Weight-Based Dosing¶
Overview¶
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:
The Two-Step Process¶
Weight-based calculations always require two steps:
- Calculate the dose — multiply weight in kg by the ordered dose per kg
- Calculate the volume — use the dose from step 1 to find how much to administer
These can be done separately or as a single unit cancellation chain.
Getting the Weight Right¶
Always Weigh 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¶
Example 1: Order: gentamicin 5 mg/kg IV Patient weight: 70 kg Stock: 40 mg/mL
Step 1 — calculate dose: [70 \cancel{\text{ kg}} \times \frac{5 \text{ mg}}{1 \cancel{\text{ kg}}} = 350 \text{ mg}]
Step 2 — calculate volume: [\frac{350 \cancel{\text{ mg}}}{1} \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 unit 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
Step 1 — convert weight: [154 \cancel{\text{ lb}} \times \frac{1 \text{ kg}}{2.2 \cancel{\text{ lb}}} = 70 \text{ kg}]
Step 2 — calculate dose: [70 \cancel{\text{ kg}} \times \frac{15 \text{ mg}}{1 \cancel{\text{ kg}}} = 1050 \text{ mg}]
Step 3 — calculate volume: [\frac{1050 \cancel{\text{ mg}}}{1} \times \frac{10 \text{ mL}}{500 \cancel{\text{ mg}}} = 21 \text{ mL}]
Or as one chain: [\frac{154 \cancel{\text{ lb}}}{1} \times \frac{1 \cancel{\text{ kg}}}{2.2 \cancel{\text{ lb}}} \times \frac{15 \cancel{\text{ mg}}}{1 \cancel{\text{ kg}}} \times \frac{10 \text{ mL}}{500 \cancel{\text{ mg}}} = 21 \text{ mL}]
Maximum Dose Checks¶
Some weight-based medications have a maximum dose — the dose should not exceed this regardless of weight.
Maximum Dose
When a maximum dose applies:
- Calculate the weight-based dose normally
- Compare to the maximum dose
- Administer the lower of the two values
Example: 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: [\text{single dose} = \frac{\text{total daily dose}}{\text{number of doses per day}}]
Example: 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} \div 3 = 266.7 \text{ mg per dose}]
Step 3 — volume per dose: [\frac{266.7 \cancel{\text{ mg}}}{1} \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
Step 1 — convert weight: [176 \cancel{\text{ lb}} \times \frac{1 \text{ kg}}{2.2 \cancel{\text{ lb}}} = 80 \text{ kg}]
Step 2 — full chain: [\frac{80 \cancel{\text{ kg}}}{1} \times \frac{4 \cancel{\text{ mg}}}{1 \cancel{\text{ kg}}} \times \frac{1 \text{ mL}}{40 \cancel{\text{ mg}}} = 8 \text{ mL}]
Problem 3
Order: ibuprofen 10 mg/kg orally, max 600 mg Patient weight: 75 kg Stock: 200 mg/5 mL How many mL?
Answer
Step 1 — 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
Step 2 — volume: [\frac{600 \cancel{\text{ mg}}}{1} \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
Step 1 — total daily dose: [60 \cancel{\text{ kg}} \times \frac{20 \text{ mg}}{1 \cancel{\text{ kg}}} = 1200 \text{ mg/day}]
Step 2 — single dose (every 12 hours = 2 doses/day): [1200 \div 2 = 600 \text{ mg per dose}]
Step 3 — volume: [\frac{600 \cancel{\text{ mg}}}{1} \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
Step 1 — convert weight: [132 \cancel{\text{ lb}} \times \frac{1 \text{ kg}}{2.2 \cancel{\text{ lb}}} = 60 \text{ kg}]
Step 2 — full chain: [\frac{60 \cancel{\text{ kg}}}{1} \times \frac{0.1 \cancel{\text{ mg}}}{1 \cancel{\text{ kg}}} \times \frac{1 \text{ mL}}{10 \cancel{\text{ mg}}} = 0.6 \text{ mL}]
Clinical Tip
For weight-based medications, document 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.