Drip Rates

Overview

When an electronic infusion pump is not available, IV fluids are administered by gravity using a drip set. In this case the flow rate is controlled manually by adjusting a roller clamp and counting drops per minute (gtt/min).

Drop Factor

The drop factor is the number of drops per milliliter produced by a specific IV tubing set. It is printed on the tubing package and must be verified before calculating.

Tubing Type	Drop Factor
Macrodrip — standard	10 gtt/mL
Macrodrip — standard	15 gtt/mL
Macrodrip — standard	20 gtt/mL
Microdrip	60 gtt/mL

Which Tubing to Use

• Macrodrip (10-20 gtt/mL) — standard fluid replacement, blood products, high volume infusions
• Microdrip (60 gtt/mL) — pediatric patients, precise low-volume infusions, critical medications

The Drip Rate Formula

\[\text{Drip Rate (gtt/min)} = \frac{\text{Volume (mL)}}{\text{Time (min)}} \times \text{Drop Factor (gtt/mL)}\]

Or using an hourly rate:

\[\text{Drip Rate (gtt/min)} = \frac{\text{Rate (mL/hr)} \times \text{Drop Factor (gtt/mL)}}{60 \text{ min/hr}}\]

Rounding Drip Rates

Always Round to Whole Number

Drip rates are always rounded to the nearest whole number — you cannot count a fraction of a drop.

Calculations from Volume and Time

Example 1: Order: 1000 mL over 8 hours Tubing: 20 gtt/mL What is the drip rate in gtt/min?

Convert time to minutes: [8 \text{ hr} \times 60 \text{ min/hr} = 480 \text{ min}]

Calculate drip rate: [\frac{1000 \text{ mL}}{480 \text{ min}} \times 20 \text{ gtt/mL} = 41.7 \text{ gtt/min}]

Round to: 42 gtt/min

Example 2: Order: 500 mL over 4 hours Tubing: 15 gtt/mL

\[4 \text{ hr} \times 60 = 240 \text{ min}\]

\[\frac{500 \text{ mL}}{240 \text{ min}} \times 15 \text{ gtt/mL} = 31.25 \text{ gtt/min}\]

Round to: 31 gtt/min

Calculations from Hourly Rate

Example 3: Rate: 125 mL/hr Tubing: 20 gtt/mL

\[\frac{125 \text{ mL/hr} \times 20 \text{ gtt/mL}}{60 \text{ min/hr}} = \frac{2500}{60} = 41.7 \text{ gtt/min}\]

Round to: 42 gtt/min

Example 4: Rate: 50 mL/hr Tubing: 60 gtt/mL (microdrip)

\[\frac{50 \text{ mL/hr} \times 60 \text{ gtt/mL}}{60 \text{ min/hr}} = \frac{3000}{60} = 50 \text{ gtt/min}\]

Microdrip Shortcut

With microdrip tubing (60 gtt/mL) the drip rate in gtt/min always equals the flow rate in mL/hr.

\[\frac{\text{mL/hr} \times 60}{60} = \text{mL/hr}\]

50 mL/hr with microdrip = 50 gtt/min always.

Counting Drops

How to Count Drip Rate

1. Use a watch with a second hand or timer
2. Count drops falling in the drip chamber for 60 seconds for most accurate count
3. Alternatively count for 15 seconds and multiply by 4, or 30 seconds and multiply by 2
4. Adjust the roller clamp to increase or decrease the rate
5. Recount after each adjustment
6. Recheck every 30-60 minutes as gravity rates can drift

Factors Affecting Gravity Drip Rates

Gravity Drip Rates Are Unreliable

Gravity drip rates can be affected by:

• Patient position and arm movement
• Height of the IV bag above the patient
• Tubing kinks or partial occlusions
• Vein pressure changes

For this reason electronic pumps are preferred whenever precision is required. Gravity sets are used when pumps are unavailable or for simple fluid replacement in stable patients.

Practice Problems

Problem 1

Order: 1000 mL 0.9% NaCl over 10 hours Tubing: 20 gtt/mL What is the drip rate?

Answer

Convert time: [10 \times 60 = 600 \text{ min}]

Calculate: [\frac{1000 \text{ mL}}{600 \text{ min}} \times 20 \text{ gtt/mL} = 33.3 \text{ gtt/min}]

Round to: 33 gtt/min

Problem 2

Order: 500 mL D5W over 6 hours Tubing: 15 gtt/mL What is the drip rate?

Answer

Convert time: [6 \times 60 = 360 \text{ min}]

Calculate: [\frac{500 \text{ mL}}{360 \text{ min}} \times 15 \text{ gtt/mL} = 20.8 \text{ gtt/min}]

Round to: 21 gtt/min

Problem 3

Rate: 80 mL/hr Tubing: 20 gtt/mL What is the drip rate?

Answer

\[\frac{80 \times 20}{60} = \frac{1600}{60} = 26.7 \text{ gtt/min}\]

Round to: 27 gtt/min

Problem 4

Rate: 30 mL/hr Tubing: 60 gtt/mL (microdrip) What is the drip rate?

Answer

Using microdrip shortcut:

30 gtt/min

Verify: [\frac{30 \times 60}{60} = 30 \text{ gtt/min} ✅]

Problem 5

Order: 250 mL over 2 hours Tubing: 10 gtt/mL What is the drip rate?

Answer

Convert time: [2 \times 60 = 120 \text{ min}]

Calculate: [\frac{250 \text{ mL}}{120 \text{ min}} \times 10 \text{ gtt/mL} = 20.8 \text{ gtt/min}]

Round to: 21 gtt/min

Problem 6

Order: 1000 mL Lactated Ringer's over 12 hours Tubing: 15 gtt/mL What is the drip rate?

Answer

Convert time: [12 \times 60 = 720 \text{ min}]

Calculate: [\frac{1000 \text{ mL}}{720 \text{ min}} \times 15 \text{ gtt/mL} = 20.8 \text{ gtt/min}]

Round to: 21 gtt/min

Clinical Tip

Always label the IV bag with the start time and expected completion time. Mark the bag with hourly volume checkpoints so you can verify the infusion is running on schedule at a glance without recalculating every time.