Weight-Based Dosing
Part of Pharmacy and Apothecary
Calculating medicine doses proportional to patient body weight — essential for pediatric dosing and accurate management of potent medicines.
Why This Matters
Medicine distributes through the body in proportion to body mass. A drug that achieves therapeutic blood concentrations at 500 mg in a 70 kg adult will achieve the same concentration at 143 mg in a 20 kg child — because the same amount of drug is spread through a proportionally smaller body. Give the child 500 mg and you risk toxicity. Give the child 143 mg as a fixed flat dose without knowing their weight, and you might be dangerously underdosing a child who weighs 30 kg instead of 20.
Weight-based dosing removes this uncertainty. By anchoring the dose to a per-kilogram figure — a dose of X mg per kg — any patient weight can be handled accurately. This is the standard in modern pediatric medicine, where it dramatically reduces dosing errors.
For the community apothecary, weight-based dosing is particularly critical for three categories: pediatric patients (where the weight variation is largest), potent medicines with narrow therapeutic windows, and medications where the margin between therapeutic and toxic doses is small.
Estimating Patient Weight Without a Scale
The most immediate challenge in a resource-limited setting is often not knowing the patient’s exact weight. A reliable scale is essential equipment — but if one is unavailable, these estimation methods provide useful approximations.
Infant weight estimation (birth to 12 months):
- Birth weight average: 3-3.5 kg
- Doubles by 5-6 months
- Triples by 12 months
- Formula: Weight (kg) ≈ Age (months) + 9 / 2 [for 3-12 months]
Child weight estimation (1-10 years):
- Simple formula: Weight (kg) ≈ (Age in years × 2) + 8
- Example: 5-year-old ≈ (5 × 2) + 8 = 18 kg
- This formula is reasonably accurate for well-nourished children at average developmental stage
Adolescent and adult estimation:
- For adults without a scale, visual estimation by an experienced practitioner is the only option
- Typical adult range: 50-90 kg for most populations
- When you genuinely cannot estimate, use a conservative assumption: 50 kg for thin adults, 60-70 kg for average-build adults
- For dosing potent medications where exact weight matters, the safest approach when uncertain is to start at the lower end of the dose range and titrate upward based on response
Tape measure methods (paediatric): The Broselow tape is a color-coded length-based weight estimation tool used in pediatric emergency medicine — a child’s length corresponds to an approximate weight. You can create a basic version: measure the child’s length and compare to a table of average heights and weights for your population. This is more accurate than age-based estimation.
| Length (cm) | Estimated Weight (kg) |
|---|---|
| 50-55 | 3-4 |
| 55-65 | 4-6 |
| 65-75 | 6-9 |
| 75-85 | 9-12 |
| 85-95 | 12-15 |
| 95-110 | 15-20 |
| 110-120 | 20-25 |
| 120-130 | 25-30 |
| 130-140 | 30-36 |
| 140-150 | 36-42 |
Calculating Weight-Based Doses
Basic formula: Dose (mg) = Dose per kg (mg/kg) × Patient weight (kg)
Then calculate volume: Volume (mL) = Dose (mg) ÷ Concentration (mg/mL)
Worked examples:
Example 1: Pediatric fever management
- Willow bark decoction: 80 mg/mL
- Standard dose: 10-15 mg/kg per dose, every 6-8 hours
- Patient: 8-year-old, estimated 24 kg
- Dose: 10 mg/kg × 24 kg = 240 mg (low range) to 15 × 24 = 360 mg (high range)
- Start at low range: 240 mg
- Volume: 240 ÷ 80 = 3 mL every 6-8 hours
Example 2: Antiparasitic treatment
- Wormwood tincture: 2 mg active compounds per mL
- Standard dose: 0.3 mg/kg per dose, twice daily, 7 days
- Patient: adult, estimated 65 kg
- Dose: 0.3 × 65 = 19.5 mg
- Volume: 19.5 ÷ 2 = 9.75 mL ≈ 10 mL twice daily
Example 3: Neonatal preparation
- Infant with fever, unable to take oral medicine (vomiting)
- Age: 4 months; weight estimated: 6 kg
- Glycerin suppository at 0.5g glycerin per suppository intended to stimulate bowel
- Pediatric rectal dose: 0.2 mL/kg glycerin
- Volume: 0.2 × 6 = 1.2 mL — use a 1g glycerin suppository (approximately 1 mL)
Dose Limits and Maximum Doses
Weight-based dosing has ceiling doses — the maximum that should be given regardless of patient weight. A very large adult (100 kg) prescribed 15 mg/kg of a medicine would receive 1,500 mg per dose. If the adult maximum safe dose is 1,000 mg, the weight-based calculation must be capped.
Always specify a maximum dose when recording a weight-based prescription: “10 mg/kg per dose, not to exceed 500 mg per dose.”
Conversely, minimum doses apply for very small patients where a weight-based dose would produce too small a volume to measure accurately. If the calculated volume is less than 0.1 mL, the preparation concentration is too dilute — either use a different concentration or switch to an age-based estimate for minimum dose.
Adjusting for Nutritional Status
Weight-based dosing assumes the measured weight is metabolically active body mass — muscle, organs, and fluid. Severely malnourished patients have a different body composition that affects drug distribution and metabolism:
Edematous malnutrition (kwashiorkor, with water retention): The patient’s weight overestimates their lean body mass. Use a lower dose than weight alone would suggest — approximately 75% of the calculated dose.
Wasting malnutrition (marasmus, very thin): Weight accurately reflects lean mass, but liver and kidney function may be reduced. Metabolism of drugs is slower. Give standard weight-based dose but lengthen the interval between doses.
Obesity (excess fat mass): Fat tissue has limited blood supply and many medicines distribute poorly into it. For most medicines, dose should be calculated on “ideal body weight” or “lean body weight” rather than actual body weight to avoid over-dosing. A rough ideal body weight formula: 50 kg + 0.9 kg per cm above 150 cm height (for adults).
Pediatric Dose Tables
Pre-calculate and post a reference table for your most-used preparations:
Willow bark decoction (80 mg/mL) — dose 10 mg/kg every 6-8 hours:
| Weight (kg) | Dose (mg) | Volume (mL) |
|---|---|---|
| 5 | 50 | 0.6 |
| 10 | 100 | 1.25 |
| 15 | 150 | 1.9 |
| 20 | 200 | 2.5 |
| 30 | 300 | 3.75 |
| 40 | 400 | 5 |
| 50+ | 500 (max) | 6.25 |
Creating a similar table for each commonly-used preparation saves calculation time during emergencies and reduces arithmetic errors at stressful moments.
Store these tables in the front of your dispensing log and review them regularly. Errors in pre-calculated tables persist — have a second practitioner verify all tables before use.