Drug Half-Life Calculator
The elimination half-life of a drug (t1/2) is the time required for the plasma concentration to decrease by half. It is derived from the elimination rate constant (ke) using the relationship t1/2 = 0.693 / ke, where 0.693 is the natural logarithm of 2. Half-life governs dosing interval selection, time to steady state (approximately 5 half-lives), and washout period. Enter ke in units of hr-1 to obtain t1/2 in hours, or adjust units accordingly.
Drug half-life formula
t1/2 = 0.693 / ke
Where ke is the first-order elimination rate constant (units: time-1). The constant 0.693 is ln(2). If ke is in hr-1, t1/2 is in hours. Steady-state is reached after approximately 5 x t1/2.
Understanding drug half-life
- After 1 half-life, 50% of the drug remains; after 2 half-lives, 25% remains; after 5 half-lives, about 3% remains.
- For drugs with linear (first-order) pharmacokinetics, half-life is constant regardless of dose.
- Half-life varies with both volume of distribution (Vd) and clearance (CL): t1/2 = 0.693 x Vd / CL.
- Drugs with long half-lives (e.g., amiodarone, digoxin) require longer washout periods before switching therapy.
- Renal or hepatic impairment can prolong half-life by reducing clearance.
Drug half-life calculator: frequently asked questions
What is drug half-life?
Drug half-life (t1/2) is the time required for the plasma concentration of a drug to fall by 50%. It is a key pharmacokinetic parameter that determines how frequently a drug must be dosed and how long it takes to reach steady state or clear the body.
What is the elimination rate constant ke?
The elimination rate constant (ke) describes the fraction of the drug eliminated per unit time. It is the slope of the log-linear plasma concentration-time curve during the elimination phase. Units are reciprocal time (e.g., hr-1).
How many half-lives does it take to reach steady state?
Approximately 5 half-lives are needed to reach more than 97% of steady-state concentration when dosing at regular intervals. This is why the time-to-steady-state calculator uses 5 x t1/2 as its estimate.
How many half-lives does it take to eliminate a drug?
After 5 half-lives, about 97% of a drug has been eliminated. After 7 half-lives, more than 99% is eliminated. A drug is considered clinically eliminated after 4 to 5 half-lives.
Can this calculator be used for clinical dosing decisions?
This tool is for educational and informational purposes only. Clinical dosing decisions must be made by a licensed healthcare professional using patient-specific data, clinical judgment, and institutional guidelines.
Official sources
- U.S. Food and Drug Administration: FDA Pharmacokinetics Guidance.
- NIH National Library of Medicine: Pharmacokinetics (StatPearls).
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.