Drug Half-Life & Steady State Calculator
Calculate drug concentration at any time after a dose, time to steady state, and loading dose using first-order pharmacokinetics.
Results
What is it?
First-order pharmacokinetics: drug concentration decays exponentially. C(t) = C₀ × e^(−ke × t) where ke = ln(2) / t½. At steady state (after ~5 half-lives), peak and trough concentrations repeat with every dose. Css_max = Cmax / (1 − e^(−ke×τ)) where τ is the dosing interval.
How to use
Enter the dose, the drug's half-life (from pharmacology references), the dosing interval, and the time after the dose at which you want to know the concentration. Also enter the volume of distribution (Vd) and patient weight to estimate initial concentration.
Example scenario
Amoxicillin 500 mg, t½ = 1.2 h, τ = 8 h, Vd = 0.3 L/kg, weight = 70 kg. Vd_total = 21 L. Cmax = 500/21 = 23.8 mg/L. At 4 h: C = 23.8 × e^(−0.578×4) = 3.0 mg/L. Time to steady state ≈ 5.2 h.
Pro tip
⚠️ For educational and clinical reference only — always verify drug dosing with a pharmacist and current formulary. Individual pharmacokinetic parameters vary widely with renal/hepatic function, age, body composition, and drug interactions. Therapeutic drug monitoring (TDM) with actual blood levels is essential for narrow therapeutic index drugs (e.g. vancomycin, aminoglycosides).