# How do you calculate drops per minute mL per hour?

Calculating the rate of intravenous (IV) fluid delivery in drops per minute or mL per hour is an essential nursing skill. IV fluids provide hydration, electrolytes, and medications to patients. Being able to accurately calculate and regulate flow rates ensures patients receive fluids safely and effectively. This article will explain how to calculate drops per minute, mL per hour, and provide example calculations and tables to help visualize the data.

## What is a Drop Factor?

The drop factor is the number of drops contained in 1 mL of solution from a particular IV administration set. Drop factors can vary based on the size and material of the IV tubing. Common drop factors are 10, 15, or 20 drops per 1 mL. The drop factor must be known in order to calculate drops per minute or mL per hour. The drop factor is determined by the manufacturer of the IV tubing and is commonly printed on the administration set packaging or on the drip chamber. If the drop factor is not labeled, it must be measured and calculated before setting an IV flow rate.

### How to determine the drop factor:

1. Fill the drip chamber to the fill line with IV solution.

2. Start the flow and count the number of drops that fall in 1 minute.

3. Catch the fluid from the tubing in a graduated cylinder or syringe for the full minute.

4. The amount of fluid collected in mL is the drip factor. For example, if 15 drops were counted and 1 mL was collected, the drop factor would be 15 drops/mL.

## Calculating Drops per Minute

The basic formula for calculating drops per minute is:

Drops per minute = Flow rate (mL/hr) x Drop factor

Example:
Order is for 125 mL/hr of IV fluids
Drop factor of tubing is 15 drops/mL

Drops per minute = Flow rate x Drop factor
= 125 mL/hr x 15 drops/mL
= 1,875 drops/minute

The flow rate would be set to deliver 1,875 drops/minute to achieve 125 mL/hr.

### Steps to calculate drops per minute:

1. Identify the ordered flow rate in mL/hr

2. Identify the drop factor

3. Multiply the flow rate by the drop factor

4. The result is the flow rate in drops per minute

## Calculating mL per Hour

The formula for calculating mL per hour from a drops per minute rate is:

mL/hr = Drops per minute / Drop factor

Example:
Current IV infusion is set at 75 drops/minute
Drop factor is 15 drops/mL

mL/hr = Drops per minute / Drop factor
= 75 drops/min / 15 drops/mL
= 5 mL/hr

Therefore, the rate of 75 drops/minute = 5 mL/hr

### Steps to calculate mL per hour:

1. Identify the IV flow rate set in drops per minute

2. Identify the drop factor

3. Divide the drops per minute by the drop factor

4. The result is the flow rate in mL per hour

## IV Flow Rates by Weight-Based Calculation

For some IV fluids, the flow rate is calculated based on the patient’s weight. This helps ensure appropriate and safe dosing.

The steps are:

1. Identify the fluid and dose in mL/kg/hr (amount per kg of body weight per hour)

2. Obtain the patient’s weight in kg

3. Multiply the dose by the patient’s weight to get the flow rate in mL/hr

4. Use the drop factor to calculate the flow rate in drops/minute

Example:
Order is 0.9% normal saline at 125 mL/kg/hr
Patient weighs 60 kg
Drop factor is 15 drops/mL

1. Dose is 125 mL/kg/hr
2. Patient’s weight is 60 kg
3. 125 mL/kg/hr x 60 kg = 7,500 mL/hr
4. Using drop factor, 7,500 mL/hr x 15 drops/mL = 112,500 drops/minute

Therefore, the flow rate would be set at 112,500 drops/minute to deliver the fluid at 125 mL/kg/hr for this patient.

## Practice Examples

### Example 1

Order: D5W at 75 mL/hr
Drop factor: 10 drops/mL

Calculate the flow rate in drops/minute:

Steps:
1. Flow rate = 75 mL/hr
2. Drop factor = 10 drops/mL
3. Drops/min = Flow rate x Drop factor
= 75 mL/hr x 10 drops/mL
= 750 drops/minute

### Example 2

Current rate set at: 45 drops/minute
Drop factor: 15 drops/mL

Calculate the flow rate in mL/hr:

Steps:
1. Drops/min = 45 drops/minute
2. Drop factor = 15 drops/mL
3. mL/hr = Drops/min / Drop factor
= 45 drops/min / 15 drops/mL
= 3 mL/hr

## IV Flow Rate By Gravity Infusion

IV fluids can also be administered by gravity, using the height of an IV bag hung from a pole to control the flow. The flow rate by gravity is regulated by:

– Height of the IV bag above the insertion site
– Tubing drop factor
– Fluid viscosity

Flow rates can be calculated using the pressure generated by the height of the fluid column. Gravity flow rates are less precise than infusion pumps and depend on external factors like temperature.

General guidelines:

Height of IV Bag Approximate Flow Rate
3 feet above insertion site 120 mL/hr
2 feet above insertion site 80 mL/hr
1 foot above insertion site 20 mL/hr

To achieve faster flow rates, elevate the bag higher above the patient’s heart. Lower the bag closer to the insertion site to decrease the flow rate. Monitor the flow closely using the drip factor and drops per minute to prevent too rapid or too slow infusion.

## Regulating and Monitoring the Flow Rate

Close monitoring of the IV flow rate is crucial for safe administration. This involves:

– Clear labeling of the IV tubing with the drop factor

– Frequent checking of the flow rate in drops/minute to ensure it aligns with the prescribed mL/hr

– Adjusting the roller clamp on the tubing as needed to achieve the correct flow rate

– Monitoring for any changes or interruptions in flow

– Monitoring the patient for therapeutic effectiveness and side effects

The IV pump or flow should be checked hourly at a minimum, including the pump settings, connections, and insertion site. For high risk medications, more frequent monitoring is indicated. Prompt action must be taken for any flow irregularities, following institutional protocols. Accurate documentation is essential.

## Special Considerations

Certain IV medications may have recommended rates for infusion. The manufacturer guidelines should be consulted. For example, some medications require infusion over a minimum time period or at a set rate.

Some patients require fluid restrictions or exact intake and output measurement, requiring more precise control of IV flow. Tube feeding may run simultaneously with IV fluids and the rates must be coordinated.

Pediatric patients and infants require weight-based IV calculation and use of pumps with pediatric settings. Neonates have very small fluid allowances and need expert guidance.

At times, an IV may be put on a “keep open” rate. This is typically 10-25 mL/hr just to keep the line open between doses or infusions. The flow rate should always be set according to protocol.

## Conclusion

Calculating IV flow rates precisely is a fundamental nursing skill that ensures patients receive appropriate fluid volume and medication doses. Determining the drops per minute or mL per hour requires knowing the prescribing order, tubing drop factor, and using simple math calculations. Minute-to-minute monitoring, vigilant assessment, and meticulous documentation are crucial when administering intravenous fluids. Consistent practice with calculation, along with a sound knowledge of intravenous therapy principles, will help nurses perfect this vital component of patient care.