How are weight-based drug doses calculated?

Weight-based doses are calculated by multiplying the dose per kilogram (mg/kg) by the patient's body weight (kg). For example, vancomycin at 15 mg/kg for a 70 kg patient = 1,050 mg. The appropriate weight type (actual, ideal, or adjusted) varies by drug.

When should you use actual vs. ideal vs. adjusted body weight?

Actual body weight (ABW): Use for most drugs and for renally-cleared drugs. Ideal body weight (IBW): Use for tidal volume calculations, aminoglycosides (loading dose), and some anesthetic agents. Adjusted body weight [IBW + 0.4 × (ABW − IBW)]: Use for vancomycin, aminoglycoside maintenance dosing, and some other drugs in obese patients.

Why is weight-based dosing important?

Weight-based dosing accounts for differences in volume of distribution and drug clearance related to body size. It is critical for drugs with narrow therapeutic indices (e.g., chemotherapy, anticoagulants, aminoglycosides) where underdosing leads to treatment failure and overdosing causes toxicity.

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Weight-Based Dose Calculator

Calculate medication doses based on patient weight using mg/kg dosing. Supports custom dose inputs and common drug presets.

Inputs

Patient Weight

Drug Preset (optional)

Dose per kg

Frequency (doses/day)

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Enter weight and dose to calculate.

Disclaimer: For educational purposes only. Always verify doses with your institution's formulary and pharmacy. Not a substitute for clinical judgment.

When to use

Weight-based dosing is essential in pediatrics and for many adult medications (antibiotics, anticoagulants, chemotherapy, sedatives). This calculator converts mg/kg (or mcg/kg, mL/kg, units/kg) dosing to a total dose based on patient weight.

Clinical pearls

Always verify maximum single and daily doses — weight-based doses can exceed safe maximums in larger patients.
Use actual body weight for most drugs; use ideal or adjusted body weight for specific medications (e.g., aminoglycosides, vancomycin in obesity).
Pediatric dosing almost always uses mg/kg; adult dosing may use fixed doses or mg/kg depending on the drug.
Round doses to practical measurable amounts (e.g., nearest 5 mg for tablets, nearest 0.1 mL for liquids).

References

Lexicomp / UpToDate Drug Dosing References.
Taketomo CK, et al. Pediatric & Neonatal Dosage Handbook. Lexicomp, 2023.

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About This Tool

What Is Weight-Based Dosing?

Weight-based dosing (mg/kg dosing) adjusts medication doses proportionally to a patient's body weight. This approach accounts for differences in drug distribution volume and metabolism across body sizes. It is the standard method for pediatric dosing and is used for many adult medications including antibiotics (vancomycin, aminoglycosides), anticoagulants (heparin, enoxaparin), and emergency medications.

How to Calculate

The formula is straightforward: Total Dose = Weight (kg) × Dose per kg. For example, a 70 kg patient receiving amoxicillin at 25 mg/kg would get 1,750 mg per dose. When a frequency is specified, the daily total is calculated as the single dose multiplied by the number of daily doses.

Which Weight to Use?

Most drugs are dosed on actual body weight (ABW). However, certain medications require ideal body weight (IBW) or adjusted body weight (AdjBW) for accurate dosing — particularly in obese patients. Aminoglycosides typically use AdjBW (IBW + 0.4 × [ABW − IBW]), while vancomycin generally uses ABW. Always consult drug-specific guidelines.

🔑 Clinical Pearls

Always check maximum dose caps — a 150 kg patient dosed at 15 mg/kg acetaminophen = 2,250 mg, exceeding the 1,000 mg single dose max.
Convert pounds to kilograms accurately: 1 lb = 0.4536 kg. A 10% error in weight = 10% error in dose.
For obese patients, consider whether the drug is lipophilic (use ABW) or hydrophilic (use IBW/AdjBW).
Pediatric doses should always be double-checked against both mg/kg and maximum adult doses.

Key References

Bauer LA. Applied Clinical Pharmacokinetics. 3rd ed. McGraw-Hill, 2015.
Taketomo CK, et al. Pediatric & Neonatal Dosage Handbook. Lexicomp, 2023.

Formula last verified: February 2026