Pharmacokinetics Calculator
Calculate Vd, half-life, clearance, and dosing from pharmacokinetic parameters.
Volume of Distribution Is Apparent, Not Real
Vd is the volume that would contain the whole dose at the measured plasma concentration. It is a proportionality constant, not an anatomical space — which is why values can vastly exceed body volume.
| Vd | Interpretation | Example |
|---|---|---|
| ~5 L | Confined to plasma | Warfarin, heparin |
| ~15 L | Extracellular fluid | Aminoglycosides |
| ~42 L | Total body water | Ethanol |
| > 100 L | Extensive tissue binding | Digoxin ~500 L, chloroquine ~15,000 L |
A Vd of 15,000 L does not mean the drug occupies that volume. It means plasma concentration is very low relative to dose because the drug is sequestered in tissue. High Vd also means haemodialysis will not remove the drug, since only the small plasma fraction is accessible.
Half-Life Is Derived, Not Fundamental
Half-life depends on both clearance and volume of distribution, so a change in half-life alone does not identify what changed. Renal impairment lowers clearance and prolongs half-life; oedema raises Vd and prolongs it too — different mechanisms requiring different dose adjustments.
| Parameter | Determines |
|---|---|
| Clearance | Maintenance dose |
| Volume of distribution | Loading dose |
| Half-life | Time to steady state and dosing interval |
That mapping is the practical core of clinical pharmacokinetics. A loading dose fills the distribution volume; a maintenance dose replaces what clearance removes. Renal impairment therefore alters the maintenance dose while leaving the loading dose unchanged.
Steady State
Reaching steady state takes about five half-lives regardless of dose. Increasing the dose raises the eventual concentration but does not shorten the time to reach it — only a loading dose does that.
Note that first-order kinetics assumes elimination proportional to concentration. Phenytoin, alcohol and salicylates saturate their elimination pathways, so half-life lengthens as concentration rises and small dose increases produce disproportionate rises.
Worked Examples
Common Mistakes
It is a proportionality constant. Values above 100 L simply mean extensive tissue binding keeps plasma concentration low.
Loading dose depends on volume of distribution; maintenance dose depends on clearance. Renal impairment alters the latter.
Half-life depends on both clearance and Vd. Oedema and renal failure both prolong it by different mechanisms.
Phenytoin, alcohol and salicylates saturate elimination. Half-life then rises with concentration and small dose increases have outsized effects.