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Drug Calculations Study Sheet

Essential Dosage Formulas - With Full Explanations

📄 6 Pages 💊 Pharmacology 📚 NCLEX-Ready

BASIC DOSAGE CALCULATIONS

These are the foundation formulas you'll use constantly in clinical practice. Master these first - they're the building blocks for all medication administration.

Desired Over Have Formula

Dose = (D / H) × Q

This is the most important formula in nursing math. D = Desired dose (what the doctor ordered), H = Have on hand (concentration available), Q = Quantity (volume or units per dose available). Think of it as: "I want THIS much, I have THAT much, so I need to give THIS many."

D — Desired dose (ordered)

H — Have on hand (available)

Q — Quantity (volume/units)

⚠️ ALWAYS check that D and H are in the SAME UNITS before calculating! Convert mg to mg, mcg to mcg first.

Example: Order: Amoxicillin 500mg. Available: 250mg/5mL. Dose = (500/250) × 5mL = 10mL

Dimensional Analysis

Start Unit × (Conversion Factors) = End Unit

Set up your calculation so units cancel out, leaving only the unit you want. This method is self-checking - if units don't cancel properly, you've set it up wrong. Many nurses prefer this because it catches errors automatically.

💡 Write out ALL units and cross them out as they cancel. If the final unit isn't what you need, flip a fraction!

IV DRIP RATE CALCULATIONS

IV calculations are critical in acute care settings. Getting these wrong can cause serious harm. Always double-check your work and use an IV pump when available.

mL per Hour (IV Pump)

mL/hr = Total Volume (mL) / Time (hours)

For IV pumps, you just need to know how many mL per hour to program. This is the simplest IV calculation - total volume divided by total time. Modern pumps do the rest.

Example: Give 1000mL NS over 8 hours. 1000mL / 8hr = 125 mL/hr

Drops per Minute (Manual/Gravity)

gtt/min = (Volume × Drop Factor) / (Time in min)

For gravity drips without a pump, you count drops. The drop factor (gtt/mL) is printed on the IV tubing package. Common factors: 10, 15, 20 gtt/mL (macrodrip) or 60 gtt/mL (microdrip).

gtt — Drops (from Latin "guttae")

Drop Factor — gtt/mL from tubing

⚠️ ALWAYS verify the drop factor on YOUR tubing package. Using the wrong drop factor is a common error!

WEIGHT-BASED DOSING

Many medications, especially in pediatrics and critical care, are dosed by patient weight. These calculations require extra care - a decimal error can be life-threatening.

mg/kg Dosing

Dose = Weight (kg) × Dose (mg/kg)

First convert patient weight to kg if needed (lb ÷ 2.2 = kg). Then multiply by the ordered dose per kg. This gives you the total dose for this specific patient.

💡 Memory trick: 1 kg ≈ 2.2 lb. A 110 lb patient weighs 50 kg. A 220 lb patient weighs 100 kg.

Example: Order: Vancomycin 15mg/kg. Patient: 70kg. 70kg × 15mg/kg = 1050mg

Safe Dose Range Check

Min Dose = Weight × Min mg/kg/day
Max Dose = Weight × Max mg/kg/day

ALWAYS check if the ordered dose falls within the safe range. Drug references give you min and max mg/kg/day. Calculate both and verify the order is between them. If the order is outside the range, CLARIFY before giving!

⚠️ This is a critical safety check, especially in pediatrics. Never skip it! You are the last line of defense before the medication reaches the patient.

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