Loading Dose Calculator (Pharmacokinetics)
Calculate a pharmacokinetic loading dose from volume of distribution, body weight, target and current plasma concentration, bioavailability (F) and salt factor (S), in mg and mg/kg.
How it works
loading dose (mg) = Vd (L/kg) × weight (kg) × (C target − C current) (mg/L) ÷ (F × S)
A loading dose fills the volume of distribution so that the target plasma concentration is reached at once instead of after four to five half-lives of regular dosing. It depends only on how widely the drug distributes (Vd), not on how fast it is cleared. Dividing by bioavailability (F) corrects for incomplete absorption and by the salt factor (S) for the fraction of a salt that is active drug. If the patient already has some drug on board, only the difference between the target and current level is loaded. Classic textbook use: theophylline, digoxin, phenytoin, amiodarone and vancomycin.
Worked example
A 70 kg adult, Vd 0.5 L/kg (Vd = 35.0 L), target 10 mg/L with no drug on board, IV (F = 1) and S = 1: loading dose = 35 × 10 ÷ 1 = 350 mg, or 5.00 mg/kg. As aminophylline (S = 0.8) the dose would be 350 ÷ 0.8 = 438 mg.
Sources: StatPearls — Loading Dose · StatPearls — Clinical Significance of Volume of Distribution in Pharmacotherapy
Frequently asked questions
Why does a loading dose depend on Vd and not on clearance?
The loading dose is the amount needed to fill the space the drug distributes into: amount = concentration × volume. Clearance determines how fast the drug is removed, so it sets the maintenance dose instead.
When is a loading dose used?
When the drug has a long half-life and an effect is needed quickly — for example amiodarone, digoxin, phenytoin, vancomycin or theophylline. Without it, steady state takes about 4–5 half-lives.
What are F and S?
F is bioavailability, the fraction of an oral or IM dose that reaches the circulation (1 for IV). S is the salt factor, the fraction of the weighed salt that is active drug — 0.8 for aminophylline (theophylline), 0.92 for phenytoin sodium.
What weight should I use?
Use the weight the drug’s Vd is referenced to: total body weight for many drugs, ideal or adjusted body weight for drugs that distribute poorly into fat (such as aminoglycosides) in obese patients.
Related calculators
Educational pharmacokinetic estimate for health professionals only — not medical advice or a prescription. Population parameters vary widely between patients; base real dosing on the product label, measured drug levels, local protocols and a clinical pharmacist.