Loading dose calculator

A loading dose is the volume of distribution multiplied by the target concentration, divided by bioavailability. Clearance plays no part, because a loading dose fills a space rather than replacing what is being removed, which is the job of the maintenance dose.

When to use the loading dose calculator

Use when a therapeutic concentration is needed faster than five half-lives would allow, which is the entire reason loading doses exist. Phenytoin, digoxin and many antimicrobials are given this way.

The loading dose formula

Loading dose = (volume of distribution in L/kg × weight in kg × target concentration in mg/L) ÷ bioavailability. For an intravenous dose, bioavailability is 1.

Worked example

Vd 0.7 L/kg, weight 70 kg, target 20 mg/L, given intravenously. Loading dose = (0.7 × 70 × 20) ÷ 1 = 980 mg.

The mistake that costs marks

The loading dose depends on volume of distribution and the maintenance dose depends on clearance, and they are calculated from different numbers for different reasons. Renal impairment changes clearance and therefore the maintenance dose, while leaving the loading dose largely unchanged, which is the distinction most often tested.

Study aid, not a clinical tool

This calculator exists so you can learn the method and check your working for an examination. It is not medical advice and not clinical decision support, and it must not be used to make a decision about a real patient. Verify any result against your local formulary, the product literature and a qualified pharmacist. Our medical disclaimer sets out the full position.

Calculators you will usually need alongside this

Most questions that reach for one of these reach for another a step later, so these are the three that most often pair with it.

• Half-life and steady state calculator — Half-life
• Creatinine clearance calculator (Cockcroft-Gault) — Creatinine clearance
• Infusion rate converter (mcg/kg/min to mL/hr) — mcg/kg/min to mL/hr

Keep going

Calculations are only half of what examinations test. The other half is knowing why a drug does what it does, which is what decides whether a number you calculated is even the right number to be calculating. Start with CYP450 inducers and inhibitors. The drug class pages cover the mechanism behind the number, or browse the rest of the free calculators and try the practice questions.