Weight-Based IV Medication Calculations

Calculating weight-based IV medications is used when a drug dose depends on the patient’s body weight. This is common with high-risk medications and continuous infusions, where accurate dosing is critical to achieve the desired effect while avoiding harm.

What You Need to Know

Weight-based calculations determine how much medication a patient receives based on their weight, usually in kilograms. These calculations are commonly used for IV infusions and are often expressed as mg/kg/hr or mcg/kg/min.

Key points:

• patient weight must be in kilograms

• the prescribed dose is given per kg of body weight

• calculations often involve multiple steps

• the final answer may need to be converted to mL/hr for an infusion pump

A quick check:

• higher patient weight = higher dose

• lower patient weight = lower dose

The Formula

There are two parts to this calculation.

Part 1: Calculate the required dose based on weight
Required dose = Prescribed rate × Weight (kg)

Examples:
mg/hr = (mg/kg/hr) × weight
mcg/min = (mcg/kg/min) × weight

Part 2 (only if needed): Convert to mL/hr using the stock concentration
mL/hr = (Required dose ÷ Stock dose) × Volume

Where:
Required dose = amount calculated from weight
Stock dose = amount of drug available
Volume = mL the stock is contained in

Worked Examples

Example 1
Order: 5 mg/kg/hr
Patient weight: 70 kg

Dose = 5 × 70 = 350 mg/hr

Answer: 350 mg/hr

Example 2
Order: 10 mcg/kg/min
Patient weight: 60 kg

Dose = 10 × 60 = 600 mcg/min

Answer: 600 mcg/min

Example 3 (including mL/hr)
Order: 5 mg/kg/hr
Patient weight: 70 kg
Stock: 500 mg in 50 mL

Step 1: Calculate dose
5 × 70 = 350 mg/hr

Step 2: Convert to mL/hr
350 ÷ 500 × 50 = 35 mL/hr

Answer: 35 mL/hr

Practice Questions

1. Order: 4 mg/kg/hr
   Weight: 80 kg

2. Order: 8 mcg/kg/min
   Weight: 75 kg

3. Order: 6 mg/kg/hr
   Weight: 60 kg
   Stock: 300 mg in 30 mL

4. Order: 2 mg/kg/hr
   Weight: 90 kg
   Stock: 400 mg in 40 mL

5. Order: 12 mcg/kg/min
   Weight: 50 kg

   (Scroll down for answers)

Answers

1. 4 × 80 = 320 → 320 mg/hr

2. 8 × 75 = 600 → 600 mcg/min

3. 6 × 60 = 360 mg/hr
   360 ÷ 300 × 30 = 36 → 36 mL/hr

4. 2 × 90 = 180 mg/hr
   180 ÷ 400 × 40 = 18 → 18 mL/hr

5. 12 × 50 = 600 → 600 mcg/min