Dosage Practice

IV Drip Rate Calculation: Drops per Minute by Gravity

Drops per minute equals the volume in mL multiplied by the drop factor (gtt/mL printed on the tubing), divided by the time in minutes. Round to a whole number, because you cannot count part of a drop.

The Formula

gtt/min = (total mL × drop factor) ÷ total minutes

From a pump rate: gtt/min = mL/hr × drop factor ÷ 60.

How to work it out

  1. 1 Find the drop factor on the tubing package: 10, 15 or 20 gtt/mL for macrodrip, 60 gtt/mL for microdrip.
  2. 2 Put the time into minutes. 8 hours is 480 minutes.
  3. 3 Multiply volume by drop factor, divide by minutes.
  4. 4 Round to the nearest whole drop. Then count drops for 15 seconds and multiply by four to check the chamber.

Worked Examples

Example 1

Order: 1,000 mL over 8 hours by gravity. Tubing drop factor 15 gtt/mL. Drops per minute?

  1. 8 hours is 480 minutes.
  2. 1,000 mL × 15 gtt/mL ÷ 480 min = 31.25.
  3. Round to 31 gtt/min.

Answer: 31 gtt/min

Example 2

An IV is running at 125 mL/hr. The pump fails and you switch to tubing with a 20 gtt/mL drop factor. Drops per minute?

  1. 125 mL/hr × 20 gtt/mL = 2,500 gtt/hr.
  2. 2,500 ÷ 60 min = 41.67.
  3. Round to 42 gtt/min.

Answer: 42 gtt/min

Try five free problems

One exact answer, rounded as asked, no partial credit. No sign-up needed.

Problem 1 of 5No sign-up needed

Order: 1,000 mL of 0.9% sodium chloride IV over 10 hr by gravity. The tubing drop factor is 60 gtt/mL. How many gtt/min will you count?

round to the nearest whole number

gtt/min

Common Mistakes

  • Leaving the time in hours

    Drops are counted per minute. 1,000 × 15 ÷ 8 gives 1,875, which is drops per hour. Divide by 480 minutes instead.

  • Drop factor upside down

    The drop factor multiplies the volume. Dividing by it gives a tiny, wrong answer.

  • Forgetting the drop factor altogether

    Volume over minutes is mL per minute. Multiply by the tubing's gtt/mL to get drops.

Questions Students Ask

What is a microdrip set?

Tubing with a 60 gtt/mL drop factor. With microdrip, gtt/min equals mL/hr, which is a handy check.

Why round to a whole number?

Because you count whole drops in the chamber. 31.25 drops a minute is not something you can set.

Keep practicing in lesson 10: Gravity Drip Rate (gtt/min)

Fresh numbers every time. Ten correct answers finish the lesson. The practice round and the first five problems of lesson 1 are free.

Open lesson 10