Half-Life Elimination
Use this calculator to estimate how much drug remains after a given time, based on half-life, and determine the time required to reach a target concentration.
Units can be mg, ng/mL, etc. Calculations are proportional.
Hours (convert minutes to hours if needed).
Hours since initial dose.
Used to estimate time to reach target (must be < initial amount).
Results
Remaining amount
35.36
35.4% of the initial 100 remains after 12 hours.
Elimination constant
k = 0.0866 h⁻¹
First-order elimination rate constant.
How to Use This Calculator
Get half-life data
Use published pharmacokinetic references (Lexicomp, FDA labels) for half-life values.
Enter initial amount
Can be dose (mg) or concentration (mg/L). The calculator works proportionally.
Estimate target levels
Optional target values help plan washout periods or therapeutic drug monitoring schedules.
Formula
Elimination constant: k = ln(2) ÷ t½.
Applicable to first-order (linear) kinetics. For zero-order or saturable kinetics (e.g., phenytoin, ethanol), use alternative models.
Frequently Asked Questions
What if dosing is repeated?
Use accumulation formulas or population PK software; this calculator models a single dose washout.
Does route of administration matter?
Half-life typically refers to the elimination phase regardless of route, assuming complete absorption.
How accurate is half-life?
Half-life varies with age, organ function, genetics, and drug interactions. Use patient-specific data when available.
Can I use this for biologics?
Yes for monoclonal antibodies that exhibit linear elimination, but note that many have long half-lives (days).
How do I convert to half-lives?
Elapsed time ÷ half-life = # of half-lives. After 5 half-lives, ~3% remains.