7.2 Medication Calculations

Accurate medication calculations are fundamental to safe nursing practice. The NCLEX tests your ability to calculate correct doses, IV rates, and weight-based dosing without a calculator for some items.

Essential Formulas

Dosage Calculation (Basic Formula):

Desired Dose
─────────────  ×  Quantity  =  Amount to Give
Have (Available)

Example: Order: Morphine 4 mg IV. Available: Morphine 10 mg/mL

• 4 mg ÷ 10 mg × 1 mL = 0.4 mL

IV Drip Rate Calculations

Formula for Gravity Infusion (drops per minute):

Total Volume (mL) × Drop Factor (gtt/mL)
───────────────────────────────────────── = gtt/min
          Time in Minutes

Common Drop Factors:

Example: Infuse 1000 mL NS over 8 hours using tubing with 15 gtt/mL drop factor

• 1000 mL × 15 gtt/mL ÷ 480 minutes = 31.25 → 31 gtt/min

IV Pump Rate Calculations

Formula for mL/hour:

Total Volume (mL)
───────────────── = mL/hour
  Time (hours)

Example: Infuse 500 mL over 4 hours

• 500 mL ÷ 4 hours = 125 mL/hour

Weight-Based Dosing

Many medications, especially in pediatrics and critical care, are dosed by weight.

Step 1: Convert weight to kilograms

• Weight in kg = Weight in lb ÷ 2.2

Step 2: Calculate the dose

• Dose = Patient weight (kg) × Dose per kg

Example: Order: Vancomycin 15 mg/kg IV. Patient weighs 176 lb.

• 176 ÷ 2.2 = 80 kg
• 80 kg × 15 mg/kg = 1200 mg

Safe Dose Range Verification

For pediatric patients, always verify the ordered dose falls within the safe range:

1. Calculate the minimum safe dose (low end × weight)
2. Calculate the maximum safe dose (high end × weight)
3. Compare the ordered dose to this range
4. If outside range, clarify with prescriber BEFORE administration

Example: Safe range for amoxicillin: 25-50 mg/kg/day. Child weighs 20 kg.

• Minimum: 25 mg × 20 kg = 500 mg/day
• Maximum: 50 mg × 20 kg = 1000 mg/day
• Order for 800 mg/day is within safe range

Dimensional Analysis Method

Dimensional analysis uses unit cancellation to solve problems systematically:

Given × Conversion Factor(s) × Conversion Factor(s) = Answer
       (Set up so units cancel)

Example: Give Heparin 18 units/kg/hour. Patient weighs 70 kg. Available: Heparin 25,000 units/500 mL.

Step 1: Calculate units per hour

• 18 units/kg/hour × 70 kg = 1260 units/hour

Step 2: Calculate mL/hour

• 1260 units/hour × (500 mL/25,000 units) = 25.2 mL/hour

Critical Conversion Factors

Insulin Calculations

Insulin is measured in units and drawn up in insulin syringes:

Sliding Scale Example:

• Blood glucose 250 mg/dL
• Sliding scale: Give 2 units for every 50 mg/dL over 150
• (250 - 150) ÷ 50 = 2 increments
• 2 increments × 2 units = 4 units regular insulin

Rounding Rules

On the Exam

• Show your work systematically
• Double-check unit conversions
• Always verify answer makes clinical sense
• If a dose seems unusually large or small, recalculate