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Calculate the loading dose required to rapidly reach a target plasma concentration: LD = Css · Vd / F, where Css is the target concentration, Vd is the volume of distribution, and F is bioavailability (1 for IV, <1 for oral). A loading dose achieves Css essentially immediately (one-compartment IV bolus) instead of waiting 4–5 half-lives for accumulation. For oral loading, divide by F. Maintenance dosing (MD = Css·CL·τ/F) must follow to sustain the concentration. Loading doses are hazardous for narrow-therapeutic-index drugs (digoxin, theophylline, antiarrhythmics) — clinical judgement required. Optional body-weight input converts the result to mg/kg. Not medical advice.
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Tool usage guide
The Loading Dose Calculator estimates the dose needed to rapidly reach a target plasma concentration using LD = Css · Vd / F. Enter the target concentration, volume of distribution, and bioavailability to calculate the loading dose in mg, with an optional mg/kg result based on body weight.
Background
A pharmacokinetic example uses a target concentration of 10 mg/L, a volume of distribution of 35 L, IV administration, and a body weight of 70 kg.
Problem
Calculate the total loading dose and express it per kilogram.
How to use
Enter Css = 10, Vd = 35, bioavailability F = 1, and weight = 70.
Css: 10 mg/L; Vd: 35 L; F: 1; weight: 70 kgOutcome
The calculator returns 350 mg, equivalent to 5 mg/kg, using LD = Css · Vd / F.
Background
An oral dose must reach a target concentration of 10 mg/L with a volume of distribution of 35 L and bioavailability of 0.9.
Problem
Determine how the oral loading dose differs from the IV calculation.
How to use
Enter Css = 10, Vd = 35, and bioavailability F = 0.9. Leave body weight blank.
It uses LD = Css · Vd / F, where Css is target concentration, Vd is volume of distribution, and F is bioavailability.
Enter 1 for intravenous administration.
Dividing by a bioavailability below 1 increases the administered dose to account for the fraction that reaches systemic circulation.
Yes. Enter optional body weight in kilograms to receive the loading dose per kilogram.
No. Loading doses require clinical judgment, especially for narrow-therapeutic-index drugs such as digoxin and theophylline.
Css: 10 mg/L; Vd: 35 L; F: 0.9Outcome
The calculator returns 388.8889 mg. The dose is higher than the 350 mg IV result because the oral calculation divides by F.