Medical Dose Weight-Based
Calculate medication doses based on body weight and concentration. Enter values for instant results with step-by-step formulas.
Formula
Dose = Body Weight (kg) × Dose per kg; Volume = Dose / Concentration
Worked Examples
Example 1: Antibiotic Dosing (Adult)
Problem:70 kg patient. Antibiotic: 10 mg/kg every 12 hours. Available as 100 mg/mL suspension. Calculate dose and volume.
Solution:Weight: 70 kg\nDose per kg: 10 mg/kg\n\nSingle dose:\n70 kg × 10 mg/kg = 700 mg per dose\n\nConcentration: 100 mg/mL\nVolume:\n700 mg / 100 mg/mL = 7 mL per dose\n\nFrequency: Every 12 hours (2×/day)\nDaily dose: 700 mg × 2 = 1,400 mg\nDaily volume: 7 mL × 2 = 14 mL\n\nAdminister: 7 mL every 12 hours\n\nWeekly dose: 1,400 × 7 = 9,800 mg\n\nVerify:\n- No maximum dose exceeded ✓\n- Normal renal function ✓\n- 12-hour spacing for even levels ✓
Result:700 mg per dose | 7 mL per dose | Twice daily | 1,400 mg/day total
Example 2: Pediatric Dosing
Problem:15 kg child (3 years old). Antipyretic: 15 mg/kg every 6 hours as needed. Max 75 mg/kg/day. Liquid: 160 mg/5 mL.
Solution:Weight: 15 kg\nDose: 15 mg/kg\n\nSingle dose:\n15 kg × 15 mg/kg = 225 mg\n\nConcentration: 160 mg / 5 mL = 32 mg/mL\nVolume:\n225 mg / 32 mg/mL = 7.03 mL ≈ 7 mL\n\nFrequency: Every 6 hours (up to 4×/day)\nMax daily: 15 kg × 75 mg/kg = 1,125 mg\nActual if given 4× = 225 × 4 = 900 mg ✓\n\n(Within maximum)\n\nPediatric note:\n- Use weight-appropriate syringe (not household spoon)\n- Round to measurable volume (7 mL, not 7.03)\n- Verify age-appropriate dosing guidelines
Result:225 mg per dose | 7 mL | Up to 4×/day | Within 1,125 mg max
Example 3: Chemotherapy (BSA-Based)
Problem:180 cm, 75 kg patient. Chemo drug: 75 mg/m² BSA. Given every 3 weeks. Calculate dose.
Solution:Height: 180 cm\nWeight: 75 kg\n\nBSA calculation (Mosteller formula):\nBSA = √((180 × 75) / 3600)\nBSA = √(13500 / 3600) = √3.75 = 1.94 m²\n\nDose per BSA: 75 mg/m²\nActual dose:\n75 mg/m² × 1.94 m² = 145.5 mg\n\nRound to practical amount: 145 mg\n\nGiven: Once every 3 weeks (21-day cycle)\n\nCycle dose: 145 mg\nMonthly dose (4.33 weeks): ~200 mg\n\nNote: BSA dosing accounts for both height and weight,\ngiving more accurate dosing than weight alone\nfor drugs with narrow therapeutic windows.
Result:145 mg per cycle | Every 21 days | BSA: 1.94 m² | Chemotherapy requires exact BSA
Frequently Asked Questions
How do weight-based drug doses work?
Weight-based dosing calculates medication amount based on patient body weight, typically mg/kg. This accounts for differences in drug distribution volume—larger patients have larger blood volumes and tissue masses, requiring more drug to achieve therapeutic levels. Formula: Dose = Weight (kg) × Dose per kg.
Why not give everyone the same dose?
Standard doses would underdose large patients (risking treatment failure) and overdose small patients (risking toxicity). Drugs with narrow therapeutic windows (small difference between effective and toxic doses) especially require weight-based dosing. Examples: chemotherapy, antibiotics, anticoagulants.
How do I calculate dose from concentration?
Volume = Dose / Concentration. Example: need 500mg, have 100mg/mL solution → 500/100 = 5 mL. For tablets: if need 25mg and have 12.5mg tablets → 2 tablets. Concentration tells you how much drug per unit volume (liquid) or per unit form (tablets).
What is a loading dose vs maintenance dose?
Loading dose: larger initial dose to quickly reach therapeutic levels. Maintenance dose: smaller ongoing dose to maintain levels. Loading dose may be 2-5x maintenance. Used when: steady-state takes long to reach, immediate effect needed. Not all drugs use loading doses.