Dosage Calculation (Desired/Have)
IV Flow Rate (Drops Per Minute)
Weight-Based Dose
Comprehensive Guide to Drug Calculations for Nursing and Pharmacy
Accurate drug calculations are the foundation of patient safety in clinical settings. Whether you are a nursing student, a practicing RN, or a pharmacist, mastering these formulas ensures that patients receive the correct therapeutic levels of medication without the risk of toxicity or under-dosing.
The Basic Formula: Desired Over Have
The "Desired over Have" method (D/H x Q) is the most common way to calculate oral and injectable medication dosages. It involves three key components:
- Desired Dose (D): The amount of medication ordered by the physician.
- Dose on Hand (H): The concentration or strength of the medication available.
- Quantity (Q): The volume (mL) or form (tablets/capsules) in which the dose on hand is supplied.
Example: A doctor orders 500 mg of Amoxicillin. The pharmacy provides 250 mg / 5 mL suspension.
Calculation: (500 / 250) × 5 = 10 mL.
IV Flow Rate Calculations
Intravenous therapy requires calculating the speed at which fluid enters the patient's bloodstream. This is measured in drops per minute (gtt/min) when using manual gravity sets, or mL/hr when using infusion pumps.
| Type of Set | Drop Factor (gtt/mL) |
|---|---|
| Macro-drip | 10, 15, or 20 gtt/mL |
| Micro-drip | 60 gtt/mL |
Weight-Based Dosing
Pediatric and high-alert medications (like Heparin or Dopamine) are often calculated based on the patient's body weight in kilograms. It is critical to ensure the weight is converted from pounds to kilograms (1 kg = 2.2 lbs) before starting the calculation.
Safety Tips for Medication Administration
- The Six Rights: Always verify Right Patient, Right Drug, Right Dose, Right Route, Right Time, and Right Documentation.
- Double Check: For high-alert medications like insulin or heparin, always have a second clinician verify your math.
- Zero Usage: Always use a leading zero (0.5 mg) and never use a trailing zero (5.0 mg), as this prevents decimal point errors.