Health📅 18 March 2025⏱ 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.
JW
James WhitfieldPersonal Finance & Maths WriterJames has written about personal finance, health metrics, and everyday mathematics for over six years. He holds a BSc in Mathematics from the University of Leeds.
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.