Volume of Distribution Calculator
Calculate the apparent volume of distribution (Vd) of a drug from dose and initial plasma concentration, or from clearance and half-life, in litres and L/kg, with what the value means.
How it works
Vd = F × dose ÷ C₀ · or Vd = CL × t½ ÷ 0.693 (= CL ÷ ke) · Vd per kg = Vd ÷ body weight
The apparent volume of distribution is the volume the drug would occupy if it were everywhere at the same concentration as in plasma. It is not a real anatomical space: a drug that binds tightly to plasma proteins (warfarin, about 0.14 L/kg) has a small Vd, while one sequestered in tissues (digoxin about 7 L/kg, chloroquine over 100 L/kg) can have a Vd far larger than the body. Vd determines the loading dose and, together with clearance, the half-life (t½ = 0.693 × Vd ÷ CL). C₀ is found by back-extrapolating the log-linear elimination phase after an IV bolus to time zero.
Worked example
An IV bolus of 500 mg (F = 1) gives a back-extrapolated C₀ of 12.5 mg/L, so Vd = 500 ÷ 12.5 = 40.0 L, or 0.571 L/kg for a 70 kg adult — about total body water. From clearance instead, 4.8 L/h × 5.8 h ÷ 0.693 = 40.2 L.
Sources: StatPearls — Clinical Significance of Volume of Distribution in Pharmacotherapy · Wikipedia — Volume of distribution
Frequently asked questions
What does a large volume of distribution mean?
That the drug leaves the blood and concentrates in tissues (fat, muscle, organs), so plasma levels are low for the amount in the body. Such drugs need larger loading doses and are poorly removed by dialysis.
What does a small volume of distribution mean?
That the drug stays mostly in the plasma or extracellular fluid, often because it is highly protein-bound or very water-soluble — for example warfarin, heparin and aminoglycosides (about 0.25 L/kg).
How is volume of distribution used clinically?
To calculate loading doses (dose = Vd × target concentration), to estimate half-life with clearance, and to judge whether dialysis can remove a drug in overdose (usually only if Vd is below about 1 L/kg).
Can Vd be larger than the body?
Yes. Because it is an apparent volume, a drug that binds strongly to tissue can have a Vd of hundreds or thousands of litres, such as amiodarone (about 60 L/kg).
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.