Use this pharmacokinetics tool to estimate how much of a drug remains in the body over time. Enter an initial amount, half-life, and elapsed time.
What is a drug half-life?
A drug’s half-life is the time it takes for the amount of that drug in the body (or bloodstream) to drop by 50%. If a medication has a half-life of 8 hours, then after 8 hours about half remains, after 16 hours about one-quarter remains, and so on.
This concept is a cornerstone of pharmacokinetics and helps clinicians plan dose timing, understand accumulation, and estimate when a drug is mostly cleared.
The formula used in this calculator
This half life drug calculator uses a standard exponential decay model:
Remaining amount = Initial amount × (1/2)(time ÷ half-life)
It assumes first-order elimination, meaning a constant fraction is removed over each interval (not a constant milligram amount).
How to use this half life drug calculator
- Enter the initial amount of drug (for example, mg after a dose).
- Enter the drug’s half-life in hours.
- Enter elapsed time since dosing.
- Optionally, enter a target amount to estimate how long it takes to fall to that level.
The output includes remaining amount, eliminated amount, percent remaining, number of half-lives passed, and common clearance landmarks such as 95% and 99% elimination times.
Quick interpretation guide
After 1 half-life
Approximately 50% remains.
After 2 half-lives
Approximately 25% remains.
After 4 to 5 half-lives
Most drugs are considered functionally cleared for many practical decisions (around 94% to 97% eliminated), although this depends on context and therapeutic window.
Why half-life matters in real life
- Dosing schedules: Helps determine how often a medication should be taken.
- Steady state: Repeated dosing usually reaches near steady state in about 4–5 half-lives.
- Washout periods: Used before switching therapies or preparing for certain tests/procedures.
- Side effect duration: Longer half-life drugs may cause prolonged effects or interactions.
Important limitations
This calculator is educational and not a substitute for medical advice. Real drug behavior can differ due to age, liver and kidney function, genetics, body composition, drug-drug interactions, protein binding, and non-linear kinetics. Some medications have active metabolites with different half-lives, which can extend clinical effects.
Frequently asked questions
Is half-life the same for everyone?
No. Population averages are often reported, but individual half-life can vary significantly.
Does “out of your system” mean zero drug left?
Not exactly. It usually means levels are low enough to be clinically negligible for a specific purpose.
Can I use this for any medication?
You can estimate with any known half-life, but always verify dosing or safety decisions with a healthcare professional.
Bottom line
A half life drug calculator is a practical way to estimate drug elimination and timing. Use it to build intuition, but rely on clinical guidance for treatment, interactions, and safety-critical decisions.