Weight-Based Dosing¶
Many medications are ordered based on the patient's body weight. This ensures the dose is proportional to the patient's size, minimizing the risk of under- or overdosing.
Weight-based orders are expressed as a dose per kilogram:
Getting the Weight Right¶
Always use weight in kg
Clinical doses are calculated using weight in kg. If the patient's weight is recorded in lb, convert first:
Never estimate weight for drug calculations — always use a measured, documented weight.
Pediatric weight verification
For pediatric patients, always verify the weight independently before calculating. A weight error in a child can result in a significant overdose.
Two-Step Method¶
Weight-based calculations always have two steps:
- Calculate the dose — multiply weight in kg by the ordered dose per kg
- Calculate the volume — use the dose from step 1 and the stock ratio
Example 1: Order: gentamicin 5 mg/kg IV Patient weight: 70 kg Stock: 40 mg/mL
Step 1 — dose: [70 \cancel{\text{ kg}} \times \frac{5 \text{ mg}}{1 \cancel{\text{ kg}}} = 350 \text{ mg}]
Step 2 — volume: [350 \cancel{\text{ mg}} \times \frac{1 \text{ mL}}{40 \cancel{\text{ mg}}} = 8.75 \text{ mL}]
Round to nearest tenth: 8.8 mL
Single Chain Method¶
The same problem as one cancellation chain:
Round to nearest tenth: 8.8 mL
Weight Conversion Required¶
Example 2: Order: vancomycin 15 mg/kg IV Patient weight: 154 lb Stock: 500 mg/10 mL
As a single chain:
Maximum Dose Checks¶
Some weight-based medications have a maximum dose — the calculated dose should not exceed it regardless of weight.
When a maximum applies
- Calculate the weight-based dose normally
- Compare to the maximum dose
- Administer the lower of the two values
Example 3: Order: ibuprofen 10 mg/kg orally, max 400 mg Patient weight: 55 kg
Calculated dose: [55 \cancel{\text{ kg}} \times \frac{10 \text{ mg}}{1 \cancel{\text{ kg}}} = 550 \text{ mg}]
550 mg exceeds the maximum of 400 mg. Administer 400 mg.
Daily Dose vs Single Dose¶
Read the order carefully
Weight-based orders may specify:
- Per dose — e.g. 5 mg/kg per dose every 8 hours
- Per day — e.g. 15 mg/kg/day divided every 8 hours
For a daily dose divided into multiple doses, divide the total daily dose by the number of doses per day to get the single dose.
Example 4: Order: amoxicillin 40 mg/kg/day orally divided every 8 hours Patient weight: 20 kg Stock: 250 mg/5 mL
Step 1 — total daily dose: [20 \cancel{\text{ kg}} \times \frac{40 \text{ mg}}{1 \cancel{\text{ kg}}} = 800 \text{ mg/day}]
Step 2 — single dose (every 8 hours = 3 doses/day): [800 \text{ mg/day} \times \frac{1 \text{ day}}{3 \text{ doses}} = 266.7 \text{ mg/dose}]
Step 3 — volume per dose: [266.7 \cancel{\text{ mg}} \times \frac{5 \text{ mL}}{250 \cancel{\text{ mg}}} = 5.3 \text{ mL}]
Practice Problems¶
Problem 1
Order: tobramycin 2 mg/kg IV Patient weight: 80 kg Stock: 10 mg/mL How many mL?
Answer
Problem 2
Order: gentamicin 4 mg/kg IV Patient weight: 176 lb Stock: 40 mg/mL How many mL?
Answer
Problem 3
Order: ibuprofen 10 mg/kg orally, max 600 mg Patient weight: 75 kg Stock: 200 mg/5 mL How many mL?
Answer
Calculated dose: [75 \cancel{\text{ kg}} \times \frac{10 \text{ mg}}{1 \cancel{\text{ kg}}} = 750 \text{ mg}]
750 mg exceeds max of 600 mg — use 600 mg
Volume: [600 \cancel{\text{ mg}} \times \frac{5 \text{ mL}}{200 \cancel{\text{ mg}}} = 15 \text{ mL}]
Problem 4
Order: vancomycin 20 mg/kg/day IV divided every 12 hours Patient weight: 60 kg Stock: 500 mg/10 mL How many mL per dose?
Answer
Total daily dose: [60 \cancel{\text{ kg}} \times \frac{20 \text{ mg}}{1 \cancel{\text{ kg}}} = 1200 \text{ mg/day}]
Single dose (every 12 hours = 2 doses/day): [1200 \text{ mg/day} \times \frac{1 \text{ day}}{2 \text{ doses}} = 600 \text{ mg/dose}]
Volume: [600 \cancel{\text{ mg}} \times \frac{10 \text{ mL}}{500 \cancel{\text{ mg}}} = 12 \text{ mL}]
Problem 5
Order: morphine 0.1 mg/kg IV PRN Patient weight: 132 lb Stock: 10 mg/mL How many mL?
Answer
Document the weight used
Record the patient's weight used for the calculation in your nursing notes. If the weight changes significantly — such as after surgery or with fluid shifts — notify the prescriber as the dose may need to be recalculated.