Weight-Based Dosing

Many medications are dosed based on the patient’s weight. This is especially critical in pediatrics, where a dose that is safe for a 30 kg child could be dangerous for a 10 kg toddler. Weight-based dosing ensures that each patient receives an amount of medication proportional to their body size.

When Nurses Use Weight-Based Dosing

• Pediatric patients — children are almost always dosed by weight
• Critical care medications — vasopressors, sedatives, and paralytics are often ordered in mcg/kg/min (notable exceptions: vasopressin is dosed in units/min, phenylephrine in mcg/min without weight-based adjustment, and dexmedetomidine in mcg/kg/hr)
• Antibiotics — many antibiotics (Gentamicin, Vancomycin, Amoxicillin) are dosed per kg
• Anticoagulants — Heparin IV boluses are weight-based (units/kg), continuous IV infusions are dosed in units/kg/hr, while subcutaneous prophylactic heparin is typically a fixed dose (e.g., 5,000 units) regardless of weight
• Chemotherapy — often dosed by body surface area (BSA) or weight

Step 1: Convert Pounds to Kilograms

In the United States, patients’ weights are often recorded in pounds (lb). Most drug references give doses in mg/kg, so you must convert first.

$\text{Weight (kg)} = \frac{\text{Weight (lb)}}{2.2}$

Example: Converting Patient Weight

A child weighs 44 lb.

$\text{Weight} = \frac{44}{2.2} = 20 \text{ kg}$

Tip: Always round weight to a reasonable precision. For pediatric patients, most facilities round to one decimal place (e.g., 15.5 kg). For neonates and infants under 10 kg, carry weight to the hundredths or even thousandths place (e.g., 3.275 kg) — small rounding errors at these low weights can cause significant dosing differences.

Step 2: Calculate mg/kg/dose

When an order says “give X mg/kg,” multiply the dose per kg by the patient’s weight:

$\text{Single Dose (mg)} = \text{Dose (mg/kg)} \times \text{Weight (kg)}$

Example 1: Single-Dose Calculation — Ibuprofen

Order: Ibuprofen 10 mg/kg PO for fever. Patient weighs 25 kg.

$\text{Dose} = 10 \text{ mg/kg} \times 25 \text{ kg} = 250 \text{ mg}$

Answer: Administer 250 mg of Ibuprofen PO.

Step 3: Calculate mg/kg/day (Divided Doses)

Many medications are ordered as a total daily dose in mg/kg/day, then divided into individual doses given at regular intervals.

$\text{Total Daily Dose (mg)} = \text{Dose (mg/kg/day)} \times \text{Weight (kg)}$

$\text{Individual Dose (mg)} = \frac{\text{Total Daily Dose}}{\text{Number of doses per day}}$

Example 2: Divided Daily Dose — Amoxicillin

Order: Amoxicillin 25 mg/kg/day PO divided q8h. Patient is a child weighing 20 kg.

Step 1: Calculate total daily dose.

$\text{Daily dose} = 25 \text{ mg/kg/day} \times 20 \text{ kg} = 500 \text{ mg/day}$

Step 2: Divide into individual doses. Since q8h means every 8 hours, there are 3 doses per day ($\frac{24}{8} = 3$).

$\text{Individual dose} = \frac{500 \text{ mg}}{3} \approx 166.7 \text{ mg per dose}$

Answer: Administer approximately 167 mg of Amoxicillin PO every 8 hours.

Example 3: Weight Conversion Plus Dosing — Cephalexin

Order: Cephalexin 25 mg/kg/day PO divided BID. Patient is a child weighing 55 lb.

Step 1: Convert pounds to kg.

$\text{Weight} = \frac{55}{2.2} = 25 \text{ kg}$

Step 2: Calculate total daily dose.

$\text{Daily dose} = 25 \text{ mg/kg/day} \times 25 \text{ kg} = 625 \text{ mg/day}$

Step 3: Divide into individual doses. BID means twice daily.

$\text{Individual dose} = \frac{625}{2} = 312.5 \text{ mg per dose}$

Answer: Administer 312.5 mg of Cephalexin PO twice daily.

Frequency Reference

Note: TID (three times a day) and q8h (every 8 hours) both result in 3 doses per day, but the timing differs. TID is typically given during waking hours (e.g., 0800, 1400, 2000), while q8h is given around the clock (e.g., 0600, 1400, 2200). Always follow your facility’s administration schedule.

Complete Workflow

For every weight-based dosing problem, follow these steps:

1. Convert weight to kg if given in pounds ($\div 2.2$)
2. Calculate the total daily dose ($\text{mg/kg/day} \times \text{kg}$)
3. Divide by the number of doses per day to get each individual dose
4. Determine the volume to administer using the available concentration (Desired/Have $\times$ Quantity)

Example 4: Full Workflow — Amoxicillin Suspension

Order: Amoxicillin 40 mg/kg/day PO divided TID. Patient weighs 33 lb. Available: Amoxicillin 250 mg/5 mL suspension.

Step 1: Convert weight.

$\frac{33}{2.2} = 15 \text{ kg}$

Step 2: Total daily dose.

$40 \times 15 = 600 \text{ mg/day}$

Step 3: Individual dose (TID = 3 doses/day).

$\frac{600}{3} = 200 \text{ mg per dose}$

Step 4: Calculate volume to give.

$\text{Volume} = \frac{200 \text{ mg}}{250 \text{ mg}} \times 5 \text{ mL} = 4 \text{ mL}$

Answer: Administer 4 mL of Amoxicillin 250 mg/5 mL PO three times daily.

Common Mistakes to Avoid

1. Forgetting to convert pounds to kilograms. Using a weight of 44 instead of 20 doubles the dose — a potentially fatal error in pediatric patients.
2. Confusing mg/kg/dose with mg/kg/day. If the reference says 25 mg/kg/day divided TID, the daily total is 25 mg/kg, not each dose. Each dose is one-third of that amount.
3. Using the wrong number of doses per day. Count carefully: q8h = 3 doses, q6h = 4 doses, BID = 2 doses. A wrong divisor changes every dose.
4. Rounding weight too aggressively. In pediatrics, small rounding errors in weight translate directly into dosing errors. Round to one decimal place.

Practice Problems

Test your understanding with these problems. Click to reveal each answer.

Key Takeaways

• Convert pounds to kilograms by dividing by 2.2 — always do this first
• For mg/kg/dose orders, multiply the dose factor by patient weight to get each dose
• For mg/kg/day orders, calculate the total daily dose first, then divide by the number of doses per day
• Always double-check frequency: BID = 2, TID = 3, q8h = 3, q6h = 4, q4h = 6
• Weight-based dosing errors are one of the most common medication errors in pediatrics — always verify your math
• When in doubt, check the calculated dose against the safe dosage range before administering

Return to Math for Nurses for more topics in this section.