Children, older adults, and clients receiving drugs with a narrow safety margin are dosed by body size rather than one fixed adult dose. Two measures are used:

• Body weight (mg/kg) — most pediatric drugs, anticoagulants, many antibiotics, sedatives, and emergency drugs
• Body surface area (mg/m²) — most chemotherapy and some other high-risk drugs

Core formulas

Read the order carefully: "mg/kg/dose" is the amount for each dose; "mg/kg/day" is the total for 24 hours and must be divided by the number of doses. Confusing the two is a classic threefold or fourfold overdose.

Older age-based rules (Clark's, Young's, Fried's) are obsolete and are not used to calculate pediatric doses.

When weight-based dosing applies: all children; neonates (weight in grams); adults receiving anticoagulants (heparin, enoxaparin), aminoglycosides, vancomycin, chemotherapy, sedation, and resuscitation drugs; clients at extremes of body weight (some drugs use ideal or adjusted body weight — follow the drug reference).

Pediatric reference doses used in this topic (verify every dose against a current pediatric drug reference in practice)

Worked examples

Example 1 — mg/kg/dose with a pound conversion. Order: acetaminophen (paracetamol) 15 mg/kg PO every 6 hours as needed for fever. Child weighs 44 lb. Available: 160 mg/5 mL.

• Weight: 44 lb ÷ 2.2 = 20 kg
• Dose: 15 mg/kg × 20 kg = 300 mg
• Volume: (300 mg ÷ 160 mg) × 5 mL = 1,500 ÷ 160 = 9.375 mL
• Rounding: oral syringe in tenths → 9.4 mL
• Safety check: 300 mg is below 1 g per dose; daily maximum 75 mg/kg × 20 kg = 1,500 mg = 5 doses of 300 mg. Every 6 hours gives 4 doses (1,200 mg) — safe.
• Dimensional analysis: 44 lb × 1 kg/2.2 lb × 15 mg/kg × 5 mL/160 mg = 9.375 mL.

Example 2 — mg/kg/day divided. Order: amoxicillin 90 mg/kg/day PO divided every 12 hours for pneumonia. Child weighs 18 kg. Available: 400 mg/5 mL.

• Daily dose: 90 × 18 = 1,620 mg/day (below the 4 g/day maximum)
• Per dose: 1,620 ÷ 2 = 810 mg every 12 hours
• Volume: (810 ÷ 400) × 5 mL = 4,050 ÷ 400 = 10.125 mL → 10.1 mL
• Check: 10.1 mL × 80 mg/mL = 808 mg (rounding difference only).

Example 3 — safe-range check (the order is too high). A drug reference lists 20–40 mg/kg/day divided every 8 hours for a drug. Child weighs 33 lb. Order: 250 mg IV every 8 hours.

• Weight: 33 ÷ 2.2 = 15 kg
• Safe range per day: 20 × 15 = 300 mg to 40 × 15 = 600 mg/day
• Safe range per dose (÷ 3): 100–200 mg
• Ordered daily total: 250 × 3 = 750 mg/day
• Decision: 750 mg/day is above the 600 mg/day maximum → do not give; hold and contact the prescriber to clarify. The nurse does not change the dose independently.
• The same process applies to a dose that is too low (for example, 75 mg every 8 hours = 225 mg/day): an underdose of an antibiotic also needs clarification.

Example 4 — dose capped by the pediatric per-dose maximum. Order: ibuprofen 10 mg/kg PO every 6 hours for fever. Adolescent weighs 45 kg.

• Calculated: 10 × 45 = 450 mg
• Check per dose: pediatric references cap ibuprofen for fever and pain at 400 mg per dose (the same as the adult OTC dose) → 450 mg exceeds it
• Check per day: 450 × 4 = 1,800 mg/day. This equals 40 mg/kg/day (40 × 45 = 1,800 mg), but many references also cap the daily total — 1,200 mg/day for OTC use. Even 400 mg × 4 = 1,600 mg/day exceeds that OTC cap.
• Decision: clarify with the prescriber before giving; give the capped dose only if the order or policy allows. (Adult prescription dosing is higher — up to 800 mg per dose and 3.2 g/day — but that is not a pediatric fever dose.)

Example 5 — body surface area. A client is 150 cm tall and weighs 54 kg. Order: a drug at 100 mg/m².

• BSA: 150 × 54 = 8,100; 8,100 ÷ 3,600 = 2.25; √2.25 = 1.5 m²
• Dose: 100 mg/m² × 1.5 m² = 150 mg
• Recheck: 1.5 × 1.5 = 2.25; 2.25 × 3,600 = 8,100. Correct.
• Chemotherapy doses are independently double-checked, often by two nurses and a pharmacist, and the height and weight used are documented.

Example 6 — maintenance IV fluid (Holliday–Segar). Child weighs 24 kg.

• Daily: first 10 kg × 100 mL = 1,000 mL; next 10 kg × 50 mL = 500 mL; remaining 4 kg × 20 mL = 80 mL → 1,580 mL/day
• Hourly "4-2-1 rule": 10 × 4 = 40; 10 × 2 = 20; 4 × 1 = 4 → 64 mL/h
• Note: 1,580 ÷ 24 = 65.8 mL/h; the two methods are approximations that differ slightly. Use the method in the order or policy, and adjust for losses, fever, and clinical status.

Adult example — enoxaparin. Order: enoxaparin 1 mg/kg SC every 12 hours. Weight 176 lb → 176 ÷ 2.2 = 80 kg → 80 mg every 12 hours (a prefilled 80 mg/0.8 mL syringe; do not expel the air bubble). If creatinine clearance is below 30 mL/min, the usual treatment dose changes to 1 mg/kg once daily — check kidney function before giving.

• Tenfold errors are the most dangerous pediatric error — a misplaced decimal point (1.0 mL read as 10 mL) or a mg/mL confusion. Small children have little margin.
• mg/kg/day given as mg/kg/dose → overdose by the number of daily doses.
• Weight in lb used as kg → 2.2-fold overdose.
• Outdated weight — infants gain weight quickly; an old weight can underdose, and an estimated weight can overdose.
• Drug-specific toxicity from overdose: acetaminophen → liver injury; opioids → respiratory depression; aminoglycosides → kidney and ear toxicity; enoxaparin and heparin → bleeding.

• Children are not small adults: immature liver and kidney function (neonates), higher body water, and developmental factors change drug handling.
• Never exceed the adult dose unless a pediatric reference specifically supports it.
• Concentration confusion: pediatric liquids come in several strengths (infant drops, children's syrup). In the US, single-ingredient infant and children's acetaminophen liquids use one concentration (160 mg/5 mL); other countries may have several. Calculate from the product in hand.
• Obesity: some drugs are dosed by ideal or adjusted weight to avoid overdose; follow the reference.
• Ibuprofen: not under 6 months; avoid with dehydration, kidney disease, or bleeding.
• Enoxaparin: caution with kidney impairment, spinal or epidural anesthesia (spinal hematoma boxed warning), and low platelets.

Listed in priority order.

1. Weigh the child in kg on admission (same scale, no heavy clothing or diaper); record kg only.
2. Calculate the safe range from a current pediatric reference and compare it with the ordered dose — per dose and per day — before every new order.
3. Hold and clarify any dose outside the safe range, or above the adult dose; document the clarification.
4. Calculate the volume, choose the right oral or 1 mL syringe, and have high-alert pediatric doses independently double-checked.
5. Administer using developmentally appropriate techniques; give oral liquids slowly into the side of the cheek.
6. Monitor for therapeutic effect and toxicity; for acetaminophen, track the total from all products in 24 hours.
7. Reweigh as ordered (often daily for infants and for children on fluids or diuretics).

• Parents should know the child's weight in kg and bring it to every visit.
• Use the dosing syringe supplied with the product, measured in mL; never a kitchen spoon.
• Check the concentration on the box every time — drops and syrups may differ.
• Do not give two products that both contain acetaminophen or both contain ibuprofen; keep a written dose log with times.
• Do not give aspirin to children or teenagers with viral illness (Reye syndrome).
• Store medicines locked and out of reach; save the poison control number.

After any dosing error: assess the child, notify the prescriber, give ordered treatment, and report the event.

• kg = lb ÷ 2.2; neonatal weight in grams ÷ 1,000
• mg/kg/dose = each dose; mg/kg/day = 24-hour total → divide by the number of doses
• Safe range: low and high mg/kg/day × kg → compare with the ordered daily total
• Outside the safe range → hold and contact the prescriber; never adjust the dose yourself
• A pediatric dose should not exceed the adult dose
• BSA (Mosteller) = √ (cm × kg ÷ 3,600); dose = mg/m² × m²
• Holliday–Segar: 100 / 50 / 20 mL/kg/day; hourly 4-2-1
• Acetaminophen 10–15 mg/kg every 4–6 h, maximum 75 mg/kg/day (4 g/day), no more than 5 doses per day
• Ibuprofen 5–10 mg/kg, not under 6 months; maximum 400 mg per dose
• Tenfold decimal errors are the most feared pediatric error — double-check high-alert doses