Antibiotic Dosing Calculator

Weight-based antibiotic dosing is a pharmacokinetic approach that calculates medication doses based on a patient's body weight rather than using fixed doses. This is essential for antibiotics like vancomycin, aminoglycosides (gentamicin, amikacin), and several beta-lactams — drugs where the dose must be precisely matched to body size to achieve therapeutic concentrations without toxicity. Underdosing leads to treatment failure and antibiotic resistance; overdosing causes nephrotoxicity, ototoxicity, and other serious adverse effects.

For obese patients, the calculation becomes more complex because drug distribution into adipose tissue varies by antibiotic. Vancomycin distributes modestly into fat, so adjusted body weight (ABW) with a factor of 0.3-0.4 is commonly used. Aminoglycosides are highly hydrophilic and distribute primarily in lean tissue, requiring ABW with a 0.4 factor. In contrast, some beta-lactams distribute well into adipose tissue and may require actual body weight dosing even in obesity.

This calculator determines the appropriate dosing weight based on ideal body weight (Devine formula), actual body weight, obesity status, and renal function. It estimates creatinine clearance using the Cockcroft-Gault equation for antibiotics requiring renal adjustment and provides loading and maintenance dose recommendations based on the selected antibiotic and clinical indication.

The calculator uses the Devine formula for Ideal Body Weight (IBW) and applies an obesity adjustment:

Vancomycin monitoring: Trough levels should be drawn before the 4th or 5th dose. Target trough of 15-20 mcg/mL for severe infections, 10-15 mcg/mL for less severe. For aminoglycosides, peak and trough monitoring guides dosing, with extended-interval (once-daily) dosing preferred for most indications.

1. Enter Patient Demographics: Weight (kg), height (cm), age, and sex — required for IBW, CrCl, and dosing weight calculations.
2. Enter Serum Creatinine: Current SCr level in mg/dL. Used in Cockcroft-Gault to estimate renal function for antibiotics requiring dose adjustment in kidney impairment.
3. Select Antibiotic and Indication: Choose the prescribed antibiotic. The indication (severe vs moderate infection) affects target dose. Severe infections (sepsis, meningitis, endocarditis) require higher initial doses.
4. Indicate Obesity Status: If the patient is obese (BMI ≥ 30), toggle this setting. The calculator will use adjusted body weight for antibiotics where obesity adjustment is recommended.
5. Review Dosing Recommendations: The calculator provides loading dose, maintenance dose, frequency, maximum dose limits, and flags if therapeutic drug monitoring is required.

Underdosing consequences: Subtherapeutic antibiotic levels are a major driver of antimicrobial resistance. When bacteria are exposed to antibiotic concentrations below the minimum inhibitory concentration (MIC), resistant subpopulations survive and proliferate. This is particularly critical for vancomycin, where trough levels below 10 mcg/mL are associated with the development of vancomycin-intermediate S. aureus (VISA) and treatment failure in MRSA infections.

Overdosing consequences: Vancomycin nephrotoxicity risk increases significantly with trough levels above 20 mcg/mL, especially when combined with other nephrotoxic agents. Aminoglycoside ototoxicity can be irreversible and is both concentration- and duration-dependent. Accurate weight-based dosing combined with therapeutic drug monitoring is the standard of care for minimizing these risks while ensuring efficacy.