Weight-Based Dose Calculator
Weight-based dosing calculates the required drug dose from a prescribed amount per kilogram of body weight. The formula is simply: Dose (mg) = Ordered dose (mg/kg) x Patient weight (kg). This approach accounts for the fact that body size affects drug volume of distribution and clearance. The calculator also computes the total daily dose when you specify how many doses per day are given, and shows per-dose amounts. Always check calculated doses against drug-specific maximum dose caps from prescribing information.
Weight-based dose formula
Dose (mg) = Ordered dose (mg/kg) x Weight (kg)
Daily dose (mg/day) = Dose x Number of doses per day
Always verify the calculated dose against the maximum dose cap for the specific drug. For drugs with significant variation by age or organ function, additional adjustments beyond weight-based calculation may be required.
Body weight considerations
- Actual body weight (ABW): used for most drugs in patients with normal body composition.
- Ideal body weight (IBW): males = 50 + 2.3 x (height inches - 60); females = 45.5 + 2.3 x (height inches - 60). Used when drug does not distribute into adipose tissue.
- Adjusted body weight (AdjBW) = IBW + 0.4 x (ABW - IBW): used for aminoglycosides and some other drugs in obese patients.
- Many pediatric drugs are dosed as mg/kg with maximum dose caps to prevent adult-equivalent doses from being exceeded before the child reaches adult weight.
- Always consult drug-specific prescribing information for the correct weight basis and maximum dose.
Weight-based dose calculator: frequently asked questions
How do you calculate a weight-based drug dose?
Multiply the prescribed dose in mg/kg by the patient weight in kg: Total dose (mg) = mg/kg x Weight (kg). For example, amoxicillin 25 mg/kg in a 32 kg child = 800 mg per dose. Always verify the calculated dose against the maximum dose specified in prescribing information.
When is weight-based dosing used?
Weight-based dosing is used for most pediatric medications, many antibiotics across all ages (e.g., amoxicillin, vancomycin, gentamicin), weight-based chemotherapy, and drugs where body size significantly affects pharmacokinetics. It accounts for variation in volume of distribution and clearance with body size.
Should actual, ideal, or adjusted body weight be used?
It depends on the drug. Many drugs use actual body weight; others specify ideal body weight (IBW) or adjusted body weight (AdjBW = IBW + 0.4 x (Actual - IBW)) for obese patients. For example, aminoglycosides use AdjBW for obese patients. Always check the drug-specific prescribing information.
What is a maximum dose cap?
A maximum dose cap limits the total dose regardless of weight, preventing excessive doses in very large or obese patients. For example, amoxicillin 25 mg/kg is typically capped at 500 mg per dose. If the calculated dose exceeds the cap, use the cap. This tool shows the uncapped calculated dose; users must apply drug-specific caps.
Can this calculator be used for clinical dosing?
This tool is for educational and informational purposes only. Clinical weight-based dosing requires professional verification against prescribing information, pharmacist review, and patient-specific assessment by a qualified healthcare provider.
Official sources
- U.S. Food and Drug Administration: FDA Pediatric Drug Development.
- NIH National Library of Medicine: Pediatric Dosing (StatPearls).
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.