Dosage Calculations

Accurate dosage calculations are essential for safe medication administration. LPN/VNs must be proficient in conversions, basic calculations, and pediatric/weight-based dosing.

Systems of Measurement

Metric System (Most Common):

Household Measurements:

Common Conversions:

• 1 g = 1,000 mg
• 1 mg = 1,000 mcg
• 1 kg = 2.2 lb
• 1 L = 1,000 mL

Basic Dosage Formula

Formula Method:

Dose to Give = (Desired Dose / Available Dose) × Quantity

Example: Order is for morphine 4 mg IV. Available: morphine 10 mg/mL

Dose = (4 mg / 10 mg) × 1 mL = 0.4 mL

Ratio-Proportion Method:

Available Dose : Quantity = Desired Dose : Unknown (X)

Example: Order: Amoxicillin 500 mg. Available: 250 mg/5 mL

250 mg : 5 mL = 500 mg : X mL
250X = 2,500
X = 10 mL

Weight-Based Dosing

Many medications, especially for children and certain adult medications, are dosed by weight.

Steps for Weight-Based Calculations:

1. Convert weight to kilograms if needed (lb ÷ 2.2 = kg)
2. Multiply weight by ordered dose per kg
3. Calculate volume to administer

Example: Patient weighs 154 lb. Order: Gentamicin 5 mg/kg/day IV divided q8h

154 lb ÷ 2.2 = 70 kg
70 kg × 5 mg/kg = 350 mg/day
350 mg ÷ 3 doses = 116.7 mg per dose (round per facility policy)

IV Flow Rate Calculations

Drops per Minute (gtt/min):

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

Drop Factors:

Example: Infuse 1,000 mL NS over 8 hours. Drop factor: 15 gtt/mL

gtt/min = (1,000 mL × 15 gtt/mL) / (8 hrs × 60 min)
gtt/min = 15,000 / 480 = 31.25 = 31 gtt/min

mL per Hour:

mL/hr = Total Volume / Time in hours

Example: Infuse 1,000 mL over 8 hours

mL/hr = 1,000 mL / 8 hr = 125 mL/hr

Reconstitution

Some medications require reconstitution (adding diluent to powder).

Steps:

1. Read package insert for diluent type and amount
2. Add diluent slowly, rotate gently (don't shake unless directed)
3. Note the resulting concentration
4. Label with date, time, concentration, and initials
5. Calculate dose based on final concentration

Example: Ampicillin 500 mg vial. Add 1.8 mL sterile water = 250 mg/mL. Order: 375 mg

375 mg / (250 mg/mL) = 1.5 mL to administer

Safe Dose Range

Before administering, verify the ordered dose falls within the safe range:

Steps:

1. Find safe dose range in drug reference (mg/kg/day or mg/day)
2. Calculate minimum and maximum safe dose for patient
3. Compare ordered dose to safe range
4. If outside range, hold medication and notify prescriber

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

Minimum: 20 kg × 25 mg/kg = 500 mg/day
Maximum: 20 kg × 50 mg/kg = 1,000 mg/day

If ordered dose is 400 mg TID (1,200 mg/day), this exceeds safe maximum—hold and verify.

Calculation Tips

Always:

• Use dimensional analysis or a consistent method
• Double-check calculations, especially for high-alert meds
• Round appropriately (usually to nearest tenth for mL)
• Include units in every step
• If answer seems unreasonable, recalculate

Common Sense Checks:

• Oral liquid doses rarely exceed 30 mL
• Subcutaneous injections rarely exceed 1 mL
• IM injections rarely exceed 3 mL
• Tablets: If calculation yields > 3 tablets, verify the dose