Pharmacokinetics

Loading Dose Calculator

Calculates the initial dose needed to reach a target plasma concentration straight away, from Vd and bioavailability.

IV loading dose

500.0mg · 7.14 mg/kg

Dose appears reasonable

Inputs

Target concentration (Cₚ)

mg/L

Therapeutic level wanted. Typical: digoxin 0.8–2.0 mcg/L, vancomycin 15–20 mg/L, phenytoin 10–20 mg/L.

Volume of distribution (Vd)

Apparent volume of distribution, in total litres.

Patient weight

Used to express the dose per kilogram.

Try an example drug

Working

LD = (Cₚ × Vd) ÷ F10 × 50 ÷ 1

IV loading dose500.0mg

Dose per kilogram7.14mg/kg

Initial concentration10mg/L

Time to therapeuticImmediate (IV)

LD = (Cₚ × Vd) ÷ F

Cₚ = target plasma concentration (mg/L); Vd = volume of distribution (L); F = bioavailability as a fraction (1 for IV). mg/L × L gives mg — so if the target is in mcg/L, the dose comes out in mcg.

Divided loading — for drugs with a long half-life, to reduce toxicity risk and allow response to be assessed (e.g. digoxin 50% initially, then 25% every 6–8 h).

Non-linear kinetics — phenytoin and theophylline follow Michaelis–Menten kinetics: small dose increases cause large Cₚ changes, so titrate carefully.

Special populations — elderly: reduced Vd and increased sensitivity; obesity: use adjusted body weight; renal failure: reduced clearance; paediatrics: different Vd per kg.

Worked example: Oral loading dose with incomplete bioavailability

A practical sheet asks for the oral loading dose of Drug X needed to reach a target plasma concentration of 8 mg/L straight away. Its volume of distribution in the patient described is 35 L, and its oral bioavailability is 80 %.

Route: Oral
Target concentration Cₚ: 8 mg/L
Volume of distribution Vd: 35 L (total)
Bioavailability F: 80 %
Dose basis: Fixed

Formula
LD = (Cₚ × Vd) ÷ F
Convert F to a fraction
F = 80 % ÷ 100 = 0.8
Amount needed in the body
Cₚ × Vd = 8 mg/L × 35 L = 280 mg (the IV-equivalent dose)
Allow for incomplete absorption
LD = 280 mg ÷ 0.8 = 350.0 mg

Oral loading dose = 350.0 mg (IV equivalent 280.0 mg).

280 mg must reach the circulation to fill 35 L to 8 mg/L; because only 80 % of an oral dose gets there, 350 mg has to be swallowed. This is an educational calculation on illustrative values — a real loading dose comes from the drug's monograph and protocol, and is confirmed with drug-level monitoring.

Common mistakes

Entering a Vd in L/kg as if it were total litres. 0.7 L/kg must first be multiplied by body weight (0.7 × 70 kg = 49 L); entering 0.7 gives a dose 70 times too small.

Entering F as a percentage in a hand calculation. 350 mg comes from dividing by 0.8, not by 80.

Mixing concentration units. A target in mcg/L (µg/L) with Vd in litres gives the dose in micrograms, not milligrams — divide by 1000 for mg.

Multiplying by F instead of dividing. An oral loading dose is larger than the IV dose, never smaller.

Using clearance or half-life to size the loading dose. They set the maintenance dose; the loading dose depends only on Vd, the target and F.

Assumptions and limitations

Assumes a one-compartment model in which the dose distributes instantly. For drugs with a slow distribution phase, levels taken too early overshoot the target.

Assumes linear kinetics. For saturable drugs the same arithmetic can badly mispredict the level reached.

Vd changes with body composition, fluid status and disease, so a population Vd may not fit an individual.

The time-to-effect line on the page is a general statement, not derived from the drug's absorption rate.

Guide: Pharmacokinetic Parameters: AUC, Clearance, Half-Life and BioavailabilityThe concept in full, with practice questions.Volume of Distribution CalculatorGet Vd in total litres from an IV dose and C₀ before using it here.Bioavailability CalculatorWork out F from AUC data instead of assuming it.Accumulation Index CalculatorSee how long repeated doses take to build up without a loading dose.

References

Rowland and Tozer's Clinical Pharmacokinetics and Pharmacodynamics: Concepts and Applications. 5th ed. Philadelphia: Wolters Kluwer; 2019.
Ducharme MP, Shargel L, editors. Shargel and Yu's Applied Biopharmaceutics & Pharmacokinetics. 8th ed. New York: McGraw Hill; 2022.

Common questions

A loading dose only has to fill the volume of distribution to the target concentration. Clearance decides how fast the drug leaves afterwards, which is the maintenance dose's job.

Multiply it by the patient's weight first. This calculator treats Vd as total litres: vancomycin 0.7 L/kg in a 70 kg patient is 49 L.

Only the fraction F of an oral dose reaches the circulation, so the dose is divided by F. With F = 50%, twice the IV dose is needed.

No — an IV dose goes straight into the circulation, so F is 1 and the bioavailability field is hidden.

mg/L with Vd in litres gives the dose in mg. Targets quoted in mcg/L (such as digoxin) give a dose in mcg; divide by 1000 for mg.

Prepared by the PharmaWallah student team. Verify results with your textbook or instructor. Report an error · Editorial policy