# Rate of Flow Calculation in Pharmacy

Rate of flow calculation and administration of continuous intravenous (IV) infusions are performed to ensure accurate delivery of medications administered via the IV route. As many of the medications delivered via continuous IV infusion have potent effects and narrow margins of safety; therefore accuracy in calculation and administration of these agents is more important and crucial for a pharmacist.

The first type of flow rate calculation is commonly known as the rate of administration, or IV rate calculation. It is generally expressed in terms of mL/hour or mL/minute -

mL/hr = vol to be infused (mL)/infusion time (hr)

mL/min = vol to be infused (mL)/infusion time (min)

The second type of flow rate calculation is the drip rate, which is the number of drops per minute delivered in that IV administration set:

gtt/min=drops/min=drips/min

On the other hand,

Drip rate = drop factor × rate of administration

Where,

Drop factor = the number of drops it takes to make up one ml of fluid (gtts)

The IV frequency, or IV schedule, is the time it takes to infuse a specific volume of solution, expressed in hours. It’s calculated by dividing the volume to be infused over the rate of administration.

q (x) hr= vol to be infused (mL)/rate of administration in (mL/hr)

#### Basic Formula of IV Flow Rate Calculation

1. To determine an unknown flow rate:

Flow rate (mL/hr) = Dose (mg/hr or mcg/hr) ÷ Concentration (mg/mL or mcg/mL)

2. To determine an unknown dose:

Dose (mg/hr) or (mcg/hr) = Flow rate (mL/hr) × Concentration (mg/mL or mcg/mL)

3. To determine the concentration of drug in 1 mL of fluid:

Concentration (mg or mcg/mL) = Total amount of drug (mg or mcg) ÷ Total volume of fluid (Ml)

#### Pharmacy Flow Rate Practice Problems

1. When Dose is unknown, diltiazem 125 mg/125 mL D 5 W is infusing at 15 mL/hr. Calculate the Dose.

Step 1: Calculate concentration of drug in 1 mL of fluid:

125 mg/ 125 mL = 1 mg/ 1 mL

Step 2: Enter known factors into the formula and solve:

Dose (mg/hr) or (mcg/hr) = Flow rate (mL/hr) × Concentration (mg/mL or mcg/mL)

Dose = 15 mL/hr × 1 mg/1 mL = 15 mg/hr.

2. A prescription reads as following — Rx: NS (Normal Saline) 1L IV 250 mL/hour. How do we find the IV frequency of a 1L IV bag?

q (x) hr= vol to be infused (mL)/rate of administration in (mL/hr)

q (x) hr = 1,000 (mL) ÷ 250 (mL/hr) = q 4 hr

3. 500 mL using 15 gtt set over 1 hour. Calculate the Drip rate.

Drip rate = drop factor × rate of administration

(500 x 15) / 60 = 125 gtts/min

4. A physician orders 200 mL of 0.9% Sodium Chloride be infused over two hours. You have a 10 gtt IV drip set. How many drops/ minute will you give?

Drip rate = drop factor × rate of administration

(200 x 10) / 120 = 17 gtts/ minute