing drop factor is 15 gtt/mL. How many drops per minute will you count?
- Time in minutes: 240 min.
- gtt/min = (1,000 mL × 15 gtt/mL) ÷ 240 min = 62.5.
- Round to the whole drop: 63 gtt/min.
Answer: 63 gtt/min
Rounding
Drops are always rounded to the nearest whole number: you cannot count part of a drop.
Common mistakes
- Dividing by hours instead of minutes.
- Using the wrong drop factor: always read it on the tubing package being used.
- Forgetting the shortcut for microdrip tubing: with 60 gtt/mL, gtt/min equals mL/hr.
Nursing safety points
- Use an electronic pump instead of gravity for high-alert drugs, potassium, heparin, insulin, vasoactive drips, and for children when your policy requires it.
- Recheck the drip rate after the patient changes position: gravity infusions speed up or slow down with the height of the bag and the arm position.
- Microdrip tubing is used for small volumes and slow rates, macrodrip for large volumes and faster rates.
Frequently asked questions
What is the formula for drops per minute?
gtt/min = volume (mL) × drop factor (gtt/mL) ÷ time (min). For 1,000 mL over 8 hours with 15 gtt/mL tubing: 1,000 × 15 ÷ 480 = 31.25, so 31 gtt/min.
What is the drop factor?
It is the number of drops that make 1 mL with a given IV tubing. It is printed on the tubing package: 10, 15 or 20 gtt/mL for macrodrip and 60 gtt/mL for microdrip.
Why does gtt/min equal mL/hr with microdrip tubing?
With 60 gtt/mL tubing, the 60 drops per mL and the 60 minutes per hour cancel out. For example, 50 mL/hr = 50 × 60 ÷ 60 = 50 gtt/min.