Health⏱ 5 min read
How Medication Dosages Are Calculated (And What Affects Them)
Doctors and pharmacists use weight-based and age-adjusted formulas to determine safe doses. Here is how the calculations work for adults and children, and why the same drug needs different doses.
Understanding how doses are calculated helps patients ask better questions and notice errors. This guide covers the main methods — it is not a substitute for professional medical advice.
Weight-Based Dosing
Many drugs are dosed in mg per kg of body weight.
Dose (mg) = Weight (kg) x Dose per kg (mg/kg)
Example: Amoxicillin for bacterial infection
Adult dose: 250-500mg, 3 times daily (standard range, not weight-based)
Paediatric dose: 25-50 mg/kg/day, divided into 3 doses
Child weighing 20kg, dose 40mg/kg/day in 3 doses:
Total daily dose: 20 x 40 = 800mg/day
Per dose: 800/3 = 267mg, rounded to 250mg (available formulation)
Weight-based dosing matters most for:
- Children (all dosing is weight-based)
- Drugs with narrow therapeutic windows (aminoglycosides, lithium)
- Renally-cleared drugs in overweight or obese patients
Renal Dose Adjustment (Creatinine Clearance)
Many drugs are cleared by the kidneys. In renal impairment,
standard doses accumulate to toxic levels.
Cockcroft-Gault equation (estimates creatinine clearance, CrCl):
CrCl (ml/min) = ((140 - age) x weight (kg)) / (serum creatinine (micromol/L) x 0.815)
Multiply by 0.85 for women.
Example: 72-year-old woman, 65kg, creatinine 120 micromol/L
CrCl = ((140-72) x 65) / (120 x 0.815) x 0.85
= (68 x 65) / 97.8 x 0.85
= 4,420 / 97.8 x 0.85 = 38.4 ml/min → moderately impaired
For drugs dose-adjusted by CrCl, the dose or frequency is reduced.
This is critical for antibiotics (gentamicin), anticoagulants (rivaroxaban),
and many diabetes drugs (metformin is contraindicated below 30ml/min).
Paediatric Dose Calculation Methods
Primary method: mg/kg (always preferred when available)
Fried's Rule (infants under 2 years, emergency use only):
Dose = (Age in months / 150) x Adult dose
Young's Rule (children 2-12 years, approximate):
Dose = (Age in years / (Age + 12)) x Adult dose
Clark's Rule (by weight, approximate):
Dose = (Weight in kg / 70) x Adult dose
These rules give ROUGH estimates only.
Always use specific paediatric dosing references (BNFc, local formulary)
and weight-based calculations where available.
The Therapeutic Window
Most drugs have a range where they are effective but not toxic:
Minimum Effective Concentration (MEC): below this, no effect
Minimum Toxic Concentration (MTC): above this, adverse effects
Therapeutic window = between MEC and MTC
Narrow therapeutic index drugs (small window, high monitoring required):
Warfarin (anticoagulant): INR target 2.0-3.0
Lithium: plasma level 0.6-1.0 mmol/L
Digoxin: plasma level 0.8-2.0 ng/ml
Gentamicin: peak and trough levels monitored
For these drugs, dosing is guided by monitoring blood levels,
not just weight — individual variation is too great for
formula-only dosing to be safe.