What Is Drop Factor in IV Tubing? Gtt/mL Explained With Worked Example

This article is study and exam practice, not for patient care. In real settings, follow the order, your facility policy, and the tubing package.

What drop factor means

Drop factor is the number of drops (gtt) it takes to make 1 mL of fluid in a specific IV tubing set. It is written as gtt/mL. The number is printed on the tubing package, so you read it rather than calculate it.

A set labeled 60 gtt/mL needs 60 drops to deliver 1 mL. A set labeled 15 gtt/mL needs 15 drops to deliver 1 mL. Larger drops mean a smaller drop factor.

Macrodrip vs. microdrip

• Macrodrip sets make large drops. Common factors are 10, 15, and 20 gtt/mL. They suit routine adult infusions and faster rates.
• Microdrip sets make small drops. The factor is 60 gtt/mL. They suit small volumes and slow, precise rates, such as pediatric patients.
• Blood tubing is commonly 10 gtt/mL, though some sets differ. Check the package.

When a question says 60 gtt/mL, it is a microdrip set. The drop rate in gtt/min then matches the mL/hr rate numerically. That shortcut only works for 60 gtt/mL tubing.

The drop rate formula

In words: multiply the volume to infuse in mL by the drop factor in gtt/mL, then divide by the time in minutes. The result is in gtt/min.

Keep the units straight. Time must be in minutes. If the order is in hours, convert to minutes first by multiplying the hours by 60. Round to the nearest whole drop, since you cannot count a fraction of a drop.

Worked example: macrodrip

A gravity infusion of 1000 mL over 8 hours (480 minutes) using 15 gtt/mL tubing:

Volume

1,000 mL

Infusion time

480 minutes

Drop factor

15 gtt/mL

Formula gtt/min = volume × drop factor ÷ minutes.

1. gtt/min = volume × drop factor ÷ minutes.
2. 1,000 mL × 15 gtt/mL ÷ 480 minutes = 31.25 gtt/min.
3. A drop cannot be split, so round to a whole number: 31 gtt/min.

Answer 31 gtt/min

Worked example: microdrip

A small-volume infusion of 50 mL over 60 minutes using 60 gtt/mL tubing:

Volume

50 mL

Infusion time

60 minutes

Drop factor

60 gtt/mL

Formula gtt/min = volume × drop factor ÷ minutes.

1. gtt/min = volume × drop factor ÷ minutes.
2. 50 mL × 60 gtt/mL ÷ 60 minutes = 50 gtt/min.
3. 50 gtt/min is exact, so no rounding is needed.

Answer 50 gtt/min

Notice the drops per minute match the mL per hour. That is the microdrip shortcut.

Worked example: IV piggyback

A 100 mL piggyback over 30 minutes using 20 gtt/mL tubing:

Volume

100 mL

Infusion time

30 minutes

Drop factor

20 gtt/mL

Formula gtt/min = volume × drop factor ÷ minutes.

1. gtt/min = volume × drop factor ÷ minutes.
2. 100 mL × 20 gtt/mL ÷ 30 minutes = 66.67 gtt/min.
3. A drop cannot be split, so round to a whole number: 67 gtt/min.

Answer 67 gtt/min

Common mistakes

• Using hours instead of minutes in the formula.
• Using the wrong drop factor, because the question gives the tubing and you assumed another.
• Forgetting to round to a whole drop.
• Applying the 60 gtt/mL shortcut to a macrodrip set.

Safety note

Gravity infusion rates drift with patient position, tubing height, and vein condition. In practice, an infusion pump is used when the drug or patient calls for it. Follow ISMP guidance and your facility policy.

Quick checklist for exam questions

1. Find the volume in mL.
2. Find the time and convert it to minutes.
3. Find the drop factor on the tubing.
4. Calculate gtt/min and round to a whole number.
5. Sanity check: a larger drop factor should give a faster drop count for the same volume and time.